Cube images ignore the wrap modes specified in the sampler.
Instead, if VK_FILTER_NEAREST is used within a mip level then
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_EDGE is used, and if
VK_FILTER_LINEAR is used within a mip level then sampling at the edges
is performed as described earlier in the Cube map edge handling section.
The first integer texel coordinate i is transformed based on the
addressModeU parameter of the sampler.
where:
j (for 2D and Cube image) and k (for 3D image) are similarly
transformed based on the addressModeV and addressModeW
parameters of the sampler, respectively.
SPIR-V instructions with Gather in the name return a vector derived
from a 2x2 rectangular region of texels in the base level of the image view.
The rules for the VK_FILTER_LINEAR minification filter are applied to
identify the four selected texels.
Each texel is then converted to an RGBA value according to
conversion to RGBA and then
swizzled.
A four-component vector is then assembled by taking the component indicated
by the Component value in the instruction from the swizzled color value
of the four texels:
where:
If λ is less than or equal to zero, the texture is said to be
magnified, and the filter mode within a mip level is selected by the
magFilter in the sampler.
If λ is greater than zero, the texture is said to be
minified, and the filter mode within a mip level is selected by the
minFilter in the sampler.
Within a mip level, VK_FILTER_NEAREST filtering selects a single value
using the (i, j, k) texel coordinates, with all texels taken from
layer l.
Within a mip level, VK_FILTER_LINEAR filtering computes a weighted
average of 8 (for 3D), 4 (for 2D or Cube), or 2 (for 1D) texel values, using
the weights computed earlier:
Finally, mipmap filtering either selects a value from one mip level or computes a weighted average between neighboring mip levels:
Anisotropic filtering is enabled by the anisotropyEnable in the
sampler.
When enabled, the image filtering scheme accounts for a degree of
anisotropy.
The particular scheme for anisotropic texture filtering is implementation
dependent.
Implementations should consider the magFilter, minFilter and
mipmapMode of the sampler to control the specifics of the anisotropic
filtering scheme used.
In addition, implementations should consider minLod and maxLod
of the sampler.
The following describes one particular approach to implementing anisotropic filtering for the 2D Image case, implementations may choose other methods:
Given a magFilter, minFilter of VK_FILTER_LINEAR and a
mipmapMode of VK_SAMPLER_MIPMAP_MODE_NEAREST:
Instead of a single isotropic sample, N isotropic samples are be sampled within the image footprint of the image level d to approximate an anisotropic filter. The sum τ2Daniso is defined using the single isotropic τ2D(u,v) at level d.
| editing-note | |
|---|---|
(Bill) EXT_texture_filter_anisotropic has not been updated since 2000, except for ES extension number (2007) and a minor speeling (sic) correction (2014), neither of which are functional changes. It is showing its age. In particular, there is an open issue about 3D textures. There are no interactions with ARB_texture_cube_map (approved 1999, promoted to core OpenGL 1.3 in 2001), let alone interactions with ARB_seamless_cube_map (approved and promoted to core OpenGL 3.2 in 2009). There are no interactions with texture offsets or texture gather. |