Приложение D: Gamma correction. Спецификация CSS1

Приложение D: Gamma correction. Спецификация CSS1

(Это приложение является информативным, а не формальным)

See the Gamma Tutorial in the PNG specification [12] if you aren't already familiar with gamma issues.

In the computation, UAs displaying on a CRT may assume an ideal CRT and ignore any effects on apparent gamma caused by dithering. That means the minimal handling they need to do on current platforms is:

PC using MS-Windows

none

Unix using X11

none

Mac using QuickDraw

apply gamma 1.39 [13] (ColorSync-savvy applications may simply pass the sRGB ICC profile [14] to ColorSync to perform correct color correction)

SGI using X

apply the gamma value from /etc/config/system.glGammaVal (the default value being 1.70; applications running on Irix 6.2 or above may simply pass the sRGB ICC profile to the color management system)

NeXT using NeXTStep

apply gamma 2.22

"Applying gamma" means that each of the three R, G and B must be converted to R'=R^gamma, G'=G^gamma, G'=B^gamma, before handing to the OS.

This may rapidly be done by building a 256-element lookup table once per browser invocation thus:

  for i := 0 to 255 do
    raw := i / 255;
    corr := pow (raw, gamma);
    table[i] := trunc (0.5 + corr * 255.0)
  end

which then avoids any need to do transcendental math per color attribute, far less per pixel.