Graphics Reference
In-Depth Information
The vertex attribute
color
used in the code example is a constant
value specified with
glVertexAttrib4fv
without enabling the vertex
attribute array 0. The
vertexPos
attribute is specified by using a vertex
array with
glVertexAttribPointer
and enabling the array with
glEnableVertexAttribArray
. The value of
color
will be the same for all
vertices of the triangle(s) drawn, whereas the
vertexPos
attribute could
vary for vertices of the triangle(s) drawn.
in a Vertex Shader
We have looked at what a vertex attribute is, and considered how to
specify vertex attributes in OpenGL ES. We now discuss how to declare
vertex attribute variables in a vertex shader.
In a vertex shader, a variable is declared as a vertex attribute by using the
in
qualifier. Optionally, the attribute variable can also include a layout
qualifier that provides the attribute index. A few example declarations of
vertex attributes are given here:
layout(location = 0) in vec4 a_position;
layout(location = 1) in vec2 a_texcoord;
layout(location = 2) in vec3 a_normal;
The
in
qualifier can be used only with the data types
float,
vec2,
vec3, vec4, int, ivec2, ivec3, ivec4, uint, uvec2, uvec3,
uvec4, mat2, mat2x2, mat2x3, mat2x4, mat3, mat3x3, mat3x4,
mat4, mat4x2,
and
mat4x3
. Attribute variables cannot be declared
as arrays or structures. The following example declarations of vertex
attributes are invalid and should result in a compilation error:
in foo_t a_A; // foo_t is a structure
in vec4 a_B[10];
An OpenGL ES 3.0 implementation supports
GL_MAX_VERTEX_ATTRIBS
four-component vector vertex attributes. A vertex attribute that is declared
as a scalar, two-component vector, or three-component vector will count as
a single four-component vector attribute. Vertex attributes declared as two-
dimensional, three-dimensional, or four-dimensional matrices will count
as two, three, or four 4-component vector attributes, respectively. Unlike
uniform and vertex shader output/fragment shader input variables, which
are packed automatically by the compiler, attributes do not get packed.
Please consider your choices carefully when declaring vertex attributes
with sizes less than a four-component vector, as the maximum number of
vertex attributes available is a limited resource. It might be better to pack
