glViewportArray

glViewportArray is a function in the OpenGL API used for setting multiple viewports. Viewports define the rectangular region on the screen where rendering occurs. Traditionally, OpenGL supported a single viewport per rendering operation. However, with the introduction of geometry shaders and tessellation shaders, it became possible to output multiple primitive streams, each potentially requiring its own viewport.

The glViewportArray function allows applications to specify an array of viewports. When used, rendering commands will write to all viewports in the array simultaneously. This is particularly useful for techniques such as stereoscopic rendering, where separate viewports can be used for the left and right eye images, or for rendering multiple layers of an image. The function takes an index, a count, and a pointer to an array of viewport structures. The index specifies the first viewport in the array to be set, and the count indicates how many viewports from that starting index will be modified. Each viewport structure defines the origin (x, y) and dimensions (width, height) of the viewport in screen coordinates. The origin is typically in the lower-left corner of the rendering window.

Using glViewportArray requires that the OpenGL context supports the GL_ARB_viewport_array extension or is using a version of OpenGL that includes this functionality natively. Incorrect usage, such as specifying an invalid index or count, can lead to undefined behavior or errors. This function is a powerful tool for advanced rendering scenarios that benefit from parallel viewport rendering.