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

Clamping Returned Values

Various types of buffers within OpenGL—most notably floating-point buffers—can store values with ranges outside of the normal [0, 1] range of colors in OpenGL. When you read those values back using glReadPixels(), you can control whether the values should be clamped to the normalized range or left at their full range using glClampColor().

void glClampColor(GLenum target, GLenum clamp);

Controls the clamping of color values for floating- and fixed-point buffers, when target is GL_CLAMP_READ_COLOR. If clamp is set to GL_TRUE, color values read from buffers are clamped to the range [0, 1]; conversely, if clamp is GL_FALSE, no clamping is engaged. If your application uses a combination of fixed- and floating-point buffers, ...