How to use glClearBuffer* in Cocoa OpenGLView to clear color buffer - objective-c

Question

I'm trying to understand how to use glClearBuffer* to change the background color in a (either single or double buffered) NSOpenGLView in Cocoa for OS X.
The following code fragment as suggested by the OpenGL Superbible fails with GL_INVALID_OPERATION:
GLfloat red[] = {1.0f, 0.0f, 0.0f, 1.0f};
glClearBufferfv(GL_COLOR, 0, red);
What do I need to supply for the second parameter?
I'm using a double buffered View extending OpenGLView.
#import "MyOpenGLView.h"
#include <OpenGL/gl3.h>
#implementation MyOpenGLView
-(void) drawRect: (NSRect) bounds
{
GLfloat red[] = {1.0f, 0.0f, 0.0f, 1.0f};
glClearBufferfv(GL_COLOR, 0, red);
GLenum e = glGetError();
// e == GL_INVALID_OPERATION after this call
// and the view is drawn in black
// The following two lines work as intended:
//glClearColor(1.0, 0.f, 0.f, 1.f);
//glClear(GL_COLOR_BUFFER_BIT);
[[self openGLContext] flushBuffer];
}
#end

Answer 1

Really? It is giving you GL_INVALID_OPEARATION?
This function is not supposed to generate that error... are you sure something earlier in your program did not create the error and you are mistaking the source?
The bigger problem however, is that using GL_COLOR as the buffer in this API call expects the second parameter to be an index into your set of draw buffers. It is unclear how your draw buffers are setup in this code, it is possible that you have GL_NONE. As there is no defined error behavior if you try to clear a draw buffer when GL_NONE is used, I suppose an implementation might choose to raise GL_INVALID_OPERATION.
In order for your current usage of glClearBufferfv (...) to make sense, I would expect to see something like this:
GLenum buffers [] = { GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1 };
GLfloat red [] = { 1.0f, 0.0f, 0.0f, 1.0f };
glDrawBuffers (2, buffers);
glClearBufferfv (GL_COLOR, 0, red);
Now this call will clear GL_COLOR_ATTACHMENT0, if you wanted to clear GL_COLOR_ATTACHMENT1, you could replace 0 with 1.