glViewportIndex

glViewportIndex refers to the OpenGL API facilities used to manage multiple viewports within a single context, enabling per-viewport specifications rather than a single global viewport. This capability is provided by the ARB_viewport_array extension and became part of OpenGL core in later revisions. It allows rendering to multiple regions of a window, each with its own viewport dimensions, origins, and optional per-viewport scissor state.

The core idea is to separate viewport state by index. For each viewport index, developers can define a distinct rectangle in window coordinates that governs how the final fragment positions map to the framebuffer. In addition to per-viewport viewports, related per-viewport state such as scissor rectangles can be controlled, enabling efficient tiling, UI rendering, or tiled GPU tasks without frequent state changes on the context. The API provides a set of functions to specify these states either individually for a single index or in batches for a range of indices.

Key functions associated with glViewportIndex describe the per-viewport workflow. For a single viewport, glViewportIndexedf or glViewportIndexedfv assigns the viewport rectangle to a specific index. For multiple viewports, glViewportArrayv (and related forms) sets a sequence of viewports starting at a given index. Corresponding mechanisms exist for scissor testing, such as glScissorIndexed and glScissorArray, to pair with the viewports.

Several considerations apply: hardware support is required (OpenGL 4.1 core or the ARB_viewport_array extension), and using multiple viewports is most beneficial when the rendering pipeline benefits from tiled draws or multi-viewport outputs. References typically point to the ARB_viewport_array specification and subsequent core OpenGL documentation.