i still don't understand how this works.

i have an effect which uses 3 texture stages:

colormap
detailmap
shadowmap

the shadow map does only contain shadows and no lighting information (white = no shadow, black = shadow). so i use modulate and not modulate2x.

for older cards i would like to render the shadow map in a second pass.

i enabled alpha blending and tried all

SrcBlend
DestBlend

combinations but nothing worked properly. what exactly is the source and what the destination? what do i have to set as colorop in the second pass? how does rendering such an effect in two passes work? do i need an alpha channel?