Question

I’m looking at an example from apple how to use vertex arrays:

typedef struct _vertexStruct
{
    GLfloat position[2];
    GLubyte color[4];
} vertexStruct;
enum {
   ATTRIB_POSITION,
   ATTRIB_COLOR,
   NUM_ATTRIBUTES };

void DrawModel()
{
    const vertexStruct vertices[] = {...};
    const GLubyte indices[] = {...};

    glVertexAttribPointer(ATTRIB_POSITION, 2, GL_FLOAT, GL_FALSE,
        sizeof(vertexStruct), &vertices[0].position);
    glEnableVertexAttribArray(ATTRIB_POSITION);
    glVertexAttribPointer(ATTRIB_COLOR, 4, GL_UNSIGNED_BYTE, GL_TRUE,
        sizeof(vertexStruct), &vertices[0].color);
    glEnableVertexAttribArray(ATTRIB_COLOR);

    glDrawElements(GL_TRIANGLE_STRIP, sizeof(indices)/sizeof(GLubyte),
        GL_UNSIGNED_BYTE, indices);
}

(source)

How does OpenGL know which one is color and which one is vertex? ATTRIB_POSITION and ATTRIB_COLOR are user defined, so opengl shouldn’t know what it means. Specifically I try to use constant color and vertex/texture arrays. If I’ll change ATTRIB_COLOR to ATTRIB_TEXTURE OpenGL won’t notice a thing, how can I do that?

Answer 1

Note: the following assumes OpenGL ES 2.0.

OpenGL neither knows nor cares about what the meaning behind any particular vertex attribute is. All it wants is a number: an attribute index.

Your vertex shader defines a number of attributes that it takes as inputs, using the attribute keyword. It is your job to connect these GLSL attributes to OpenGL attribute indices, using glBindAttribLocation before linking the program. So attribute 0 is only the position if you bind the shader position attribute to attribute 0.