Editing Alpha compositing (section)

== Gamma correction ==
[[File:Mix lazy.png|thumb|Alpha blending, not taking into account gamma correction]]

[[File:Mix precise.png|thumb|Alpha blending, taking<br> into account gamma correction]]

The RGB values of typical digital images do not directly correspond to the physical light intensities, but are rather compressed by a [[gamma correction]] function:

: <math>C_\text{encoded} = C_\text{linear}^{1/\gamma}</math>

This transformation better utilizes the limited number of bits in the encoded image by choosing <math>\gamma</math> that better matches the non-linear human perception of luminance.

Accordingly, computer programs that deal with such images must decode the RGB values into a linear space (by undoing the gamma-compression), blend the linear light intensities, and re-apply the gamma compression to the result:<ref>{{cite web|url=https://www.youtube.com/watch?v=LKnqECcg6Gw| archive-url=https://ghostarchive.org/varchive/youtube/20211122/LKnqECcg6Gw| archive-date=2021-11-22 | url-status=live|title=Computer Color is Broken|author=Minute Physics|date=March 20, 2015|website=[[YouTube]]}}{{cbignore}}</ref><ref>{{cite web |title=What every coder should know about gamma |url=https://blog.johnnovak.net/2016/09/21/what-every-coder-should-know-about-gamma/ |last=Novak|first=John|date=September 21, 2016}}</ref>{{Failed verification|date=October 2022}}

:''<math id="C_o = \left(\frac{  C_a^\gamma \alpha_a + C_b^\gamma \alpha_b (1 - \alpha_a)  }{\alpha_o}\right)^{1/…" qid="Q281055">C_o = \left(\frac{  C_a^\gamma \alpha_a + C_b^\gamma \alpha_b (1 - \alpha_a)  }{\alpha_o}\right)^{1/\gamma}</math>''

When combined with premultiplied alpha, pre-multiplication is done in linear space, prior to gamma compression.<ref>{{cite web |title=Gamma Correction vs. Premultiplied Pixels – Søren Sandmann Pedersen |url=http://ssp.impulsetrain.com/gamma-premult.html |website=ssp.impulsetrain.com}}</ref> This results in the following formula:

:<math>C_o = \left( C_a^\gamma + C_b^\gamma (1 - \alpha_a) \right)^{1/\gamma}</math>

Note that the alpha channel may or may not undergo gamma-correction, even when the color channels do.