[FFmpeg-devel] [PATCH] Add a gamma flag to exr loader to avoid banding

Jimmy Christensen jimmy at ghost.dk
Tue Apr 29 15:58:03 CEST 2014

On 29/04/14 15:18, Ronald S. Bultje wrote:
> Hi,
> On Tue, Apr 29, 2014 at 2:49 AM, Jimmy Christensen <jimmy at ghost.dk> wrote:
>> 'm not sure that I like that it's default with gamma 2.2. Personally I
>> would rather have it defaults to gamma 1.0 (no changes).
> The gamma here is a little different from what you probably think it is,
> it's the linear XYZ (that's the actual perceived rays of light shooting in
> your eye as a scientific metric) vs. exponential RGB/YUV (which are just
> matrix transformations of each other), such as sRGB or bt709 or whatever.

Huh? The gamma correction in the patch has nothing to do with the XYZ 
colorspace. As far as I can see it's a simple powf() applied.

> It's not the gamma _over_ the non-linear YUV/RGB (which was already
> non-linear in itself), which is the kind of operation that you get when you
> apply a gamma in e.g. gimp or photoshop on an image.
> For typical gamma values for colorspaces, see e.g.
> http://www.brucelindbloom.com/index.html?Eqn_RGB_to_XYZ.html - 2.2 is a
> pretty typical default.

My point is that you can never assume that the input OpenEXR's are gamma 
1.0. Not to mention when you start to mix in ACES sometime in the near 

Assuming stuff which is not part/dictated of the standard is a bad idea 

- Jimmy

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