Re: [Gimp-developer] [Gimp-user] Time to fork BABL and GEGL

[Elle Stone <ellestone ninedegreesbelow com>]

On 11/19/2014 06:13 PM, Simon Budig wrote:
> Hi Elle.
>
> Elle Stone (ellestone ninedegreesbelow com) wrote:

> > In case you don't understand this, HDR sRGB data is still *bounded* by the
> > sRGB xy chromaticities. It's only unbounded along the Y axis. There are NO
> > negative channel values in HDR sRGB data unless the *user* chooses to do
> > something odd, in which case the *user* is responsible for fixing the
> > results.
>
> I think you are missing that "unbounded" also allows *negative* R, G or B
> coordinates. That makes the colors imaginary in the sRGB color space,
> but it still is possible to convert them to all other RGB color spaces.

No I am not missing anything.

When you convert from a wider gamut color space to unbounded sRGB, the 
out of gamut colors are expressed using at least one and perhaps two 
negative channel values.

When you edit HDR scene-referred images, the RGB channel values can be 
greater than 1.0. They never go negative unless the user chooses to do 
something odd like subtract red from black.

HDR scene-referred data is bounded by the xy coordinates of the RGB 
working space chromaticities.

Unbounded sRGB isn't bounded by anything.

See

"Models for image editing: Display-referred and scene-referred"
http://ninedegreesbelow.com/photography/display-referred-scene-referred.html

> Unbounded sRGB can express all the colors XYZ can express.

Of course it can.

> Transformations from userRGB to sRGB to userRGB are lossless.

Of course they are lossless, to the limits of the precision used to do 
the conversion.

Please read the article I just linked to above. And then read this article:

LCMS2 Unbounded ICC Profile Conversions
http://ninedegreesbelow.com/photography/lcms2-unbounded-mode.html

There is a great big huge difference between HDR scene-referred and 
unbounded sRGB that none of you seem to understand. But you really do 
need to understand the difference between scene-referred and unbounded 
sRGB. That difference really does matter.

Elle