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154 Chapter 17: Rendering
Breaking low-res vertices causes the indentations to render in
the normal bump map. However, projection now misses some
areas of the geometry, so the cage needs to be adjusted further.
Flipped Seams in Ta ngent S pace
When you use tangent space for your normal
bump map, usually the values used for the tangents
areconsistentbetweenthehardwareshader,
renderers, and third-party applications. However ,
sometimes portions of the geometry are flipped,
relative to each other , causing discontin uities when
you render or u se a h ardware shader. Tangent
space is the default option for normal bump
mapping, and it is the method you should use
for objects that both move and deform, such as
animated characters. This problem does not ar ise
when you use the other coordinate spaces: world,
screen, or local.
For example, in the following scene, the pants
show discont inuities. They are flipped relative to
each other, as the arrows show.
Flipped seams in rendering of pants
Left: Left side shows a discontinuous red stripe
Right: Right side shows a discontinuous blue stripe
Thearrowsshowhowthemapsaremisaligned.
When you render a normal bump map, you can
generally see flipped areas as showing a “flare” of
reddish color to the right, and of bluish color to
the left.
Uncorrected normal bump map shows blue and red “flares.”
The solution is to use the UVW Unwrap modifier’s
Flip Horizontal command for those sub-object
selections that show flaring, or an excess of red
areas.