Technical Q&A QA1502

Ensuring hardware accelerated rendering for your OpenGL application

Q: How do I select a hardware accelerated renderer and prevent it from falling into the software renderer?

A: The Mac OS X implementation of OpenGL allows an application to gracefully switch to a software renderer as a failure recovery system when the hardware resources are not sufficient for a given operation. While most applications will benefit from this recovery there are certain applications that might want to disable this feature.

Performance driven applications, like games, need to stay in hardware accelerated renderers to achieve and maintain an acceptable frame rate.

A simple way to select a hardware accelerated renderer without the choice of software fallback is by selecting the following pixel format attributes:

For CGL:

kCGLPFAAccelerated and kCGLPFANoRecovery

For AGL:

AGL_ACCELERATED and AGL_NO_RECOVERY

For NSOpenGL:

NSOpenGLPFAAccelerated and NSOpenGLPFANoRecovery

Using the *noRecovery pixel format attribute will not prevent use of software vertex processing in such cases where the drawing request exceeds the hardware resources.

Listing 1 Creating a Pixel Format Object with Accelerated and NoRecovery attributes.

- initWithFrame:(NSRect)frameRect

    NSOpenGLPixelFormatAttribute attrs[] = {

          NSOpenGLPFANoRecovery,

          NSOpenGLPFAColorSize, 24,

            NSOpenGLPFADepthSize, 16,

            NSOpenGLPFADoubleBuffer,

            NSOpenGLPFAAccelerated,

};

    long rendererID;

    NSOpenGLPixelFormat* pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attrs];

    // Report the renderer ID that this pixel format binds to

    // CGLRenderers.h contains a list of known renderers and their corresponding RendererID codes.

    // Also at http://developer.apple.com/documentation/GraphicsImaging/Reference/CGL_OpenGL/

    [pixelFormat getValues:&rendererID forAttribute:NSOpenGLPFARendererID forVirtualScreen:0];

    NSLog(@"NSOpenGLView pixelFormat RendererID = %08x", (unsigned)rendererID);

    self = [super initWithFrame:frameRect pixelFormat:pixelFormat];

    return self;

The Techniques for Choosing Attributes section of the OpenGL Programming Guide for Mac OS X provides additional information on this subject.

Determining if your current drawing state will execute in the GPU

CGL provides two parameters for checking whether the system is using the GPU for processing: kCGLCPGPUVertexProcessing and kCGLCPGPUFragmentProcessing. To check vertex processing, pass the vertex constant to the CGLGetParameter function. To check fragment processing, pass the fragment constant to CGLGetParameter.

Listing 2 Using CGL to check whether the GPU is processing vertices and fragments.

BOOL gpuProcessing;

GLint fragmentGPUProcessing, vertexGPUProcessing;

CGLGetParameter (CGLGetCurrentContext(), kCGLCPGPUFragmentProcessing,

                                         &fragmentGPUProcessing);

CGLGetParameter(CGLGetCurrentContext(), kCGLCPGPUVertexProcessing,

                                         &vertexGPUProcessing);

gpuProcessing = (fragmentGPUProcessing && vertexGPUProcessing) ? YES : NO;

The above listing will set gpuProcessing to YES for the currently bound shader if it will execute in the GPU.

Document Revision History

Date	Notes
2007-07-18	New document that using the NoRecovery pixel format attribute and how to check whether vertex and fragment processing is executed on the GPU or CPU