Fragment Shaders - Praise for OpenGL ES 30 Programming Guide - page 284

of the fragment. There are a number of algorithms where it can be
useful to know the window coordinates of the current fragment. For
example, you can use the window coordinates as offsets into a texture
fetch into a random noise map whose value is used to rotate a filter
kernel on a shadow map. This technique is used to reduce shadow map
aliasing.
• gl_FrontFacing —A read-only variable that is available in the
fragment shader. This boolean variable has a value of true if the
fragment is part of a front-facing primitive and false otherwise.
• gl_PointCoord —A read-only variable that can be used when rendering
point sprites. It holds the texture coordinate for the point sprite that is
automatically generated in the [0, 1] range during point rasterization.
In Chapter 14, “Advanced Programming with OpenGL ES 3.0,” there is
an example of rendering point sprites that uses this variable.
• gl_FragDepth —A write-only output variable that, when written
to in the fragment shader, overrides the fragment's fixed-function
depth value. This functionality should be used sparingly (and only
when necessary) because it can disable depth optimization in many
GPUs. For example, many GPUs have a feature called Early-Z where
the depth test is performed ahead of executing the fragment shader.
The benefit of using Early-Z is that fragments that fail the depth
test are never shaded (thus saving performance). However, when
gl_FragDepth is used, this feature must be disabled because the GPU
does not know the depth value ahead of executing the fragment
shader.
Built-In Constants
The following built-in constants are also relevant to the fragment shader:
const mediump int gl_MaxFragmentInputVectors = 15;
const mediump int gl_MaxTextureImageUnits = 16;
const mediump int gl_MaxFragmentUniformVectors = 224;
const mediump int gl_MaxDrawBuffers = 4;
const mediump int gl_MinProgramTexelOffset = -8;
const mediump int gl_MaxProgramTexelOffset = 7;
The built-in constants describe the following maximum terms:
• gl_MaxFragmentInputVectors —The maximum number of fragment
shader inputs (or varyings). The minimum value supported by all
ES 3.0 implementations is 15.