OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Viewport

To tell OpenGL where to display the rectangular viewing cone, use:

void glViewport(GLint x, GLint y, GLint width, GLint height);

Defines a pixel rectangle in the window into which the final image is mapped. The x and y parameters specify the lower-left corner of the viewport, and width and height are the size of the viewport rectangle, in pixels. By default, the initial viewport values are (0, 0, winWidth, winHeight), where winWidth and winHeight specify the size of the window.

The underlying windowing system of your platform, not OpenGL, is responsible for opening a window on the screen. However, by default, the viewport is set to the entire pixel rectangle of the window that’s opened. You use glViewport() to choose a smaller drawing ...