Graphics Reference
In-Depth Information
Example 5-1
Sample Vertex Shader
#version 300 es
uniform mat4 u_matViewProjection;
layout(location = 0) in vec4 a_position;
layout(location = 1) in vec3 a_color;
out vec3 v_color;
void main(void)
{
gl_Position = u_matViewProjection * a_position;
v_color = a_color;
}
As with uniform variables, the underlying hardware typically places limits on
the number of attribute variables that can be input to a vertex shader. The
maximum number of attributes that an implementation supports is given by
the
gl_MaxVertexAttribs
built-in variable (it can also be found by querying
for
GL_MAX_VERTEX_ATTRIBS
using
glGetIntegerv
). The minimum
number of attributes that an OpenGL ES 3.0 implementation can support
is 16. Implementations are free to support more, but if you want to write
shaders that are guaranteed to run on any OpenGL ES 3.0 implementation,
you should restrict yourself to using no more than 16 attributes. We cover
attribute limitations in more detail in Chapter 8, “Vertex Shaders.”
The output variables from the vertex shader are specified with the
out
keyword. In Example 5-1, the
v_color
variable is declared as an output
and its contents are copied from the
a_color
input variable. Each vertex
shader will output the data it needs to pass the fragment shader into one
or more output variables. These variables will then also be declared in the
fragment shader as
in
variables (with matching types) and will be linearly
interpolated across the primitive during rasterization (if you want more
details on how this interpolation occurs during rasterization, jump to
Chapter 7, “Primitive Assembly and Rasterization”).
For example, the matching input declaration in the fragment shader for
the
v_color
vertex output in Example 5-1 follows:
in vec3 v_color;
Note that unlike the vertex shader input, the vertex shader output/fragment
shader input variables cannot have layout qualifiers. The implementation
automatically chooses locations. As with uniforms and vertex input
attributes, the underlying hardware typically limits the number of vertex
shader outputs/fragment shader inputs (on the hardware, these are usually
