[Bf-compositor] Clamping (Mix, Math, RGB Curves)

[Troy Sobotka]

On May 23, 2014 4:46 AM, "Bartek Skorupa (priv)" <
bartekskorupa at bartekskorupa.com> wrote:

> In many cases when using nodes we'd like to clamp output values to visual
range (or to range between 0.0 and 1.0).

There is no “visual range” in Blender's scene referred internal model. All
values are valid, and 1.0 has no special meaning.

The default sRGB output tone response curve only considers 0..1.0, but
there is also one that preserves approximately 2 and a bit stops more
(values of 4.5 IIRC).

The transform to a display referred output is a creative decision on what
values will become diffuse white. There is no real idea of white in a scene
referred model, and again, 1.0 has no significance.

> Some nodes like Mix or Math nodes have the checkbox for this and it's
fantastic.

Would seem like a bug for scene referred data. Difficult to deal with HDR
IBL work.

> However in many cases we have a need for clamping just "whites" or just
"blacks".

There is no value that is white in a scene referred model. LDR display
referred imagery may have a previous diffuse white located at 1.0 when
transferred as display linear, but it is merely achromatic in a scene
referred model.

> RGB Curves node in compositor doesn't clamp and when used in Cycles
materials it clamps, so we have inconsistency. I'm aware that those are
different nodes even though look almost exactly the same.

Not sure why the Cycles one would clamp, and would seem a bug if it did.

With respect,
TJS
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