OpenGL 4.2 (Compatibility Profile) - April 27, 2012

3.10. TEXTURING 330
Major Axis Direction Target s c t c m a
+r x TEXTURE_CUBE_MAP_POSITIVE_X −r z −r y r x
−r x TEXTURE_CUBE_MAP_NEGATIVE_X r z −r y r x
+r y TEXTURE_CUBE_MAP_POSITIVE_Y r x r z r y
−r y TEXTURE_CUBE_MAP_NEGATIVE_Y r x −r z r y
+r z TEXTURE_CUBE_MAP_POSITIVE_Z r x −r y r z
−r z TEXTURE_CUBE_MAP_NEGATIVE_Z −r x −r y r z
Table 3.23: Selection of cube map images based on major axis direction of texture
coordinates.
3.10.10 Cube Map Texture Selection
When cube map texturing is enabled, the ( s t r ) texture coordinates are treated
as a direction vector ( )
r x r y r z emanating from the center of a cube. The
q coordinate is ignored. If the q coordinate is negative, then the values of the
direction vector become undefined. At texture application time, the interpolated
per-fragment direction vector selects one of the cube map face’s two-dimensional
images based on the largest magnitude coordinate direction (the major axis direction).
If two or more coordinates have the identical magnitude, the implementation
may define the rule to disambiguate this situation. The rule must be deterministic
and depend only on ( )
r x r y r z . The target column in table 3.23 explains how
the major axis direction maps to the two-dimensional image of a particular cube
map target.
Using the s c , t c , and m a determined by the major axis direction as specified in
table 3.23, an updated ( s t ) is calculated as follows:
s = 1 ( )
sc
2 |m a | + 1
t = 1 ( )
tc
2 |m a | + 1
Seamless Cube Map Filtering
Seamless cube map filtering is enabled or disabled by calling Enable or Disable,
respectively, with the symbolic constant TEXTURE_CUBE_MAP_SEAMLESS.
When seamless cube map filtering is disabled, the new ( s t ) is used to find a
texture value in the determined face’s two-dimensional image using the rules given
in sections 3.10.11 through 3.10.12.
OpenGL 4.2 (Compatibility Profile) - April 27, 2012