From QB64.org wiki
_glClear: clear buffers to preset values
- mask that indicate the buffer OR buffers to be cleared. The three masks are _GL_COLOR_BUFFER_BIT, _GL_DEPTH_BUFFER_BIT, and _GL_STENCIL_BUFFER_BIT.
- _glClear sets the bitplane area of the window to values previously selected by _glClearColor, glClearDepth, and _glClearStencil.
- Multiple color buffers can be cleared simultaneously by selecting more than one buffer at a time using _glDrawBuffer.
- The pixel ownership test, the scissor test, dithering, and the buffer writemasks affect the operation of _glClear. The scissor box bounds the cleared region. Alpha function, blend function, logical operation, stenciling, texture mapping, and depth-buffering are ignored by _glClear.
- _glClear takes a single argument that is the bitwise OR of several values indicating which buffer is to be cleared. The values are as follows:
- The value to which each buffer is cleared depends on the setting of the clear value for that buffer.
If a buffer is not present, then a _glClear directed at that buffer has no effect.
_GL_INVALID_VALUE is generated if any bit other than the three defined bits is set in mask.
- _glClearBuffer, _glClearColor, _glClearDepth, _glClearStencil
- _glColorMask, _glDepthMask, _glStencilMask
- _glDrawBuffer, _glDrawBuffers