Perspective correction
with DxO ViewPoint

  Corrections on the
image geometry



  
Foreword

DxO ViewPoint offers the tools necessary to optimized correction
of the photographed scenes geometry.

This tutorial is mainly a reflection on the
perspective correction in photography.

Your comments and comments are welcome: tuto.dxo [at] free.fr
Gérard THOMAS - April 2019
Collaboration, translation and layout by Pascal PELÉ

Translation is checked by Endre Tollar and Erica de Vries :)


 


The different versions of Viewpoint

Distorsion correction
Manual distortion correction

Horizon correction
Cases where it is not necessary - Cases where it is essential

Automatic Horizon correction

Horizon Manual correction
Using a vertical element -
Using a horizontal element
Identifying the skyline

Perspective correction
Should we totally correct the perspective ?
Planning for perspective correction at the time of the shot

Automatic Perspective correction and Cropping
Limiting one of the directions; vertical or horizontal - Changing the intensity of the correction

Perspective correction using lines and points
Perspective correction by the verticals / horizontals - By rectangle - 8 points tool

Manual correction of the Perspective
Up / Down Correction - Left / Right Correction - H/V ratio

Volume Deformation correction
Automatic correction - Manual correction
Correction by Diagonal  - Type Horizontal /
Vertical

Using creative corrections
Give a playful aspect  - Work on an image for an aesthetic or artistic rendering

The Miniature effect








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The different versions of ViewPoint

He has only one license for the three applications:
* A version integrated in DxO PhotoLab (and DxO OpticsPro)
* An independent version (standalone)
* An external module  (plug-in) for Photoshop, Photoshop Elements, Lightroom et Aperture

Whatever the medium, the tools are identical.


* In the ViewPoint version integrated in DxO PhotoLab, the tools are available via two palettes.
- "Geometry" palette: Horizon, cropping and distortion settings (basic in PhotoLab)
- "ViewPoint" palette: The other settings specific to ViewPoint

Only this version benefits from parametric processing without leaving the raw workflow, which allows it to resume corrections after registration.


* The ViewPoint independent version opens JPEG or TIFF images (8 and 16 bits).

It is convenient for correcting direct DCs JPEG files.


* How to Develop a Photo and Correct Perspective with ViewPoint, Plug-In for Adobe:
Photoshop - Photoshop Elements and Aperture

ViewPoint's Opening Sequence in Photoshop (for a raw):
- Photoshop: Open a raw file (via Camera Raw)
- Camera Raw: Developing the photo and returning to Photoshop
- Photoshop: Calling the ViewPoint module and automatic transmission of an RGB image 
- ViewPoint: Image correction and return to Photoshop



Opening sequence of ViewPoint plugin for Lightroom:
- Lightroom: Open and develop a raw file
- Lightroom: Call the ViewPoint module and transmit an RGB file (JPEG or TIFF (8 and 16 bits)
  Prefer 16bit TIFF export without compression
- ViewPoint: Image correction 
- ViewPoint: Save RGB file and return to Lightroom
 


The tutorial illustrations are made with ViewPoint standalone to facilitate reading for those not used to the PhotoLab interface.

There are several ways to manipulate the sliders.
- Enter a numerical value directly on the keyboard
- Click on the cursor, then move it with the mouse wheel in predetermined steps
- Move the cursor and hold it down
  PhotoLab only: Clicking and dragging the cursor allows you to finetune the values
- Double-clicking the cursor brings it back to its default value


Resetting corrections:
-
ViewPoint: Click on the "U-turn" arrow at the top right of each of the palettes
- Photolab:
Double-click on the cursor, or the cancel icon for the perspective palette



ViewPoint integrated in PhotoLab

  ViewPoint standalone version



ViewPoint plug-in for Lightroom


ViewPoint plug-in for Photoshop

 
Distorsion correction

Before correcting the image geometry, it is necessary to pre-correct lens distortion.


The lens distortion is all the more important as the focal length is low.
A wide angle will be more affected by distortion than a long focal lens. However, the wide angle lens precisely requires straightening the horizon and / or correct the perspective. Hence the need to carefully execute this step for a perfect result.




Integrated PhotoLab Version
This step is transparent since performed automatically for the supported lenses.

Lense modules management is a basic function of PhotoLab, see tutorial: Efficiency with DxO PhotoLab.

Distortion correction for a 16mm lens (24x36 format).



Distorsion correction


Independent version plug-in for PS / PSE / LR
ViewPoint accesses the lense database characterized by DxO and has the same automatic distortion correction as PhotoLab
.

This can justify the use of ViewPoint to benefit from this fix.


One of several cases occurs at the opening of an image based on the presence or absence of EXIF allowing ViewPoint to know which lens was used.

* The module is already present and the correction is applied
* All the necessary elements have been found in the EXIF. A dialog box opens, the module is imported and the correction is applied
* There is no EXIF metadata, or they do not allow to determine which lens was used.
ViewPoint asks "Open original image": the file (JPEG or raw) from the camera. When the module is loaded, the DC model, the lens and the original image file name are displayed, and the correction is automatically available.
* The elements have been found in the EXIF, but there is no corresponding module for the purpose. In this case, only the manual setting is possible (AUTO button is gray)



Opening ViewPoint


   
Manual distortion correction

ViewPoint provides tools according to the three types of distortion. 
- Barrel distortion
- Pincushion
distortion
-
Taken from fisheye

* A monument taken from the front: The distortion is often clearly visible and easily correctable
* A landscape without particular visual cues (buildings, monuments). As the perspective correction is rarely used, it may be preferable to not correct anything.


Correcting the distortion is an obligation before a perspective and / or volume correction.

A monument taken from the front with a 28mm wide-angle lens (24x36 format).
Referring to the paving stones line at the edge of the image, there would be a slight barrel distortion. The examination of other photographs, taken at the same time, shows that the middle of the paved passage is slightly curved and that the 4 central studs have their vertex located on the same straight line. It is therefore on these studs that the correction must be based.
One has to be sure of the reference points.




Manual correction of the distortion



The distortion is constant for a given focal length (except for some zooms at short distance). A simple trick is to take a photograph of a geometric shape with straight lines (door frame, ...) and placing it closer to the edge of the frame and record the correction values.
This is enough for a fixed focal length. For a zoom, it will be necessary to repeat the operation for the principal focal lengths
.

This manipulation is longer to describe than to implement.

Photo from a 35mm vintage lens, with a complex "mustache" distortion.
The complete correction of the DxO modules quality is obviously not possible in this case. Manual correction is a very acceptable alternative and far superior to no correction. 
Examine a test picture by using the approach above. With such a barrel distortion, the best result is obtained with a value of 30. Some defects remain in the angles that can not be fully corrected, but the damage is reduced to a minimum. The correction values thus achieved will be extended for all photos.




PhotoLab Elite users can create a partial preset of these corrections and apply it to each new photo of that lens.

(continued from the comment on the illustration) Scan from a slide taken with this lens. Barrel deformation is not immediately noticeable. The correction values determined in the previous procedure are applied to all pictures taken with this lens.
 


Permanent correction of the distortion


Horizon correction

The next step consists in correcting the horizon. This correction seems simple but must be very precise in case we need to work on the perspective.  


Cases where it is not necessary to correct the horizon
* When the photographer has intentionally applied an important angle while shooting            
There must be no ambiguity about the original intention so that correcting does not make any sense. This is a common case in action photo reports and street photos where spontaneity generally prevails
.
* If there is no horizontal mark present in the frame
This is a common case for indoor shots taken
from above or low angle shots. The vertical should be aligned with the main subject, even if it is off-center. One rarely corrects the perspective in this situation, which does not preclude volume deformation correction.
* When there are no horizon references or exact verticals in the image
It is common in the forest that trees lean slightly in the same direction, but this is perceived more naturally than a corrected image with vertical trees.

Cases where correcting the horizon is essential

Virtually all other cases and especially.
* When the frame has a pure horizontal (in particular seascapes)

This is the "sea that empties" syndrome where a slightly flawed horizon can be experienced as unbearable by some spectators.
* The photography of architectural elements (cityscapes, monuments, etc ...)
* If the subject is mirrored according to a natural horizontal plane (water plane, reflecting surface) 
* When the next step is to manually correct the perspective (see chapter)

 



Automatic Horizon correction

The Automatic horizon correction is a particularly effective tool when ViewPoint finds sufficiently horizontal or vertical elements in the picture, even of small size.

The presence of verticals is particularly successful in the correction.


In palette "Horizon" 
- ViewPoint: "Auto"
Button 
-
PhotoLab: Magic wand 

The grid display allows visualizing horizon defects and their magnitude before correction, and the validity of the proposed recovery after correction.

Display the Grid
- ViewPoint: Toolbar Button (G Key)
- PhotoLab: Menu: View - Grid
(Ctrl/Cmd+ G)


There is no need to correct the horizon if you plan to use Perspective AutoCorrect since it will take care of it by itself.

A slightly leaning sea horizon. The automatic result is perfect.

Here, the horizon is centered on the frame and, as a result, little affected by possible optical distortion defects. When it is close to one of the lower or higher edges, it can sometimes be a very important distortion.




Marine Horizon











ViewPoint sometimes fails to perform a satisfactory automatic correction. Most often this failure is due to "disruptive elements".
E.g. the presence of oblique lines without other vertical or horizontal references, or a majority of non-parallel vertical lines in the picture. The correction error is often very obvious.


This is a significant example in which, despite the presence of buildings in the background, ViewPoint does not seem to appreciate the monumental modern sculpture!



Failure of Horizon Auto correction


Horizon Manual Correction

It is necessary to manually correct the horizon both when the result of the auto step is unsatisfactory and as a preliminary to a manual correction perspective.

The difficulty is to find the right element truly horizontal on which to rely.
In our contemporary environment, vertical lines are most common.
Finding a vertical passing through the middle of the photo is relatively easy, especially in urban photography or monuments.
 







A typical example where the vertical is directly accessible on the subject, the horizontal can not be determined directly. 



Verticals are more common





Successfully correcting the horizon
It is important to determine which reference elements can be used in the image.
Using a horizontal element is not always indicated, since the perspective effect quickly loses the horizontality of the lines. It is often better to rely on the vertical elements of the image.


Using a vertical element
Rigorous photographers look for the median vertical element as close as possible to the vertical axis of the image. If there is not exactly one in the middle, it is often possible to interpolate a virtual line between two elements both sides.


The reference element must of course be perfectly vertical in reality. If it is also the case in the image, it means that the horizon does not need to be corrected.

The most classic cases where vertical elements are not lacking.
The shorter the vertical reference, the more accurate the placement of the marks needs to be.




Using a vertical element


ViewPoint: in palette "Horizon"
- "Vertical level" button - The tool is positioned in the middle of the image
- Adjust each end. Press SHIFT simultaneously to slow down

PhotoLab: in palette "Geometry / Horizon" OR the upper toolbar        
- "Horizon" button - The tool, a horizontal line, is centered on the image.        
- Ignore the displayed level line and click on the first reference point
- Hold down the mouse button and set the other end of the new level line that appears. ViewPoint understands that this is a vertical level and considers it as such!


This procedure works for both versions

There is no magnifying in PhotoLab. This absence is compensated by zooming (Ctrl + scroll wheel) on the selected area. it's just as fast and accurate.


Illustration with a complex image.
In this photo, nothing seems made by a plumb-line, but it is necessary to correct the horizon before subjecting it to the Perspective correction. The problem is to find a "real" vertical. Do not be influenced by the lower part of the ditch walls.
Vertical tool display shows that the image axis is almost exactly on the massive entrance front gate, which is likely to be really vertical. It is thus possible to make a fine adjustment of the vertical level.
The correction angle displayed is 0 °!
This means that the image, despite appearances, is perfectly aligned horizontally. On the other hand it will be necessary to correct the perspective.



Complex Image







The reflection on a horizontal plane
An object and its reflexion on water body are strictly located on the same vertical. The horizon setting need to be perfect.
The principle is identical to the previous examples. Point an element towards the middle of the image,  not necessarily a line, and connect it to its reflection.        
In almost all cases, this operation must be followed by a straightening of perspective so that all objects that are not exactly in the middle of the image have their vertical reflections.



The reflection on a horizontal plane






Using a horizontal element


In palette "Horizon"
- "Horizontal level" button - same procedure as previously


A body of water whose opposite bank is located far away.
This photo is the typical subject for adjusting the manual horizon recovery. Thebank is located at a distance compatible with the sea horizon.
The automatic correction does not work because of the absence of vertical lines and the "horizon" line of the bank is disturbed by the waves and boats. The eye succeeds without problem to draw an exact line. The reference line (opposite bank) does not occupy the entire image, but its length is sufficient for perfect correction.



It is sometimes desirable to correct a troublesome "pseudo horizon", a distant line leaning in reality, but which gains to be rectified.



Use a horizontal element



Identifying the skyline


In reality the skyline is exactly at eye level, straight in front of you.
In photography, where the sensor is often inclined, the horizon is rarely on the line of sight (median height of the image).

Defining the horizontal of the photo consists in aligning with a support line which, in reality, is exactly on the skyline. This support line can occupy any position on the height of the photo but that has consequences on the vertical lines.

This position depends on the down / up orientation of the line of sight:
* Case 1- If it is pointing down (shot from above), the line of sight rises to the top of the picture and the vertical lines point naturally downwards
* Case 2- If the line of sight is horizontal (strictly vertical case), the horizon line passes exactly in the middle of the photo.
* Case 3- If it points upward, the horizon goes down. The low angle shot is the most frequent case for photos of buildings / monuments. Note the verticals that point upward.

The difficulty is to find the correct really horizontal element which is often more complicated to discover than a vertical one.




 Definition of the skyline



  



Two essential rules for identifying the skyline:
- Rule 1: It is located at the photographer's eye level (in the same horizontal plane)               
- Rule 2: It is defined by the convergence to infinity of coplanar lines*  of landscape elements
   * lines located on the same horizontal or vertical plane 






The rules of the skyline; the convergence of the lines is verified.

Here the horizon line is already known but the line convergence would allow finding the horizon line if this one was not visible.




The rules of the skyline




To correct the skyline, we must first identify its position in the image!
* The simplest case - a building seen strictly from the front. In this situation, all facade horizontal lines are parallel to one another and to the horizon line.
Any of these lines can be used to correct the horizon.
* A building or cityscape viewed from any angle - locate the position of the skyline





This building fits this criterion well.
This is a slightly low angle performed shot, the skyline is approximately located in the middle of the platform height  but can not be defined exactly. A significant horizontal facade alignment is used to correct the horizon.




Front building









When the skyline is not visible, locate it.

First clue according to rule 1:
A person is close to the front on the right. The skyline is probably at his eye level since the photo was taken with the camera at eye level.
The horizon line would thus be at the height of the first window bar.
Observation:
On the other hand, the two other people in the center of the image are
clearly below. Either the soil goes down here or the reference is not good.
Second clue according to rule 2:
The virtual lines of the recent paving (probably flat) converge a little below the first bar.
Verification:
All first
window bars are not on the same line: the skyline is not located exactly on these bars, but a little below.
Taking the bottom of the windows as a reference, we realize that they are all located on the same straight line: It is the skyline !




Locating the skyline 1
















Locate the skyline using the parallel planes of the steps of a staircase.

This neo-renaissance building is, at first glance, not very conducive to this exercise. But it has a very useful element in many similar cases; a staircase. When the steps are horizontal, one of these steps will be close to (or on) the skyline, it is the first one whose upper face is no longer visible.

The solution is to take this step as the reference, then, from this point, to find the elements located on the same plane. All those above  and below this step are not aligned.




Locating the skyline 2



Locate the skyline on topics of varying shape.

A 15th century castle. With its round towers and a strong low angle shot, finding the skyline seems complicated.
In fact, on the first tower, just look for the line where mortar between the stones passes from a concave line to a convex line: it is the horizon line !




All the horizontal elements, whatever their orientation, located on the skyline are on the same plane and therefore appear merged on the same line.
On the contrary, they will appear to follow divergent lines or elliptical arcs if they are on different planes than the horizon line.
This applies to all forms (parallelepiped, cylinder, sphere, cone ...).


However, it must be ensured that the contiguous elements are on the same horizontal level, which is not always the case on historic buildings.


On the last example, the two towers of the entrance do not have their stone beds at the same level, which is why the skyline is in the middle of a bed on one side and on a joint of the other.



Locating the skyline 3


Perspective correction






While human vision is binocular, a camera is monocular. We can add that man has a particularly powerful image processor: the brain. 
The rules of perspective are known since the Renaissance and painters apply them ever since.
The verticals are always represented as genuinely vertical (parallel) and the
horizontals as vanishing (non-parallel).

Accepting a high or low angle vision with vanishing verticals is a recent issue which appeared with the invention of photography. Although the first photographers took great care to avoid it by using tilted view cameras.
On the other hand, the cinematographic and advertising iconography has used and abused this vision of high / low angle shots so it may seem natural ... but it is not at all!
Before the appearance of correction software and in the absence of a shift lens, the effect was inevitable.

Should we totally correct the perspective?
Logically yes, because that is how the human brain sees it.
For centuries, painters painted scenes with
parallel verticals, and no one affirmed it was unnatural.

Monument photographers use tilt-shift lenses for this purpose.
Today images are everywhere and we should consider in what position the image is presented to us.
For a photo presented flat on a table, the brain naturally expects vanishing verticals whereas displayed on a wall, parallel verticals are consistent with normal vision.


Totally corrected, or not, remains the choice of the author of the photo.
Totally correcting the perspective is sometimes necessary: descriptive or architectural photo, panoramic preparation, etc ...
For the other cases, the intended use the image needs to be considered, the output format, and of course personal taste!

Totally correcting the prints to be exposed is advisable

When choosing a partial correction a totally perfect correction is required before modulating.
A simplified and / or unsymmetrical correction is quickly detected upon reading.


Planning for perspective correction at the time the shoot
It is important to foresee corrections at the time of shooting, taking into account the loss of information on the edges.

Points to be respected (see illustration "Foreseeing correction at the time of shooting"):
* The virtual lines "framing the subject" (in red on the illustration) at each side must imperatively be contained in the "lower limit of the frame ". In other words, keep a margin (in orange) with the edges of the image. Thus, all horizontal lines will cross within the corrected image frame.
* Do not adjust the subject to the edges of the image. Provide a margin adapted to the subject and its environment.
* Adjust horizontality as good as you can, for example by ensuring that a vertical element of the subject in the center of the frame is vertical in the viewfinder.

Watch the particularities of the building
When shooting historical monuments, it is important to identify all severe defects of verticality and misalignments that may exist.
Another frequent trap, buttresses and flying buttresses are usually of variable section depending on their height, and thus not vertical.

On this church view (Foreseeing correction at the time of shooting) some elements seem to be flawed.
The transept facade and the bell tower are not aligned in reality
Here, one should not rely on the buttresses to define the vertical, but on the unaligned facade at the back.


Example of a photo poorly adapted to the perspective correction.
If the idea was to include the left and right towers
in the correction, with "lack of air" on each side,  this didn't work.
This shot does not meet the criteria for a successful perspective correction.




  Planning correction at the time of shooting

Shot taken too close

   
Automatic Perspective correction and Cropping



Reminder: Automatic perspective correction does not require prior horizon correction.
If the horizon has been changed automatically, the position of the horizon cursor reset.


Success of the automatic perspective correction depends on several criteria.
* The presence of enough vertical and / or horizontal components
Absence of disturbing elements (remarkable neither horizontal nor vertical lines)

When these conditions are met, ViewPoint gives very good results.


In palette "Perspective" 
- ViewPoint: "Auto" button
- PhotoLab: Magic wand


This corrected image does not pose any particular problems.
The subject has enough components to ensure success. The limited extent of recovery leads to a small loss of information to cropping.
The left most part has a horizontal alignment difference (subsidence) which ViewPoint respected.



Auto perspective correction



A successful automatic correction may truncate a large part of the subject.
This is a very common situation where ViewPoint crops to the narrowest part of the image (usually at the bottom since most buildings photos are low-angle shots).
Cropping is required and sometimes setting a different image ratio
as well.

In palette "Crop" / "Gemetry - Crop" 
- ViewPoint: Choose "Custom" from the drop-down list, then enter the width / height ratio
- PhotoLab: Choose or enter the width / height value directly in the "Aspect ratio"

ViewPoint only: The checkbox "Constrain to image" forces the frame to remain in the image area.


This illustration requires considerable perspective correction. Whereas there is no problem with ViewPoint AutoCorrect,, Auto Crop hides the upper part of the subject.
The original 3:2 ratio has been preserved but with a bit of a compromise at the bottom and top of the image. The ratio choice sometimes depends on external constraints, such as the consistency of a series or the compatibility with a standard print format. A 4:3 format is often a good option. In cases of very severe adjustments, the 1:1 ratio is often the only option.


It is possible to keep the entire image, black areas included, for later editing in Photoshop (or a pixel editor, e.g. Affinity Photo).



Auto Perspective correction and manual cropping



Limiting the auto correction to one of the directions; vertical or horizontal
  
This limitation in the directions makes it possible to preserve the automatic
correction of the perspective.


In palette "Perspective" 
- ViewPoint: "Auto Correct" drop-down list
- PhotoLab: "Auto mode" - Choose the desired direction:
   - Verticals & horizontals (default)
   - Verticals only
   - Horizontals only








In this illustration, verticals correction only is the one that comes closest to an exact correction.



Limit auto correction to one of the directions Vertical or Horizontal



Changing the intensity of the correction
By default, ViewPoint makes a entire automatic correction (100%), that is to say the directions are perfect. It is possible to reduce its effect with the slider "Intensity".


In
palette "Perspective" 
- Acting on the "Intensity" slider


This is relatively common situation of the limitations of automatic correction. It seems successful when it does not conform to the rules of perspective. Or, it is not pleasant to the eye.

If it is not an essential image, reducing the intensity of the automatic correction leads to an acceptable compromise.



The illustration shows intensity changes to 100% - 80% - 50%
Shooting is not perpendicular to the subject, so there should be horizontal vanishing (here to the right). 



Changing the intensity of the correction


Perspective correction using lines and points


Perspective correction by the verticals


In palette "Perspective"  
- ViewPoint:
Button "Force vertical parallels"
- PhotoLab: Button "Force parallels"
ViewPoint uniquement: The Tab[ulation] key is used to navigate from one marker to another..

* The selected straight lines should preferably be on the same vertical plane for a better correction quality (same facade of building, ...).









The edges of the facades are on the same vertical plane while far away. 
For more accuracy, it is better to be on verticals located approximately equidistant from the point of view. The image is then finalized by adjusting the crop.



Perspective correction by the verticals



Perspective Correction by Horizontal
Contrary to the correction by verticals which is in common use, correction by horizontals is limited to a few cases. By straightening the vanishing (who become thus parallel) it does not respect the rules of perspective.
Its scope concerns topics framed almost from face (but not quite). The vertical axis of shooting (
high or low angle shot) must also be sufficiently limited not to present vertical vanishing too important (otherwise it is better to use the correction 8 points).



In palette "Perspective"  
-
ViewPoint: Botton "Force horizontal parallels" 
- PhotoLab: Botton "Force parallel" - Rotate the vertical lines by a quarter turn




The illustration shows both the interest of such a correction, a more balanced view of the subject and its limits, the deformation induced on the tower of the background.



Perspective Correction by Horizontal


  
Perspective correction by Rectangle

This tool is intended to straighten any square or rectangular subject taken at any angle. What conduit to move from one point of skew view in a strictly frontal view.

In palette "Perspective"
-  "Rectangle"
button

The recovery of the perspective necessarily leads to a deterioration of the quality of the image in the most stretched parts. Although ViewPoint behaves pretty well in this usage. It is better to limit to reasonable angles of shooting.


* It is imperative that the subject to straighten is flat.
This only concerns a limited number of cases, but for which the function is of great interest. In particular because the original proportions of the subject (Width / Height aspect ratio) are restored correctly.
* The photographer must provide the recovery in a perfect rectangle when shooting. It is recommended to align the side of the subject nearest with the edge of the frame to maintain image quality.
* In addition for short-range views taken at a large angle, there may also be problems with insufficient depth of field between the near and far edges.

Typical case of rectangle correction; photography of exhibited works.



Perspective correction by rectangle



Perspective correction 8 points
This tool combines the possibilities of correction by the verticals (perspective correction) and correction by the horizontals (horizontal recovery for a subject taken "almost" Front) with the advantage that it can take into account plans different.
The field of application concerns the subjects with a scene mainly frontal shooting
from above or, more generally, low angle shooting.
The typical case is a courtyard with a central facade "almost" front view and facades in return.



In palette "Perspective"
- "8 points" button


The interior low angle shooting view was not taken exactly in the axis, hence the horizontal vanishings on the transept. The 8 points correction allows you to straighten both the verticals and horizontals lines of the transept, including the lack of horizontality to shooting.



Perspective correction 8 points 1







One particular use of the 8 points correction is to not use the four lines of the tool. The most often only one line is enough to define the horizontal for a fast and very satisfactory result.

In the Illustration, both verticals are imposed on the ends of the facades, which improves the straightening accuracy. The lower horizontal line is set on the skyline, and the upper horizontal is offset (without changing its direction) towards the top of the image to reduce its influence to a minimum.



Changing the intensity of the perspective correction using lines and points

As for the automatic perspective, it is possible to reduce the effect of the corrections with the slider "Intensity" for more realism.



Perspective correction 8 points 2


Manual correction of the perspective


When the auto result is simply not acceptable. It is primarily intended for expert users who wish to have control over the entire correcting process.

A successful manual correction gives the best possible result, which can be very close to what allows a tilt and shift lens. 
These manual settings can also be used to fine-tune an imperfect automatic correction by only using one of these components (up / down + left / right). 



The manual perspective correction must be preceded by the horizon correction.

Up / Down Correction
This is the essential adjustments of the perspective correction used in most cases!
The image swings around its median horizontal axis.

In palette "Perspective"
- "Up / Down" tool
   - to the left (Top): the top of the image becomes more wide; straightening of a low angle shooting (most common case)
   - to the right (Bottom): the bottom of the image expands; straightening a low angle shooting


It is interesting to straighten by disabling the cropping tool to see the extent of the effect.


Observe the phenomenon by applying the correction AND activating at the same time the cropping:
- Move the cursor to the left. The impression is to enlarge the image in its upper part when in fact it is also compressed in its lower part.
 
* It is strongly recommended to use the grid to refine the accuracy based on the side elements.
Ensure a symmetrical correction where the verticals are perfectly corrected on the left and on the right. If this is not the case, it means that the skyline is not perfectly horizontal on the image.
A slight defect can be corrected by refining the angle value (Palette Horizon / Angle).
* Horizon and perspective corrections can greatly reduce the usable area of the original image. It is possible to recover some by cloning "neutral" areas such as the sky. The crop is then airier.
This is done with Photoshop (Fill > ‘Content aware’) in addition to ViewPoint plugin.

In the first illustration, it is to finalize the image after the horizon correction and to replace a dark part out of original shooting.

The second illustration shows a case where automatic correction and correction by verticals lines fail because the verticals references are near and on the same side of the image. The Up / Down correction is perfectly adapted, the few verticals available are enough for a perfect correction.



Up / Down Correction 1

Up / Down Correction 2



Left / Right Correction

The Left / Right correction is used primarily for facade (building / monuments) photos when the horizontal line of sight is not perfectly perpendicular to the facade.
This situation occurs frequently for tall monuments when the farthest point available for shooting is not in the axis of the facade. The horizontal lines then present unwanted vanishing lines.
 

This correction must occur after the Top / Bottom perspective correction and must always be moderate to remain natural. 
Other uses for more creative renderings
are possible without the limitations of a realistic one, and then correction can be much more important.

In palette "Perspective"
- "Left / Right" tool

   - to the left: the image is settled up on the right. The height of the subjects to the right of the image decreases
   - to the right: inverse effect. The objects on the right side of the image attach greater importance

* This correction also serves to rebalance the proportions between two characters of which one is in withdrawal.

The point of view is slightly to the right of the subject: the horizontals have a vanishing point to the left while it is expected that to be really horizontal. After correcting the vertical perspective, the Left / Right correction straightens the horizontal lines. The goal is to inscribe the facade in a rectangle instead of a trapeze.



Left / Right Correction



Horizontal / Vertical ratio

This tool modifies the proportions of relative width / height of an image. In most cases, straightening the perspective with ViewPoint's tools is keeping the subject's proportions. 
Its effect is very (too) fast. It is to be used sparingly to make it natural.

In palette "Perspective"
- tool "H / V ratio"
   - to the left, squeeze the image that loses a little height
   - to the right stretches the image, always on the pitch

* Changing the H / V ratio affects the image's initial ratio. Restore it with the crop tool.

The use can be diverted in order to refine the silhouette of the model in wide shot 

It is tempting to increase the height of the building by modifying the report H / V to make the feel of the shooting (related to our vision in relief).
The rectified image is perfectly in line with the actual proportions of the facade. But the height of the towers seems diminished for a logical reason of perspective. Placed away from the main facade, they are perceived lower.
In this case, we can act on the H / V ratio to increase the height. It's a compromise between reality and feeling.


The final destination of the image and the way to looking at it may affect the need to modified the aspect ratio.
For this example, a web screen view or an A4 print may justify a slight low angle stretch to emphasize the impression of height. But this is neither necessary and not desirable for a large format print to be exposed in optimal conditions, where the eye is placed at the bottom of the image as in reality.



Horizontal / Vertical ratio

  
Volume Deformation correction 

Volume deformation change in the appearance of the components of the photo that are increasingly stretched towards the edges of the image. It is characteristic of a wide-angle lens usage and increases with the decrease of the focal length used. We usually start noticing below 28mm (24x36 format) and it becomes more and more visible later.

This distortion is cumulative with the other deformations related to the use of the wide-angle lens
* The distortion is a specific characteristic of the lens by design
* The leakage effect of the vertical and horizontal planes depending on the rules of the perspective






This is not an optical defect. This deformation is normal on wide-angle pictures taken, and more with an ultra-wide angle lens. Its magnitude is a function of the distance between the point of view and the first planes framed when this distance is very small proportionally to the framed field.
If we could look at a print (or a screen view) by positioning the eyes exactly in the same relative position as the camera when shooting (so very close, the nose stuck on it), we would not perceive any deformation. But we never look at an image like this because of the minimal distance necessary for normal vision.

* The choice between Diagonal and Horizontal / Vertical corrections determines the visual relevance of the correction depending of the photo content.
* The volume deformation is not linear. Zero in the center of the image, it becomes more and more intensive by moving away to become maximum on the edges of the image. The correction obviously takes it into account. 

Comparison of the two corrections, Cropping disabled.
Since the skyline is located on the median horizontal axis of the image, it is not deformed by the Diagonal correction as well as the vanishing substantially joining the center. The other straight lines are more and more curved as we
approach the edges. Horizontal / Vertical correction preserves the verticals of the deformation. But the perception of volumes is a little less good.



  Comparison of the two types of corrections





Automatic
correction
The Auto correction is the normal operation of this tool. ViewPoint directly assigns the correct correction value. It can be refined with the sliders but it is rarely necessary.


Unlike the automatic distortion correction, there is no requirement to have the corresponding optical module. To achieve this, ViewPoint simply needs to find the information it needs in EXIF metadata.



Manual
correction
If there is no EXIF metadata (or if they are not correctly filled in), ViewPoint indicates it in the palette "Volume Deformation". ViewPoint applies a standard correction.



Both cases are in the absence of lens module.



  Procedure in the absence of lens module


  
Correction by the Diagonal

This is the most accurate. It starts from the center of the image with an amplitude that grows as it moves away from it.
It is particularly adapted to the subjects having spherical volumes in the broad sense (characters, heads, etc.). A representative case is the selfie photo.
On the other hand, it distorts the straight lines by bending them. The deformation is maximum for the lines along the edges of the frame.


- In palette "Volume deformation" - "tool type: Diagonal"
The correction is automatic for the cases defined above.
The default value (150) is a complete correction. It can be decreased for a more natural rendering.



Illustration with a subject adapted to Diagonal type correction, placed at the edge of the frame. Focal length 21 mm (format 24x36).
The real proportions of the subject are perfectly restored. The structured background was chosen to show the deformation undergone. It is always better to have a neutral background in terms of geometric shapes. In this example it is not very recognizable and of secondary importance.
For this type of correction, the magnitude of the induced deformations can lead to a severe reframing that should be anticipated at the shooting. This is not completely the case here where the headdress mounts on the top edge of the frame while keeping the original ratio. Moving to a free crop allows it to aerate slightly.




Correction by the Diagonal


  
Correction type Horizontal / Vertical

The correction settles the image from the median vertical axis towards the edges for landscape format pictures (along the horizontal axis for portrait format pictures).
There is no curvature of straight lines. It is preferred for urban photography, monuments, interiors of buildings, etc.
It is not exact but better accepted on these types of subjects.


In case of doubt about the correction to be applied according to the subject, trying the rendering of the two types of corrections helps to determine the most adapted. 
  
- In palette "Horizontal / Vertical" - "Volume distortion"
Here again the correction is automatic if ViewPoint can access the EXIF data.

- ViewPoint automatically presents the right choice according to the orientation of the original image.
In case of a wrong proposition, the values (100 and 0) should be reversed.
The value is 100 by default for the selected correction direction (horizontal or vertical) and 0 for the other direction. This is an optimized correction for the majority of situations.
It is preferable not to modify (or very little) the correction to zero because of the antagonism between the two directions of adjustment.

 
A typical subject for this correction. Focal length 22 mm (format 24x36).
The correction restores their normal appearance to elements near the edges. The problem that usually arises is the choice of cropping after correction.
Two options are possible: keep the maximum of the image area with a free crop, or lose a little by keeping the original ratio which is the solution chosen here, with a
upward displacement to improve the cropping.



Correction type Horizontal / Vertical








Beware of "the optical illusion"!
The default display of an image in ViewPoint (as in other software) is achieved by adapting the largest dimension of the image to the available display area. We can get the impression by applying the correction that it is done in the opposite direction to that expected.
For example, for a horizontal image like this, the display shows an image that keeps the same width and increases in height, while in reality its height remains fixed and its width decreases.
This can be verified by reading the dimensions displayed in pixels.. 
 


The good solution is to use the dual view: Before / After.
With PhotoLab, we can avoid this confusion by displaying the image at a fixed (zoom) size smaller than the display window.


 
Principle of correction Horizontal / Vertical and logical direction of corrections.
 


Logic of the meaning of corrections




Application of volume corrections to an image
We often address the volume distortion correction at the end of the process. This position is well suited for images requiring diagonal correction. On the other hand, those with a Horizontal / Vertical type of correction are often subject to a correction.
This correction is, in fact, a prerequisite for horizon and perspective corrections.



This is not contradictory with the operating principle of parametric software such as Photolab or ViewPoint where each setting can be made separately, and modified at any time before the final output. Some settings condition others.

 


Further corrections for a complete treatment. Focal length 17 mm in 24x36:
On the final image, the cars in the foreground have returned to their normal proportions.




Applying volume corrections to an image


Using creative corrections

It is possible to divert ViewPoint's geometric corrections to produce an image that fits a more personal vision.

By definition there is no recipe. Everything depends on the rendering you want to achieve. It is however possible to classify them in two families:
- the corrections diversion with playful goal
- the more creative use, with aesthetic or artistic aim

Give a playful aspect to an image

If ViewPoint corrects the distortions, it is also possible to accentuate it for a funny effect.
For example, applying a fisheye effect to an image by combining the pincushion distortion and volume distortion corrections with maximum intensity, or the opposite by creating a pseudo 3D effect from a fisheye lens correction .

Since we start from a flawless image, the distortion corrections lead to reverse deformations; for example, a barrel deformation is created using the pincushion correction.

Illustration of these two proposals. The first to give a comic book effect, the second to create a more delirious relief effect.



Giving an image a playful aspect




Work on an image for an aesthetic or artistic rendering
The goal is not to have an absolutely faithful presentation of the scene but a personal interpretation remains however plausible. This obviously requires a fairly strict approach and it is often desirable to have thought the final effect from the shooting.





An image where the final result has been thought as soon as the shot is taken.
Highlight the sculpture framed by the "555" and its reflection on the neighboring building facade. These elements conditioned the point of view, and the facades were straightened to have a rectilinear vision. The choice of a 16/9 format cropping allows to place the sculpture in the crossing of one thirds, the image highlight.
Photo Pascal Pelé




Working an image for an aesthetic or artistic rendering


The miniature effect

The miniature effect allows to a "mock-up rendering" with a very small depth of field as would be the case with a real mock-up proxi-photo shooting. It also allows rendering simulation of a tilt lens where the sharp area of depth of field can be placed in the frame at the desired location and with any orientation.

The miniature effect has a fairly wide scope since it can be used to highlight a sharp subject by creating a fuzzy environment. It can also be used to accentuate pre-existing blur areas.
This effect excels with subjects that can be conceived in a miniature mock-up.
Landscapes, mostly urban, are especially suitable for shot from above, with
a limited sky zone.

Typical case where the rendering model can be obtained with sharp / blur areas in accordance with a conventional lens, or those of a tilt lens.
In the first case the zones succeed each other horizontally from the first plane of the image. It can be considered more authentic because it conforms to the usual vision of such an image.
In the second case we can choose their orientation. It is often more spectacular when the components of the image agree.



The miniature effect


ViewPoint: In palette "Miniature effect"
PhotoLab: In alette "DxO ViewPoint"
- Activate the tool

The lines delimiting each of the sharp / blur areas are displayed at their default position.
- Clicking the center point moves the group of lines.
- Click on the point of a line in solid line allows to act on its position and orientation
By moving the mouse away from the graphical display area the lines disappear to judge the effect obtained.

The behavior of the tool depends on the options to check:
- Symmetrical position
- Symmetrical or independent blur zones


ViewPoint: Mouse over the point of the lines to display a slider that modulates the intensity of the blur
PhotoLab: The button for activating the symmetric blur areas is placed between the two intensity sliders

The list of choices "Blur shape" adjusts the bokeh quality by simulating the rendering according to the number of diaphragm lamellae and its effect on the transitions.
It is better to choose the 100% zoom screen display to judge.
The default option "Circular" is a good starting point.

Illustration of the mock-up rendering where it is necessary to adapt the sharp and transition zones to the geometry of the plane. The top line is wedged as closely as possible to limit areas of the background that will remain sharp while including the entire main subject.
Important: the subject must be completely clear or the effect will be destroyed. Thus irregular subjects of shape or having significant growths are not suitable. This would be the case here if the empennage had not been included in the net area.
To accentuate the rendering, the transition zones have been reduced and the intensity of the blur increased.



Miniature effect tool








Miniature effect usability
These illustrations show some varied cases of the miniature effect usability.
For some, the intensity of the blur has been exaggerated to highlight the effect.


The intensity must be adapted according to the image destination; strong for the web with a small screen display, softer for a large print.



Possible uses of Miniature effect







Completing the miniature effect

For some subjects, it is possible to go further in the model rendering with DxO FilmPack.



The idea was to simulate, as realistically as possible, miniature railway networks photos as they could be seen in 1970s specialized magazines.
ViewPoint was used to slightly accentuate the high angle perspective and for the miniature effect, then FilmPack to regain the typical mainstream color due to the use of daylight films under tungsten lighting, and add the slide cover.



Completing the miniature effect










Using the miniature effect on a classic image

It is possible to take advantage of this effect to improve a classic image whose main subject is to be highlighted.
For a more natural rendering, the transitions zones will be increased appreciably and the intensity of the blur will be clearly diminished.


Here the foreground is quite present and is of secondary interest. It is dimmed by dosing the intensity of the blur so that the effect remains credible. Adjusting the blur shape on 6 blades allows to keep a little more structured blur, also more realistic.
The sky has also been treated keeping clear the most interesting area.



Using the miniature effect on a classic image






DxO ViewPoint 3 - Gérard THOMAS et Pascal PELE - April 2019

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