Graphics Reference
In-Depth Information
sRGB Texture Formats
Another texture format introduced in OpenGL ES 3.0 is sRGB textures.
sRGB is a nonlinear colorspace that approximately follows a power
function. Most images are actually stored in the sRGB colorspace, as the
nonlinearity accounts for the fact that humans can differentiate color
better at different brightness levels.
If the images used for textures are authored in the sRGB colorspace but are
fetched without using sRGB textures, all of the lighting calculations that
occur in the shader happen in a nonlinear colorspace. That is, the textures
created in standard authoring packages are stored in sRGB and remain in
sRGB when fetched from in the shader. The lighting calculations then are
occurring in the nonlinear sRGB space. While many applications make
this mistake, it is not correct and actually results in discernibly different
(and incorrect) output image.
To properly account for sRGB images, an application should use an sRGB
texture format that will be converted from sRGB into a linear colorspace
on fetch in the shader. Then, all calculations in the shader are done
in linear colorspace. Finally, by rendering to a sRGB render target, the
image will be correctly converted back to sRGB on write. It is possible
to approximate sRGB → linear conversion using a shader instruction
pow(value, 2.2)
and then to approximate the linear → sRGB conversion
using
pow(value, 1/2.2)
. However, it is preferable to use a sRGB texture
where possible because it reduces the shader instructions and provides a
more correct sRGB conversion.
Table 9-9
Valid sRGB Sized Internal Format Combinations for
glTexImage2D
internalFormat
format
type
Input Data
R
F
GL_SRGB8
GL_RGB GL_UNSIGNED_BYTE
8/8/8 SRGB
X
GL_SRGB8_ALPHA8 GL_RGBA GL_UNSIGNED_BYTE
8/8/8/8 RGBA
X
X
Depth Texture Formats
The final texture format type in OpenGL ES 3.0 is depth textures. Depth
textures allow the application to fetch the depth (and optionally, stencil)
value from the depth attachment of a framebuffer object. This is useful in
a variety of advanced rendering algorithms, including shadow mapping.
Table 9-10 lists the valid depth texture formats in OpenGL ES 3.0.
