Hints provided in this section are intended to help you correctly
use HP's implementation of OpenGL.
4D Values |
 |
When specifying 4D values, such as vertices, light positions,
etc, if possible supply a w
value that is not near the floating point limits of MINFLOAT
or MAXFLOAT.
Using w values
near the floating point limits increases the likelihood of floating
point precision errors in calculations such as lighting, transformations,
and perspective division.
Also, performance will be best when 4D positions are normalized
such that w is
1.0.
For best accuracy and performance, if you want to specify
some 4D position like (0.0, 0.0, 5e10, 1.5e38), instead use the
equivalent normalized position (0.0, 0.0, 3.33e-28, 1.0).
If a light position must be specified with a w
value that is near the floating point limits, consider setting
to ensure that lighting occurs in Eye space. This will eliminate
an extra transformation of the light position, giving the best possible
solution.
Texture Coordinates |
|
When using non-orthographic projection, keep in mind the texture
coordinates will be divided by w
as an intermediate calculation. HP's implementation of OpenGL estimates
that for VMD, the texture coordinates used in perspective projections
will have only five significant digits of precision. Therefore,
when you have texturing close to a window edge and the decomposition
of the primitive causes the vertices to have very closely-spaced
texture coordinates after perspective projection, you may see loss
of texturing precision. This loss of precision may make the texture
primitive seem locally smeared.
Printable version
