This fixes the issue with objects and terrain being darker than they should be. As it turns out, most levels have not normalized light direction which happens to make light brighter and this is the expected result. To keep in line with GL14 engine, newer engines should use the length of the vector to make light brighter.
If CONFIG_QUALITY_SHADOWS is defined (which it always is) then the
fragment shader code that samples the shadow map will take five samples
in a cross shape around the point to be sampled, to apply antialiasing.
Currently, the offset of these samples is hardcoded to 0.00025× the
shadow map resolution. This is very inconsistent: if the shadow map
resolution is 128×128, then these samples are 0.032 texels apart, which
is a waste of four texture samples, and essentially means that no
antialiasing is applied. If the shadow map resolution is 8192×8192, then
these samples are 2.048 texels apart, which causes visual artefacts
around shadow edges, instead of giving smoother shadows.
The correct thing to do is to always sample exactly one texel away from
the original position. This is easy in GLSL 3.30, as it includes a
textureOffset function which offsets a texture fetch by an exact number
of texels. This is faster than manually calculating an offset ourselves,
it fixes visual artefacts at high resolutions, and it properly applies
antialiasing at low resolutions.
Tomasz Kapuściński