perspective correction

A review of SKRWT – keystone correction for IOS

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For a few years now, I have been using  SKRWT, an app that does perspective correction in IOS.

The goal was to have some way of quickly fixing issues with perspective, and distortions, in photographs. The most common form of this is the keystone effect (see previous post) which occurs when the image plane is not parallel to the lines that are required to be parallel in the photograph. This usually occurs when taking photographs of buildings where we tilt the camera backwards, in order to include the whole scene. The building appears to be “falling away” from the camera. Fig.1 shows a photograph of a church in Montreal. Notice, the skew as the building seems to tilt backwards.

  • original
  • corrected
Fig. 1: Correction of keystone effect

The process of correcting distortions with SKRWT is easy. Pick an image, and then a series of options are provided in the icon bar below the imported picture. The option that best approximates the types of perspective distortion is selected, and a new window opens, with a grid overlaid upon the image. A slider below the image can be used to select the magnitude of the distortion correction, with the image transformed as the slider is moved. When the image looks geometrically corrected, pressing the tick stores the newly corrected image.

Fig. 2: The simple process of correcting a vertical distortion

Using the SKRWT app, the perspective distortion can be fixed, but at a price. The problem is that correcting for the perspective distortion requires distorting the image, which means it will likely be larger than the original, and will need to be cropped (otherwise the image will contain black background regions).

Here is a third example, of Toronto’s flatiron building, with the building surrounded by enough “picture” to allow for corrective changes that don’t cut off any of the main object.

  • before
  • after
Fig. 3: Correcting the flatiron building

Overall the app is well designed and easy to use. In fact it will remove quite complex distortions, although there is some loss of content in the images processed. To use this, or any similar perspective correction software properly, you  really have to frame the building with enough background to allow for corrections – not so you are left with half a building.

The sad thing about this app is something that plagues a lot of apps – it has become a zombie app. The developer was suppose to release version 1.5 in December 2020, but alas nothing has appeared, and the website has had no updates. Zombie apps work while the system they are on works, but upgrade the phone, or OS, and there is every likelihood it will no longer work.

Fixing photographs (e.g. travel snaps) (i)

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When travelling, it is not always possible to get a perfect photograph. You can’t control the weather – sometimes it is too sunny, and other times there is not enough light. So the option of course is to modify the photographs in some way, fixing what is considered “unaesthetic”. The problem lies in the fact that cameras, as good as they are, don’t always capture a scene the way human eyes do. Your eyes, and brain correct for many things that aren’t possible with a camera. Besides which we are all tempted to make photographs look brighter – a legacy of the filters in apps like Instagram. Should we fix photographs? It’s one of the reasons the RAW file format exists, so we can easily modify an images characteristics. At the end of the day, we fix photographs to make them more aesthetically pleasing. I don’t own a copy of Photoshop, so I don’t spend copious hours editing my photographs, it’s usually a matter of adjusting the contrast, or performing some sharpening.

There is of course the adage that photographs shouldn’t be modified too much. I think performing hundreds of tweaks on a photograph results in an over-processed image that may not really represent what the scene actually looked like. A couple of fixes to improve the aesthetic appeal?

So what sort of fixes can be done?

1︎⃣ Fixing for contrast issues

Sometimes its not possible to take a photograph with the right amount of contrast. In an ideal world, the histogram of a “good” photograph should be uniformly distributed. Sometimes, there are things like the sky being overcast that get in the way. Consider the following photo, which I took from a moving train using shutter-priority with an overcast sky.

A lack of contrast

The photograph seems quite nice right? Does it truly reflect the scene I encountered? Likely not quite. If we investigate the histogram (the intensity histogram), we notice that there is one large peak towards the low end of the spectrum. There is also a small spike near the higher intensity regions, most likely related to the light regions such as the sky.

So now if we stretch the histogram, the contrast in the image will improve, and the photograph becomes more aesthetically pleasing, with much brighter tones.

Improving contrast

2︎⃣ Fixing for straight lines

In the real world, the lines of buildings are most often straight. The problem with lenses is that they are curved, and sometimes this impacts the form of photograph being acquired. The wider the lens, the more straight lines converse to the centre of the image. The worse case scenario are fish-eye lenses, which can have a field of view of up to 180°, and result in a barrel distortion. Take a photograph of a building, and the building will appear distorted. Human eyes compensate for this with the knowledge that it is a building, and its sides should be parallel – they do not consciously notice converging vertical lines. However when you view a photograph, things are perceived differently – it often appears as though a building is leaning backwards. Here is an photograph of a building in Bergen, Norway.

An image askew, and corrected.

Performing a perspective correction creates an image where the vertical lines of the building are truly vertical. The downside is of course that the lower portion of the image has been compressed, so if the plan is to remove distortion in this manner, make sure to allow enough foreground in the image. Obviously it would be better to avoid these problems when photographing buildings.

In-camera keystone compensation (Olympus) (ii)

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So I took some photographs using the Olympus keystone compensation on a trip to Montreal. Most of them deal with buildings that are leaning back, which is the classic case when trying to photograph a building. The first set deal with some landscape photographs. In both these photographs I could not move any further back to take the photographs, and both were taken with the Olympus 12-40mm, set as wide angle (12mm or 24mm full frae equivalent).It was possible to correct both images, without loosing any of the building.

keystone correction of photographs
Originals (left), keystone corrected (right)

The second case deals with portrait format photographs. In both cases it was slightly more challenging to make sure the entire picture was in the frame, but doing it in-situ it was possible to assure this happened. Doing in post-processing may result in the lose of a portion of the photograph. In the lower image I had enough leeway to position the keystone-corrected frame in such a manner that the building is surrounded by ample space.

keystone correction of photographs
Originals (left), keystone corrected (right)

Compensating for perspective distortion often comes at a price. Modifying the geometry of a photograph means that less will fit in the photograph. Taking a photograph too close to a building may mean something is cut off.

Horizontal keystone correction can sometimes be more difficult, because the distortion is usually a compound distortion. In the example below, the photograph was taken slightly off-centre, producing an image which is distorted both from a horizontal and a vertical perspective.

keystone correction
Complex distortion

Is there a loss in aesthetic appeal? Maybe. Food for future thought.

In-camera keystone compensation (Olympus) (i)

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The Olympus OM-D EM5 Mark IIhas a completely cool feature they call keystone compensation. It’s a kind-of weird name – but dig a little deeper and you run into the keystone effect  which is the apparent distortion of an image caused by projecting it onto an angled surface. It basically makes a square look like a trapezoid, which is the shape of an architectural stone known as a keystone. Now normally when you take a photograph of a building, this effect comes into play. Reducing the keystone effect is called keystone correction. There are special lenses that remove this distortion, i.e. tilt-shift lenses. Now Olympus has introduced an algorithm which compensates for the keystone effect. Here is an example of keystone correction (distortion is shown as the opaque pink region).

keystone correction
Keystone correction before (left) and after (right)

Olympus has introduced an algorithm on some of their cameras (e.g. EM5ii) which compensates for the keystone effect. First, you have to enable Keystone Correction in “Shooting Menu 2”.

Olympus EM-5(ii)
Turning on keystone correction on an Olympus EM-5(ii)

Then it’s a simple matter of using the front or rear dial for correction. The front dial is used to horizontal correction, and the rear dial is used for vertical correction. Note that it doesn’t allow for both types of keystone compensation to be used at the same time. If you decide to change from vertical to horizontal correction, you have to reset the vertical component to 0. Frame the shot and adjust the effect in the display using the front and rear dial. Select the area to be recorded using the directions buttons (surrounding the OK button).

keystoneOLY4
Keystone correction screen

The only trick is using the INFObutton to switch between keystone compensation and making adjustments to exposure compensation. In fact if you are using keystone correction often, I would program it into one of the function buttons.

Keystone Compensation mode enables keystone distortion to be corrected when shooting architecture and product photography without resorting to tilt-shift lenses or post-processing corrections in Photoshop.