OpenGL 4.2 (Compatibility Profile) - April 27, 2012

3.10. TEXTURING 330
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.
When seamless cube map filtering is enabled, the rules for texel selection in
sections 3.10.11 through 3.10.12 are modified so that texture wrap modes are ignored.
Instead,
• If NEAREST filtering is done within a miplevel, always apply wrap mode
CLAMP_TO_EDGE.
• If LINEAR filtering is done within a miplevel, always apply wrap mode
CLAMP_TO_BORDER. Then,
– If a texture sample location would lie in the texture border in either u
or v, instead select the corresponding texel from the appropriate neighboring
face.
– If a texture sample location would lie in the texture border in both u
and v (in one of the corners of the cube), there is no unique neighboring
face from which to extract one texel. The recommended method to
generate this texel is to average the values of the three available samples.
However, implementations are free to construct this fourth texel
in another way, so long as, when the three available samples have the
same value, this texel also has that value.
The required state is one bit indicating whether seamless cube map filtering is
enabled or disabled. Initially, it is disabled.
3.10.11 Texture Minification
Applying a texture to a primitive implies a mapping from texture image space to
framebuffer image space. In general, this mapping involves a reconstruction of
the sampled texture image, followed by a homogeneous warping implied by the
mapping to framebuffer space, then a filtering, followed finally by a resampling
of the filtered, warped, reconstructed image before applying it to a fragment. In
the GL this mapping is approximated by one of two simple filtering schemes. One
of these schemes is selected based on whether the mapping from texture space to
framebuffer space is deemed to magnify or minify the texture image.
OpenGL 4.2 (Compatibility Profile) - April 27, 2012