glVertexAttribDivisor

glVertexAttribDivisor is a function in the OpenGL API used for instanced rendering. It allows developers to specify how much a vertex attribute should be incremented for each instance drawn. When instanced rendering is enabled, multiple copies of the same geometry are drawn in a single draw call. Vertex attributes are typically per-vertex data, such as position or color. However, instancing requires certain attributes to vary per instance rather than per vertex.

The glVertexAttribDivisor function takes two arguments: the index of the vertex attribute and the divisor. The divisor specifies that after every 'divisor' instances, the attribute will advance to its next value. A divisor of 0 means the attribute value is constant across all instances, meaning it will be the same for every single copy of the geometry. A divisor of 1 means the attribute value will advance for every instance. For example, if you have a vertex color attribute with a divisor of 1, the first instance will use the first color, the second instance will use the second color, and so on.

By setting different divisors for different vertex attributes, you can control which aspects of the geometry change between instances. This is extremely useful for rendering large numbers of similar objects, such as trees in a forest, particles in a simulation, or elements in a UI. It significantly reduces the overhead of making many individual draw calls, leading to substantial performance improvements. The corresponding function to reset the divisor to its default value (0) is glVertexAttribDivisor(index, 0).