i've got a car paint shader and I wanna make it look like rain is hitting and travelling down the car (for a closeup shot). I have my carpaint shader and i plug this into the layered shader. simple solid blinn with a colour ramp.
i now have a second fully transparent blinn. I'm using some footage of water trickling down a windscreen that looks really effective and I've tiled and looped it in aftereffects. this layer has an alpha channel and its exported as an iff sequence. its then attached to the blinn as a bump map. refractions are turned on in this blinn too.
now I want to layer this over the top so my droplets-blinn creates refractions on the paintwork surface beneath where the droplets are so I layer in the shader appropriately and go to render with mental ray but I've now got a prob - its as if the transparent blinn appears to refract right through the car - I can see the other side of the bodywork thru the paint and its refracted. this cant be right surely I just expect the blinn/droplets to refract the paintwork not look right thru. can anyone suggest what I might be doing wrong?
I'm aware layered shaders are slower but i'm prepared to wait if I get the right refractions
I've supplied some pics beneath of what I'm trying to accomplish - this is done using composited layers in afx and a fake refraction. its quite effective from a distance but up close its not quite right.