Credits
go to Phil Bedard and Francis Bernier
for the model and texture.
If
you think of your 3d mesh as a sculputure,
then you can think of your mesh's texture
map as your sculpture's paint job. It
fills in all the colors and details
that geometry can't. The only problem
is, while you can paint directly onto
a sculpture, a texture map has to be
a flat image. Perhaps a better analogy,
then, would be if you had to paint on
a sheet of paper, cut it up into little
pieces, and paste them onto your statue.
That's
the essence of what a texture map is
- a flat drawing that's cut up, rotated,
and fitted onto your mesh. A UVW map
is the template that tells your 3D software
exactly how to cut up the texture map
and where to place it on your object.
If
you've painted texture maps in the past,
you probably know that bad UVW mapping
coordinates can make the texturing process
a major pain. I'll explain in this tutorial
how to actually go about setting up
UVW coordinates, but it helps to know
ahead of time what can make a UVW map
good or bad. There are a few main points
to keep in mind, but if you stick to
the analogy of gluing paper to a statue,
you should do alright.
First,
you usually want to break your UVW map
into as few pieces as possible. This
makes drawing the texture map a lot
easier, because you have fewer seams
that you have to make sure line up,
and it's easier to keep track of which
part of the map goes onto which part
of the object.
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Second,
you want to minimize stretching. Stretching
is what happens when there's less texture
detail on one polygon than on the next.
If you go back to the wallpaper analogy,
stretching would mean the wallpaper
just doesn't fit right on your statue
- about your only option is to crumple
it up a bit to fit the shape correctly.
Of course in your 3D program, it doesn't
crumple, it acts a lot more like you
had drawn your texture on a rubber balloon.
If it doesn't fit perfectly, you have
to stretch it to cover the whole form,
and your paint job will look distorted.
Third,
avoid overlapping mapping coordinates.
You can't paint two different things
on the same part of the texture map.
The exception is when you have two parts
of your object that will be getting
the same texture map anyway - this is
especially common when you have a symmetric
object, or an object with repeating
details. In the space ship image above,
only half of the bottom of the ship
is textured, but both halves of the
object make use of the same texture
space. The engines also use the same
texture map as each other.
Fourth,
you want to make sure your mapping coordinates
use the texture space as efficiently
as possible. If you're restricted, say,
to a 256x256 image for your texture,
any large gaps between mapping coordinate
clumps are just wasted space - if you
could scale all your coordinates up
a little bigger, and rearrange them
to fill in the space (without overlapping,
of course) all your details would be
a little bit higher resolution.
Finally,
it often helps if your mapping coordinates
all map the same amount of texture space
into 3D space. If one three-meter panel
has 64 texels mapped to it, every other
three-meter panel should use 64 pixels.
It looks strange if one part of an object
has a lot more detail than another,
and it's much easier to match up seams
if the texture resolution is consistent.
There are exceptions to this, of course
- when mapping a character, for instance,
the head and face often get more detail
than the rest of the body, since they're
much more important.
In
practice, you have to balance these
guidelines against each other to create
an efficient UVW map. You also need
to keep in mind that UVW mapping an
object is an art form in its own right,
though an incredibly dull one that you
want to spend as little time on as possible.
There's no "perfect" set of
mapping coordinates for an object, and
you can always do a better job. The
trick is to decide what's good enough
for your project, and get there as quickly
as possible.
