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

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

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.


"Models for image editing: Display-referred and scene-referred"

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

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.


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