glDrawArraysInstanced

glDrawArraysInstanced is an OpenGL function that renders multiple instances of a sequence of vertices described by the currently bound vertex array state. It enables instanced rendering, where a single set of vertex data is drawn many times, potentially with per-instance variation supplied by the shader.

The signature is void glDrawArraysInstanced(GLenum mode, GLint first, GLsizei count, GLsizei instancecount). mode specifies the primitive type to render (such as GL_POINTS, GL_LINES, GL_TRIANGLES). first and count define the range of vertices to render within the enabled arrays, starting at index first and containing count vertices. instancecount indicates how many instances to render. If instancecount is zero, nothing is drawn.

Per-instance variation is typically achieved by attributes configured to advance once per instance. This is done with glVertexAttribDivisor, where a divisor of 0 means the attribute advances per-vertex, and a divisor of 1 or higher advances per instance. In the vertex shader, the built-in variable gl_InstanceID provides the index of the current instance, which can be used to index into per-instance data or drive per-instance calculations.

Common uses include rendering large numbers of similar objects, such as particles, foliage, or crowds, with per-instance transforms or colors driven by per-instance data. This approach reduces CPU overhead and draw calls compared to issuing a separate draw for each instance.

Compatibility and variants: glDrawArraysInstanced is available in core OpenGL since version 3.1 (via ARB_draw_instanced). There are related functions such as glDrawElementsInstanced for indexed drawing and, in extended forms, variants that support a base instance offset. A typical example draws several instances of a mesh by supplying instance-specific data through instanced vertex attributes and using gl_InstanceID in the shader.