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Ensuring a Single Renderer for a Display

In some cases you may want to use a specific hardware renderer and nothing else. Since the OpenGL framework normally provides a software renderer as a fallback in addition to whatever hardware renderer it chooses, you need to prevent OpenGL from choosing the software renderer as an option. You either need to specify the no recovery attribute for a windowed drawable object or use a full-screen drawable object. (The full-screen attribute always implies not to use the software renderer as an option.)

Limiting a context to use a specific display, and thus a single renderer, has its risks. If your application runs on a system that uses more than one display, then dragging a windowed drawable object from one display to the other will likely yield a less than satisfactory result. Either the rendering will fail, or OpenGL uses the specified renderer to copy the drawing to the second display. The same unsatisfactory result happens when attaching a full-screen context to another display. If you choose to use the hardware renderer associated with a specific display, you need to add code that detects and handles display changes.

The three code examples that follow show how to use each of the Apple-specific OpenGL APIs to set up a context that uses a single renderer. Listing 7-2 shows how to set up an NSOpenGLPixelFormat object that supports a single renderer. The attribute NSOpenGLPFANoRecovery specifies to OpenGL not to provide the fallback option of the software renderer.

Listing 7-2  Setting an NSOpenGLContext object to use a specific display

#import <Cocoa/Cocoa.h>

+ (NSOpenGLPixelFormat*)defaultPixelFormat

{

    NSOpenGLPixelFormatAttribute attributes [] = {

                        NSOpenGLPFAScreenMask, 0,

                        NSOpenGLPFANoRecovery,

                        NSOpenGLPFADoubleBuffer,

                        (NSOpenGLPixelFormatAttribute)nil };

CGDirectDisplayID display = CGMainDisplayID ();

// Adds the  display mask attrib for selected display

attributes[1] = (NSOpenGLPixelFormatAttribute)

                    CGDisplayIDToOpenGLDisplayMask (display);

return [[(NSOpenGLPixelFormat *)[NSOpenGLPixelFormat alloc] initWithAttributes:attributes]

                                       autorelease];

}

Listing 7-3 shows how to use AGL to set up a context that uses a single renderer. The attribute AGL_NO_RECOVERY specifies to OpenGL not to provide the fallback option of the software renderer.

Listing 7-3  Setting an AGL context to use a specific display

#include <AGL/agl.h>

GLint attrib[] = {AGL_RGBA, AGL_DOUBLEBUFFER, AGL_NO_RECOVERY, AGL_NONE};

GDHandle display = GetMainDevice ();

AGLPixelFormat aglPixFmt = aglChoosePixelFormat (&display, 1, attrib);

Listing 7-4 shows how to use CGL to set up a context that uses a single renderer. The attribute kCGLPFAFullScreen ensures that OpenGL does not provide the fallback option of the software renderer.

Listing 7-4  Setting a CGL context to use a specific display

#include <OpenGL/OpenGL.h>

CGLPixelFormatAttribute attribs[] = { kCGLPFADisplayMask, 0,

                                 kCGLPFAFullScreen,

                                 kCGLPFADoubleBuffer,

                                  0 };

CGLPixelFormatObj pixelFormat = NULL;

long numPixelFormats = 0;

CGLContextObj cglContext = NULL;

CGDirectDisplayID display = CGMainDisplayID ();

// Adds the  display mask attrib for selected display

attribs[1] = CGDisplayIDToOpenGLDisplayMask (display);

CGLChoosePixelFormat (attribs, &pixelFormat, &numPixelFormats);

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