GLSL Spec

8 Built-in Functions
Syntax
genType fwidth (genType p)
Description
Returns the sum of the absolute derivative in x and y
using local differencing for the input argument p, i.e.,
abs (dFdx (p)) + abs (dFdy (p));
8.12.2 Interpolation Functions
Built-in interpolation functions are available to compute an interpolated value of a fragment shader input
variable at a shader-specified (x, y) location. A separate (x, y) location may be used for each invocation of
the built-in function, and those locations may differ from the default (x, y) location used to produce the
default value of the input.
For all of the interpolation functions, interpolant must be an input variable or an element of an input
variable declared as an array. Component selection operators (e.g., .xy) may not be used when specifying
interpolant. If interpolant is declared with a flat or centroid qualifier, the qualifier will have no effect on
the interpolated value. If interpolant is declared with the noperspective qualifier, the interpolated value
will be computed without perspective correction.
Syntax
float interpolateAtCentroid (float interpolant)
vec2 interpolateAtCentroid (vec2 interpolant)
vec3 interpolateAtCentroid (vec3 interpolant)
vec4 interpolateAtCentroid (vec4 interpolant)
Description
Return the value of the input interpolant sampled at a
location inside the both the pixel and the primitive
being processed. The value obtained would be the
same value assigned to the input variable if declared
with the centroid qualifier.
float interpolateAtSample (float interpolant,
int sample)
vec2 interpolateAtSample (vec2 interpolant,
int sample)
vec3 interpolateAtSample (vec3 interpolant,
int sample)
vec4 interpolateAtSample (vec4 interpolant,
int sample)
Return the value of the input interpolant variable at
the location of sample number sample. If
multisample buffers are not available, the input
variable will be evaluated at the center of the pixel.
If sample sample does not exist, the position used to
interpolate the input variable is undefined.
float interpolateAtOffset (float interpolant,
vec2 offset)
vec2 interpolateAtOffset (vec2 interpolant,
vec2 offset)
vec3 interpolateAtOffset (vec3 interpolant,
vec2 offset)
vec4 interpolateAtOffset (vec4 interpolant,
vec2 offset)
Return the value of the input interpolant variable
sampled at an offset from the center of the pixel
specified by offset. The two floating-point
components of offset, give the offset in pixels in the x
and y directions, respectively. An offset of (0, 0)
identifies the center of the pixel. The range and
granularity of offsets supported by this function is
implementation-dependent.
154