this next tutorial makes use of the BLEND material channel
my scene is a couple of guys fighting, one guy grabs a spear and hits the other guy with it - i wanted to show that both combatants start off with clean costumes, and the guy that get's hit with the spear shows a bloody costume - but i wanted in one continous take - so here's how i did it
first watch the
video of the effect here - notice the relatively clean armor, he turns away and a spear hits him in the back, blood is splattered all over his armor
to do this is fairly simple - we will use the BLEND material channel and the TIMELINE
1) go to UPPER - MODIFY - click on the OPEN icon for the BLEND material channel
load a bloody texture - the way this channel works is anything white becomes transparent and the colored items get's applied (using Multiply blend mode)
2) once loaded, set the material STRENGTH to 0
here's the guy as he turns to run away - clean armor

make sure the blend material strength is set to zero up until the point where the spear hits him, then adjust the material strength to 100 - use the timeline for more accuracy of placement
so for example, of the spear hits him at frame 150, make sure frame 0 is at zero strenght and frame 149 is at zero strenght, then a frame 150, adjust the strenght to 100
so here's our character with a spear stuck on his back, blend material strength set to 100 at the appropriate keyframe

so there you are - a simple way to show injury using the blend channel - you can use this effect for houses. show them clean then as fire ravages them, slowly turn dark
