On 12.09.2010 11:37, Rupert Weber wrote:
> Just discovered that the clipping in the new layer modes needs more work.
>
> It's not a problem with colors that originated in sRGB, but the layer
> modes can easily construct colors outside sRGB, or even
> invisible/virtual colors.
> The current behavior is different from floating point/GEGL implementation.
>
> Especially when composing e.g. in Color mode onto a white layer, results
> differ. A red onto white will become yellow (which isn't necessarily
> worse than the red from floating point, as it seems to be closer to
> correct lightness -- but it's different.)

btw, the red returned from the current GEGL implementation is too dark:
the projection eventually converts to linear light "RGB u8" [1] and
these values get fed directly to the screen, which in contrast expects
gamma-corrected values -- that is, at least when color management is turned off.

By numbers:
       sRGB  1 0 0                     red (color mode) over
       sRGB  1 1 1                     white

gives LCH(ab) (D65):
       LCH   53.24   104.55   40       red
       LCH   100     0         %       white

the result color is certainly out of gamut:
       LCH   100     104.55   40             super-bright red
=     RGB     3.11    0.45    0.21          linear light  [2]
=  R'G'B'     1.64    0.70    0.50          gamma-encoded [2]


with the GEGL projection enabled, GIMP currently delivers
              (1.0     0.45    0.21) * 255  =  255 115  55

using proper gamma handling the result, truncated to sRGB, will be:
              (1.0     0.70    0.50) * 255  =  255 179 128


hopefully i'm not needlessly complicating things,
yahvuu



[1] to quick'n'dirty test the gamma-correct conversion,
     gegl/gimp-gegl-utils.c:61  can be modified to:
         case 3:
           return babl_format ("R'G'B' u8");   /* former ("RGB u8")  */
         case 4:
           return babl_format ("R'G'B'A u8");  /* former ("RGBA u8") */


[2] babl clamps the results of LCH->RGB conversions to the [0 ..1] interval:
     extensions/CIE.c:1286



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