Graphics Reference
In-Depth Information
This assignment is implementation specific; that is, it can vary from one
OpenGL ES 3.0 implementation to another. An application can query the
assigned binding by using the
glGetAttribLocation
command.
GLint
glGetAttribLocation
(GLuint
program
,
const GLchar *
name
)
program
program object
name
name of attribute variable
glGetAttribLocation
returns the generic attribute index that was
bound to the attribute variable
name
when the program object defined by
program
was last linked. If
name
is not an active attribute variable, or if
program
is not a valid program object or was not linked successfully, then
-1
is returned, indicating an invalid attribute index.
The vertex data specified using vertex arrays are stored in client memory.
This data must be copied from client memory to graphics memory when
a draw call such as
glDrawArrays
or
glDrawElements
is made. These
two commands are described in detail in Chapter 7, “Primitive Assembly
and Rasterization.” It would, however, be much better if we did not have
to copy the vertex data on every draw call, but instead could cache the
data in graphics memory. This approach can significantly improve the
rendering performance and also reduce the memory bandwidth and
power consumption requirements, both of which are quite important
for handheld devices. This is where vertex buffer objects can help. Vertex
buffer objects allow OpenGL ES 3.0 applications to allocate and cache
vertex data in high-performance graphics memory and render from this
memory, thereby avoiding resending data every time a primitive is drawn.
Not only the vertex data, but also the element indices that describe
the vertex indices of the primitive and are passed as an argument to
glDrawElements
, can be cached.
OpenGL ES 3.0 supports two types of buffer objects that are used for
specifying vertex and primitive data:
array buffer objects
and
element array
buffer objects
. The array buffer objects specified by the
GL_ARRAY_BUFFER
token are used to create buffer objects that will store vertex data. The
element array buffer objects specified by the
GL_ELEMENT_ARRAY_BUFFER
