Graphics Reference
In-Depth Information
program object, they share the same set of uniform variables. Therefore,
if a uniform variable is declared in the vertex shader and also in the
fragment shader, both of those declarations must match. When the
application loads the uniform variable through the API, its value will be
available in both the vertex and fragment shaders.
Uniform variables generally are stored in hardware into what is known
as the “constant store.” This special space is allocated in the hardware
for the storage of constant values. Because it is typically of a fixed
size, the number of uniforms that can be used in a program is limited.
This limitation can be determined by reading the value of the
gl_MaxVertexUniformVectors
and
gl_MaxFragmentUniformVectors
built-in variables (or by querying
GL_MAX_VERTEX_UNIFORM_VECTORS
or
GL_MAX_FRAGMENT_UNIFORM_VECTORS
using
glGetintegerv
). An
implementation of OpenGL ES 3.0 must provide at least 256 vertex
uniform vectors and 224 fragment uniform vectors, although it is free to
provide more. We cover the full set of limitations and queries available
for the vertex and fragment shaders in Chapter 8, “Vertex Shaders,” and
Chapter 10, “Fragment Shaders.”
In Chapter 4, “Shaders and Programs,” we introduced the concept of
uniform buffer objects. To review, uniform buffer objects allow the storage
of uniform data to be backed by a buffer object. Uniform buffer objects
offer several advantages over individual uniform variables in certain
situations. For example, with uniform buffer objects, uniform buffer data
can be shared across multiple programs but need to be set only once.
Further, uniform buffer objects typically allow for storage of larger amounts
of uniform data. Finally, it can be more efficient to switch between
uniform buffer objects than to individually load one uniform at a time.
Uniform buffer objects can be used in the OpenGL ES Shading Language
through application of uniform blocks. An example uniform block
follows:
uniform TransformBlock
{
mat4 matViewProj;
mat3 matNormal;
mat3 matTexGen;
};
