The raw images of my camera, an Olympus E-M5 III, show a band of darker pixels at the extreme right - and only there. This area is cropped away on out-of-camera JPEGs, but other than for the somewhat darker tone, the area looks perfectly usable.
Strangely, the dark band is not always visible. So far I have no understanding under which conditions it appears. (But it appears often enough.)
Any ideas? I’d be grateful for suggestions about the origin of the phenomenon and how to best deal with it. (I think I could define a crop preset, but then I’d have to manually check the border of each image. Alternatively, I could try to configure darktable to get rid of the border even before the pipeline, but then I will be throwing perfectly good pixels away.)
I get this with some cameras. It is not really a fault in my view. It is just DT is not cropping the edge pixels that the camera crops. It was so common with my Canon G16 that I made a crop preset that I could apply to the image to remove the few pixels. Others may have a more informative answer for you.
Probably of no help, but some makers if not all use sensor border pixels for special purposes. For example, my raw viewer can include or exclude masked pixels from the review image.
I can only speculate that those dark pixels could be used to detect the onset of over-exposure. Such pixels are included all over the Foveon F23 sensor, not in the borders.
A common issue with Darktable camera database. Some time ago I actually submitted a pull request to dump these perfectly good pixels in raw files from my G9 because the predefined crops were unreliable respectively to the various camera modes and their combinations.
Having a closer look at the issue, the darker band appears also in an image converted using the dcraw tool. That image has the resolution of 5240 * 3912 which, according to dxomark, is the (full) resolution of the sensor.
The width of the band is exactly 32 pixels throughout.
Is there a way to trim these pixels early on (before lens correction module), without affecting the work of the lens correction module? The crop module operates on the lens-corrected image, so it’s not ideal.
There is indeed. Search the forum for the “allow_editing_crop” workaround, and you will then get the expert crop controls in the rawprepare (raw black/white point) module.
So I’ve been using the solution suggested by @kmilos (=using the raw black/white point module to get rid of the column of darker pixels) for some time now, and it works well for this particular problem. However, it introduces another problem: it prevents the raw black/white point module from honoring the BlackLevel EXIF MakerNote tags. This introduces a color cast in (heavily) lifted shadows.
The problem is that on my camera (Olympus E-M5 III) the EXIF BlackLevel varies from image to image. It seems to depend not only on ISO, but also on things like shutter mode (mechanic vs electronic) and probably yet other parameters.
Is there a way in darktable to create a preset that only modifies some values of a module? So far I was not able to find one.
It does not seem possible to dynamically adjust module defaults upon import using Lua. (See here for details.)
In the end, I defined a shortcut that sets the right crop to 32. This is possible directly within the GUI. Now I can apply the crop whenever necessary (curiously, the band of pixels is not always present). This is not a perfect solution, but it will do.
