Elle: Things are confusing now because the code isn't finished. A lot of things will change before 2.10 is released. Rest assured, we won't keep everything this way. On Sun, Jul 7, 2013 at 5:23 PM, Elle Stone <l.elle.stone@xxxxxxxxx> wrote: > Gimp from git allows the user to choose which image precision to use > while editing the image. The choices used to be 8-bit, 16-bit, and > 32-bit integer, plus 16-bit and 32-bit floating point. The choices > have recently doubled: for each image precision choice, the user must > choose whether to work in a "linear" or a "gamma" color space. The > default seems to be "gamma" rather than "linear". > > I did some experimenting to see what these new "precision" choices might mean: > > The practical result of choosing "linear" vs "gamma" as your image > "precision" depends on the image's actual RGB color space TRC (whether > linear or true sRGB TRC or some other TRC) and it also depends on > which operations you want to perform. Limited testing suggests that at > present it affects blend modes but not blurring or gradients. > > CASE A: the image is in the regular sRGB color space > > If the image is already in the regular sRGB color space, then choosing > "gamma" means for example that a soft-light blend of an image over > itself will look like it was done in the regular sRGB color space. But > choosing "linear" means it will look like the blend was done in a > linear gamma version of the sRGB color space and the result will be > darker than it would be if done in the regular sRGB color space. In > both cases gradients look like regular sRGB gradients. And in both > cases, blurring looks like it was done in a linear gamma version of > the sRGB color space. > > CASE B: the image in a linear gamma version of the sRGB color space > > If the image is already in a linear version of the sRGB color space, > then choosing "gamma" means that a soft-light blend of an image over > itself will look like it was done in a *linear* gamma version of the > sRGB color space. And choosing "linear" means it will look like the > blend was done in a gamma=0.45 (approx) version of the sRGB color > space. In both cases gradients look like linear gamma sRGB gradients. > And in both cases blurring looks like it was done in a gamma=0.45 > version of the sRGB color space. > > CASE C: the image is in some color space other than sRGB > > Some color management background: > > The "gamma" precision refers to the regular sRGB matrix color space > tone response curve. However, the sRGB matrix color space is not a > true gamma color space. Its tone response curve (TRC) is composed of a > linear section at the toe and a true gamma=2.4 section everywhere > else. > > A linear gamma version of the sRGB color space is the regular sRGB > color space with the regular sRGB TRC replaced with a gamma=1.0 TRC. > > RGB matrix working spaces like sRGB or CIE-RGB or a ProPhoto-like > color space, etc, are defined primarily by three things: their RGB > Primaries, their Tone Response Curve, and their white point. Putting > white point aside (lcms takes care of that behind the scenes), a color > space's Primaries determine the reddest red, bluest blue, and greenest > green that color space can contain. A color space's TRC determines how > fast a color goes from dark to light as the RGB values go from 0 to > max white (255 for 8-bit integer, 65535 for 16-bit integer, 1.0 for > floating point). > > So linear sRGB is not the only linear gamma color space. You can have > linear gamma versions of any working space. And you can also have, for > example, a color space with the CIE-RGB primaries and the regular sRGB > tone response curve. ProPhoto-like color spaces usually have a > gamma=1.8 TRC but you can create a linear gamma version of a > ProPhoto-like color space, and a version with the regular sRGB TRC. > And so on. > > What happens if your image happens to be in a color space other than > the regular sRGB color space? > > The Gimp "gamma" and "linear" precision code assumes the image has the > regular sRGB TRC. The closer a color space's TRC is to the regular > sRGB TRC, the more the resulting tonalities will be like case A above. > The closer the color space's TRC is to being linear, the more the > result's will be like CASE B above. And results will be more or less > wrong to the extent that the color space's TRC deviates from the sRGB > TRC. > > May I ask what is the point or reason for this new and confusing > option of "linear" and "gamma" precision? I say "confusing" because it > changes the mathematically expected behavior given the TRC of the > image color space. > > I should probably confess that for my own image editing I use a > version of babl/gegl/Gimp where the babl conversions between linear > and regular sRGB are eliminated. That way I can choose which color > space I want to work in and be confident of the results: linear gamma > working space, linear gamma results; higher gamma working space, > higher gamma results. I fear this "out" will not be viable for long. > > Elle, as usual, puzzled > _______________________________________________ > gimp-developer-list mailing list > List address: gimp-developer-list@xxxxxxxxx > List membership: https://mail.gnome.org/mailman/listinfo/gimp-developer-list _______________________________________________ gimp-developer-list mailing list List address: gimp-developer-list@xxxxxxxxx List membership: https://mail.gnome.org/mailman/listinfo/gimp-developer-list
