The gamma values for some of the PixelFormats seem wrong

sadlame commented 5 years ago

Summary

I believe that some portion of the System.Windows.Media.PixelFormats are not correctly described.

Why I opened an issue instead of a PR

I can demonstrate with some piece of working code that there is some inconsistency in the doc, but due to the lack of deep understanding I can be only sure about a subset of the dubious part, so I opened this issue to ask for help instead of opening a PR.

Demo setup

For the demo, I wrote a simple ShaderEffect that takes a 1-D texture as input and renders the grayscale intensity of each pixel as a waveform. The 1-D texture is filled with a linear gradient from black to white, so when the gamma value is 1.0, the output image should be a linear ramp connecting the bottom-left corner and the top-right corner of the window.

I performed a small set of experiments on this demo program while changing the content of the 1-D texture.

Experiment 1

I associated with the 1-D texture a WriteableBitmap initialized with PixelFormats.Gray16 and observed a linear ramp, which means the gamma value of the texture was probably 1.0.

Then I associated another one initialized with PixelFormats.Gray32Float and observed what looks like an sRGB gamma ramp. This suggests that the gamma value of PixelFormats.Gray32Float is not actually 1.0, contrary to what is described in the doc.

Experiment 2

I performed the same experiment with PixelFormats.Rgba64 and PixelFormats.Rgba128Float, observing the gamma ramp of all the 4 color channels respectively.

The red, green and blue channels resulted in what looks like an sRGB gamma ramp just like what I got from PixelFormats.Gray32Float in both pixel formats, while I got a linear ramp for the alpha channel in both pixel formats.

This result suggests two things:

In both experiment 1 and 2, non-linear gamma ramps were correctly displayed as being non-linear and vice versa, because usually the gamma ramp is only applied to RGB channels and not alpha.
Neither PixelFormats.Rgba64 nor PixelFormats.Rgba128Float have a gamma value of 1.0 contrary to what is described in the doc.

The part I am not certain about

I conducted two simple experiments whose results strongly suggest errors in the doc, but I am not completely certain about this.

I used ShaderEffect for the experiments and I did not test any other means of image output, so it could be a bug in the texture transfer from CPU memory to VRAM or something and the gamma values are correctly reflected when GPU (i.e. DirectX) is not involved, in which case it is the internal code of WPF that needs a bugfix, not the doc.

However, I found an MSDN forum post from 2009 which describes a similar inconsistency.

The author states that only the float formats have a gamma value of 1.0 and others do not which is not consistent with what I got. Nevertheless it does suggest that there is some inconsistency.

Conclusion

I believe the two experiments only point to a part of the problem.

For example, PixelFormats.Rgb48 is simultaneously described as being an sRGB format and having a gamma value of 1.0. It does not make much sense because usually, as the MSDN Forum post above suggests, sRGB color space is associated with a gamma value of 2.2.

It seems to me that the entire entry is half-baked, unless I am misunderstanding things at a very fundamental level. I would be very glad if somebody who knows the WPF internals could review this and provide us with the correct information about PixelFormats.

I am willing to publish the code I used for the demonstration if asked. I did not do so in advance because there might be a better and more simple way already.

Thank you in advance.

mairaw commented 5 years ago

Thank you for your feedback @sadlame.
@merriemcgaw @Tanya-Solyanik @RussKie can one of you help with this issue for https://docs.microsoft.com/en-us/dotnet/api/System.Windows.Media.PixelFormats?

RussKie commented 5 years ago

@mairaw in WinForms we use System.Drawing.Imaging.PixelFormat which comes from CoreFx. The issue is referencing System.Windows.Media.PixelFormats which is defined in WPF space. In this case we need to seek help from @vatsan-madhavan and his team.