glScissorArray

glScissorArray is an OpenGL command that specifies scissor rectangle values for a contiguous group of viewport array elements. It is defined in the ARB_viewport_array extension and has become part of the core OpenGL specification since version 4.0. The function prototype is

void glScissorArray( GLuint first, GLsizei count, const GLint *v );

The first parameter identifies the starting index of the viewport array that will receive scissor information, while count indicates how many consecutive elements are updated. The array pointed to by v must contain 4 × count integer values; each group of four values represents the x and y coordinates of the lower‑left corner of the rectangle, followed by its width and height. The coordinates and dimensions are expressed in window‑space pixel units, exactly like those used by the legacy glScissor command. Values are interpreted as integer offsets; when the GL component is running in a fixed‑function pipeline the rectangles are applied after the viewport transformation and before fragment rendering.

When glScissorArray is executed the GL sets the scissor state for the selected viewport array elements and associates each rectangle with its respective viewport. Subsequent rendering calls that target a specific viewport will clip fragments to the rectangle assigned to that viewport index. Clipping occurs after depth and stencil tests but before blending. This gives applications fine control over which parts of the window should receive fragments when drawing into multiple viewports.

Typical usage involves calling glScissorArray once per frame after glViewportArray or glBegin? After setting up viewport positions, an application can define per‑viewport scissor boxes to restrict output to specific screen regions, for instance to render a HUD on a small area while the main scene occupies the rest of the window.

When the GL_SCISSOR_TEST capability is disabled, calling glScissorArray has no effect on rendering, although the state is still stored. If the specified index range exceeds the maximum number of supported viewports, GL_INVALID_VALUE is generated. The function has no side effects on other viewport‑dependent state. Documentation recommends that the application check the value of GL_MAX_VIEWPORTS; glScissorArray with a count of zero is permitted and performs no updates.