Alpha and gamma

SDL Development
#1

It appears to me that SDL does alpha blending without
concern for gamma. If so, this is a bug. Alpha blending
may only be done with linear light (gamma==1.0) data.

This shows up as ugly dark edges around objects that
are blitted with fuzzy edges, and as semi-transparent
objects being too dark. Try light grey with dark grey.

The SDL function for adjusting gamma is rather useless
for fixing this, for two reasons:

  1. it doesn’t work with all video drivers
  2. for good looks, gamma==1.0 needs at least 10 bits/channel

Gamma adjustment using a 12-bit lookup table seems to
go pretty fast, even when using floating point for the
linear data.

So… how about having SDL handle this stuff better?

#2

The SDL function for adjusting gamma is rather useless
for fixing this, for two reasons:

  1. it doesn’t work with all video drivers
  2. for good looks, gamma==1.0 needs at least 10 bits/channel

SDL just plugs into the underlying video API for gamma settings.

Gamma adjustment using a 12-bit lookup table seems to
go pretty fast, even when using floating point for the
linear data.

So… how about having SDL handle this stuff better?

Do you have a proposed patch?

See ya,
-Sam Lantinga, Software Engineer, Blizzard Entertainment