Decal z-fighting on walls

[neoplan]

Decals are awesome,
but I run into one problem that I can't figure out why it happens. Dependant on the players vie angle, some decals in my map start to show z-fighting.
This usually only happens on axial walls and not on floors. Anyone has similar experience or in idea how to solve this?



Here is the shader:

textures/amt-fs5_decal/cretebase3_decal
{
qer_editorimage textures/amt-fs5_decal/cretebase3_decal.tga
qer_trans 1
surfaceparm trans
surfaceparm nomarks
surfaceparm nonsolid
polygonOffset
{
map textures/amt-fs5_base/cretebase3.tga
blendFunc GL_SRC_ALPHA GL_ONE_MINUS_SRC_ALPHA
rgbGen identity
}
{
map $lightmap
rgbGen identity
blendFunc filter
}
}

[obsidian]

polygonOffset works best with blendFunc add, filter, GL_DST_COLOR GL_ZERO (multiplicative), or GL_ZERO GL_ONE_MINUS_SRC_COLOR (inverse-multiplicative). So these blendFuncs are best for burn marks on walls, glowing bits, etc. It doesn't like multi-staged shaders with alpha-channels very much.

You can try using a sort value (try 6 or 7) and see if that works for you.

Otherwise, you might be better off leaving polygonOffset out and make the decal a "floater", sticking it on a detail brush or patch and floating the surface just above the wall behind it. You'll be venturing into less than 1 grid sizes, so if using a brush make sure to set it DETAIL.

Generally it's a good idea to add "surfaceparm detail" to your decal shaders anyway.

[neoplan]

Thanks for your suggestions. I tried different blend modes but the result was always too dark or it was fullbright.

The sort values have the problem that weapon marks will be drawn behind the decals.

[mrd]

Otherwise, you might be better off leaving polygonOffset out and make the decal a "floater", sticking it on a detail brush or patch and floating the surface just above the wall behind it. You'll be venturing into less than 1 grid sizes, so if using a brush make sure to set it DETAIL.

This is what I always did, worked fine for me.