GlInstanceIDs

GlInstanceIDs is a feature in OpenGL that allows for instanced rendering. Instanced rendering is a technique used to draw multiple copies of the same geometry with variations in their transformations, colors, or other attributes. This is achieved by providing per-instance data to the vertex shader.

The core idea behind GlInstanceIDs is to render a single mesh multiple times in a single draw call, rather than issuing separate draw calls for each instance. This significantly reduces CPU overhead, especially when dealing with a large number of identical objects.

To use GlInstanceIDs, you typically bind an array buffer containing per-instance data. This data might include translation vectors, rotation matrices, scaling factors, or color values for each instance. When you make a draw call with the instanced rendering mode enabled (e.g., glDrawArraysInstanced or glDrawElementsInstanced), the vertex shader receives a special variable, gl_InstanceID. This gl_InstanceID is an integer that uniquely identifies the current instance being rendered, starting from 0 and incrementing for each instance.

Within the vertex shader, you can use gl_InstanceID to access the corresponding per-instance data from your bound buffer. For example, you might use gl_InstanceID as an index into an array of transformation matrices to apply a unique transformation to each instance of your geometry. This allows for efficient rendering of scenes with many identical objects, such as foliage, particles, or crowds of characters.