Shaders and Programs - Praise for OpenGL ES 30 Programming Guide - page 93

queries. You can find the list of active attributes using the GL_ACTIVE_
ATTRIBUTES query. You can find the properties of an attribute using
glGetActiveAttrib . A set of routines are then available for setting up
vertex arrays to load the vertex attributes with values.
However, setting up vertex attributes really requires a bit more
understanding of primitives and the vertex shader than we are ready to
delve into right now. Instead, we dedicate an entire chapter (Chapter 6,
“Vertex Attributes, Vertex Arrays, and Buffer Objects”) to vertex attributes
and vertex arrays. If you want to find out how to query for vertex attribute
info, jump to Chapter 6 and the section Declaring Vertex Attribute Variables
in a Vertex Shader .
Shader Compiler
When you ask OpenGL ES to compile and link a shader, take a minute
to think about what the implementation has to do. The shader code is
typically parsed into some sort of intermediate representation, as most
compiled languages are (e.g., an Abstract Syntax Tree). A compiler must
then convert the abstract representation into machine instructions
for the hardware. Ideally, this compiler should also do a great deal
of optimization, such as dead-code removal, constant propagation,
and more. Performing all this work comes at a price—and this price is
primarily CPU time and memory.
OpenGL ES 3.0 implementations must support online shader compilation
(the value of GL_SHADER_COMPILER retrieved using glGetBooleanv
must be GL_TRUE ). You can specify your shaders using glShaderSource ,
as we have done so far in our examples. You can also try to mitigate
the resource impact of shader compilation. That is, once you have
finished compiling any shaders for your application, you can call
glReleaseShaderCompiler . This function provides a hint to the
implementation that you are done with the shader compiler, so it can
free its resources. Note that this function is only a hint; if you decide to
compile more shaders using glCompileShader , the implementation will
need to reallocate its resources for the compiler.
void glReleaseShaderCompiler (void)
Provides a hint to the implementation that it can release resources
used by the shader compiler. Because this function is only a hint, some
implementations may ignore a call to this function.