The paint shader (continued)
The curve looks like this:
It's essentially turning the linear 0 -> 1 output of the sampler info's facing ratio into a 0 -> 2 exponential value.
The solution I settled on was a fairly simple one, using masks to define where the mud would lie on the car, and then mixing through to a mud shader in those areas. The mud shader was mapped separately and could be tiled and varied without affecting the paintwork.
I use curves instead of ramps to do this as I'm used 3D Studio Max's way of doing things, but really it makes no difference how it's done.
Because all my muddy areas were defined by masks I could tweak the actual look of the mud over the whole car without having to keep going back to Photoshop. I mapped the two types of mud over the entire car as a Tri-planar projection, and then experimented with adding gradients to have the mud at the bottom of the car darker (wetter) than at the top.
Many of the settings on my shaders were connected to a control object in the scene, by changing those settings I could alter the look of the paint and mud instantly. These global controls really sped up the process at the end, even if they made duplicating shading networks a pain.
An early experiment using a different type of mud. This looked like someone had chucked a bucket of wet concrete on the bonnet.
As you'd expect rendering took a long long time. My frames were averaging one hour on a 3Ghz machine, and I ended up rendering an animated sequence of about 1000 frames! I was lucky to get the use of the machines in work.
Mental ray had some serious flickering and crawling issues. Setting the final gathering min and max radius correctly really helped prevent this. According to the mental ray information I found the max radius should be set to 10% of the scene width, and the min radius should be 10% of that. I ended up with 20max and 2min
I also played with the BSP settings to speed up render time, I've no idea why but I had most luck with BSP size set to 16, BSP depth set to 60 and BSP max memory set to 0.
Some renders still had a little flickering so I had to render out small fix up patches to place over those areas.
So there you have it, a little bit of behind the scenes info on the shading and rendering of my Mini Cooper.
Click here to see more of the car on the author's site!