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

Copying Data from the Framebuffer

It is possible to read part of the framebuffer into a texture object, and then use it in subsequent rendering. To do this, use either the glCopyTexImage1D() or glCopyTexImage2D() functions whose prototypes are as follows:

void glCopyTexImage1D(GLenum target, GLint level,                         GLint internalFormat, GLint x, GLint y,                         GLsizei width, GLint border);void glCopyTexImage2D(GLenum target, GLint level,                         GLint internalFormat, GLint x, GLint y,                         GLsizei width, GLsizei height,                         GLint border);

Copy pixels from the current read framebuffer into the texture currently bound to target of the active texture unit.