GLVIEWPORT

glViewport is an OpenGL function that defines the viewport, the rectangular region of the window where the final image is rasterized, and sets the mapping from normalized device coordinates to window coordinates. This mapping determines how clip space coordinates are converted to screen-space pixels during rasterization.

The function signature is void glViewport(GLint x, GLint y, GLsizei width, GLsizei height). The parameters x and y specify the lower-left corner of the viewport in window coordinates, while width and height specify the dimensions of the viewport in pixels. If width or height is zero or negative, the viewport has zero area and no fragments are produced for rendering.

In practice, OpenGL maps normalized device coordinates, which range from -1 to 1 in all three axes, to window coordinates within the rectangle [x, x+width] by [y, y+height]. Specifically, an ndc coordinate of (0,0) maps to the center of the viewport, and depth ndc values map to the depth range, which is subsequently transformed by glDepthRange to the [0,1] depth interval. The viewport origin is the lower-left corner, consistent with window coordinates in OpenGL.

The viewport is commonly updated when a window is created or resized to match the current window size or a desired subregion. It is also used when rendering to textures or when multiple viewports are needed, though multi-viewport rendering requires additional extensions or API features. Rendering can be further restricted within the viewport by the scissor test, whose rectangle is defined in window coordinates and overlies the viewport region.

Overall, glViewport controls where rendering occurs in the window and how clip-space coordinates are converted to screen-space positions. It is one of the fundamental state settings for configuring the output viewport in OpenGL.