I
decided fairly early on that the car
should be muddy and dirty. This mud
would need to be textured onto the
car from bitmaps, which would mean
huge file sizes if I wanted to render
close-ups. I needed a system to allow
me to tile and reuse my mud textures
so they could stand up to extreme
close ups.
Painting
textures into simple Photoshop files
and just applying them to the car
wasn’t going to work.
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
separated the dirt into two layers
a dark wet mud and a thinner dry dust
spray, each layer had its own mask
for each body panel. The mud itself
was mapped globally to the whole car
with a tri-planer projection map.
This way the mud would run from one
panel to another neatly.
Mostly
importantly this approach meant that
I could deal with the look of the
mud separately from its placement.
Painting
masks
Once
I’d UV mapped each body panel
I was really to paint my dirt masks.
I used Maya’s 3D paint system
to paint myself guides on each panel,
and then exported these TGA’s
out to Photoshop and painted more
finished versions.
I
also exported the UV coordinates at
the same scale and put them into the
Photoshop file, this gave me exact
reference of where my paint strokes
would appear on the car in Maya.
I
then worked on two masks for each
panel, a basic dry dirt mask and a
wet splatter mask:
The
black areas define where the mud will
sit and the white where the paint
will be.
All
the masks were hand painted and assembled
from various sources. I used a lot
of concrete texture maps layered up
and manipulated to create the basic
dirt masks.
The
paint shader
The
red paint shader was fairly simple,
although given the strange way mental
ray’s FG system seems to deal
with shaders it took a lot of tweaking
to look right. All the lighting in
the scene came from a HDRI map.
The
shader is a simple red Blinn with
the diffuse component turned right
down to 0.4. The reflectivity and
specular colour have a sampler info
node passing through a curve to really
push the specularity when the metal
bends away from camera.
