I have several shots with a plain background, but the brightness is not exactly even. I had taken a single shot of only background and thought that maybe it would be possible to correct the brightness based on this shot.
I discovered the external raster mask option in Darktable, and thought, that might be just the right tool. So I exported the image as a PFM file, and then imported it on the same image file as an external raster mask. Then I went to the exposure module and selected that mask, with blending mode lightness. In my idea, if I would adjust the exposure, I would find a way that perfectly corrects it to have a completely homogenous image brightness.
However, I did not manage to achieve this. Is this possible with my approach? I also tried inverting the mask, and flipping it up and sideways, turning it to a black and white image. All did not help. Any advice is appreciated.
I attached a jpg of the image, if anybody wants to try it.
That’s correct, that’s the image of the background. However, I should be able to apply this image as a mask on itself to obtain a perfectly homogenous image, that was my idea. If that works, I could then apply the same processing on the whole batch of images that include objects.
If I understand correctly you want to do a vignetting correction for your lens. This might be better achieved using the lens correction module or creating and second instance of exposure with a vignetting profile created using an ellipse mask, which can then be saved as a preset to apply to other images shot under the same condition. This first option with lens correction presets for vignetting is what I have done in my version of DT.
If you apply an image to itself with the composite module and use divide blend mode you will get a white image and you can bump that down with the fulcrum but I am not sure if that gives you what you want but it will be uniform white or grey…
