[Rawstudio-dev] Luminance curve - explainations and plan ?

Edouard Gomez ed.gomez at free.fr
Tue Apr 22 00:31:50 CEST 2008


First of all, congrats for 1.0.

Now i'd like to bring back a subject about the curve
as it's something that still bothers me.

The current implementation can be simply considered as a table lookup
matching R->R', G->G', B->B', all color components using the same

So for grey toned pixels (where R~G~B) this results in what we are
used to, the curve modifies more or less the luminosity of the
pixels. This works quite ok for the usual S-Curves for boosting
contrast. However for more exagerated curves, this completly
alterates the colors (shift in hue).

So as i see it, the current curve is buggy, and mislead the user.

So I propose again to work on a good curve implementation:
 - Luminance curve: chnges only the luminosity of the pixels
   respecting as closely as possible what humans consider
 - Independent Red, Blue, Green curves for cross processing
   or any other "after effect".
Now I'd like to discuss the current pixel flow so I can try
to understand the logic behind the current rawtudio's pixel
 step 1 - Substract black point and shift pixel to cover the
          entire 16bit range
 step 2 - Apply exposure/saturation/temperature/hue
          using a 4x4 matrix
 step 3 - Apply curve
 step 4 - Apply contrast
 step 5 - Apply gamma or color profile
 (step 3/4/5 are in fact all done through a common lookup table)

When i submitted my patch concerning luminance curve, i remember
it's been refused because its effect was too soft. Someone advised
me to apply curve after gamma.

I think this suggestion was done considering the fact, human eyes
perceive as linear luminosity changes, changes that follow more a
gamma curve than a pi/4 sloped line. However this approach brings
two problems:
 - for my patch to work i used to compute f = y'/y, and then do
   r' = f*r, g' = f*g, b' = f*b which are linear operations on
   linear color components, because luminance is computed from
   linear components, not gamma corrected ones. So there's some
   kind of discrepency between the pregama and postgamma
 - Doing it post gamma, means i cannot efficiently implement
   luminance curves because i cannot simply use the 4x4 matrix
   that is used in step 2 to do Y computation for free.

I really would like to get a clean picture of the perfect pixel
flow we want to have in Rawstudio. However i see big speed slow
downs if i implement it that way.

What's your opinion ? Could we have some description of the pixel
flow, explaining step by step each change, and why it's done in
linear color space or gamma corrected one ?

Extra bonus question, are we right concerning the resize operation
that should be performed pre gamma correction ? See 

PS: i did not discuss the RGB curves because these ones can be
    implemented at choice on pregamma or postgamma pixels. It's an
    arbitrary lookup anyway.

Edouard Gomez

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