Graphics Reference
In-Depth Information
on the kind of color computations being performed and the range and
precision required for the operations being performed. We believe that
highp
will most likely be the default precision used for most operations in
a vertex shader; thus we use
highp
as the default precision qualifier in the
examples that follow.
gl_MaxVertexUniformVectors
describes the maximum number of
uniforms that can be used in a vertex shader. The minimum value for
gl_MaxVertexUniformVectors
that must be supported by any compliant
OpenGL ES 3.0 implementation is 256
vec4
entries. The uniform storage
is used to store the following variables:
• Variables declared with the uniform qualifier
• Constant variables
• Literal values
• Implementation-specific constants
The number of uniform variables used in a vertex shader along with the
variables declared with the
const
qualifier, literal values, and implementation-
specific constants must fit in
gl_MaxVertexUniformVectors
as per the
packing rules described in Chapter 5, “OpenGL ES Shading Language.” If
these do not fit, then the vertex shader will fail to compile. A developer might
potentially apply the packing rules and determine the amount of uniform
storage needed to store uniform variables, constant variables, and literal
values. It is not possible to determine the number of implementation-specific
constants, however, as this value will not only vary from implementation to
implementation but will also change depending on which built-in shading
language functions are being used by the vertex shader. Typically, the
implementation-specific constants are required when built-in transcendental
functions are used.
As far as literal values are concerned, the OpenGL ES 3.0 Shading
Language specification states that no constant propagation is assumed.
As a consequence, multiple instances of the same literal value(s) will
be counted multiple times. Understandably, it is easier to use literal
values such as
0.0
or
1.0
in a vertex shader, but our recommendation
is that this technique be avoided as much as possible. Instead of
using literal values, appropriate constant variables should be declared.
This approach avoids having to perform the same literal value count
