Technical Q&A QA1542

QuickTime Texture Visual Context - kQTVisualContextNotAllowedErr with non-accelerated graphics cards

Q:  I'm working with QTCoreVideo101 but it never returns a VisualContextRef. Calling QTOpenGLTextureContextCreate always fails with error -9459. I have a Mac Pro system with a third party video card for Final Cut Pro. How can I create a QuickTime OpenGL Texture Visual Context?

A: I'm working with QTCoreVideo101 but it never returns a VisualContextRef. Calling QTOpenGLTextureContextCreate always fails with error -9459. I have a Mac Pro system with a third party video card for Final Cut Pro. How can I create a QuickTime OpenGL Texture Visual Context?

The most likely reason for the -9459 (kQTVisualContextNotAllowedErr) error is the third party video card. If the card presents itself as a video device but does not support accelerated surfaces QTOpenGLTextureContextCreate will fail.

To successfully create a QuickTime OpenGL Texture Visual Context, the non-accelerated display(s) may be masked out by creating a display mask (simply a bitmask used to specify a set of attached displays) that only identifies the displays supporting accelerated surfaces. This can be done using Quartz Display Services APIs such as CGDisplayUsesOpenGLAcceleration and CGDisplayIDToOpenGLDisplayMask.

This display mask (with the non-accelerated display(s) masked out) is then used when creating the OpenGL Pixel Format object. This Pixel Format object is used when creating the OpenGL Context and when calling QTOpenGLTextureContextCreate.

Listing 1 demonstrates how to build the display mask, while listing 2 demonstrates how to use the returned display mask when creating the NSOpenGLPixelFormat object.

Listing 1  Return the Accelerated Display Mask.

static CGOpenGLDisplayMask GetTheAcceleratedDisplayMask(void)
{ 
    const CGDisplayCount maxDisplays = sizeof(CGOpenGLDisplayMask) * 8; // the number of displays

    CGDirectDisplayID displays[maxDisplays]; // represents the unique identifier for an attached display
    CGDisplayCount display, displayCount;
    CGOpenGLDisplayMask displayMask = 0; // bitmask used in OpenGL to specify a set of attached displays

    // get the list of displays that are active (or drawable)
    if (CGGetActiveDisplayList(maxDisplays, displays, &displayCount) == kCGErrorSuccess) {

        // does the display support acceleration
        for (display = 0; display < displayCount; display++) {

            if (CGDisplayUsesOpenGLAcceleration(displays[display])) {
                // it does, so get the OpenGL display mask for the display and update our display mask
                displayMask |= CGDisplayIDToOpenGLDisplayMask(displays[display]);
            }
        }
    }

    // return our display mask that now has all the non-accelerated displays filtered out
    return displayMask;
}

In listing 1, we need the active display list. An active display is connected, awake and available for drawing, while a display is considered online simply when the frame buffer is connected to a monitor.

In a hardware mirroring set, only the primary display is active.

Listing 2  Using the Display Mask by Subclassing NSOpenGLView

@interface MyOpenGLView : NSOpenGLView
{
    ...

    QTVisualContextRef mQTOpenGLTextureVisualContext;
}
    ...

- (QTVisualContextRef)visualContext;
- (void) setVisualContextRef:(QTVisualContextRef)inVisualContext;

    ...
@end

@implementation MyOpenGLView

+ (NSOpenGLPixelFormat *)defaultPixelFormat
{
    // create an NSOpenGLPixelFormat object with the appropriate display mask
    CGOpenGLDisplayMask myDisplayMask = GetTheAcceleratedDisplayMask();

    if (0 != myDisplayMask) {
        NSOpenGLPixelFormatAttribute attributes[] =
        {
            NSOpenGLPFADoubleBuffer, // double buffered pixel format
            NSOpenGLPFAAccelerated,  // hardware accelerated renderer
            NSOpenGLPFANoRecovery,   // disable all failure recovery systems
            //** bit mask of supported physical screens **//
            NSOpenGLPFAScreenMask, (NSOpenGLPixelFormatAttribute)myDisplayMask,
            (NSOpenGLPixelFormatAttribute)0
        };

        return [[(NSOpenGLPixelFormat*)[NSOpenGLPixelFormat alloc]
                                                            initWithAttributes:attributes] autorelease];
    } else {
        return [super defaultPixelFormat];
    }
}

- (id)initWithFrame:(NSRect)frameRect
{
    return [super initWithFrame:frameRect pixelFormat:[MyOpenGLView defaultPixelFormat]];
}

- (void)prepareOpenGL
{
    ...

    if (nil != [self openGLContext]) {
        long swapInterval = 1;

       // sync with screen refresh to avoid tearing
       [[self openGLContext] setValues:&swapInterval forParameter:NSOpenGLCPSwapInterval];
    }

    ...

    // create the QuickTime Texture Visual Context for rendering to my custom NSOpenGLView
    if (NULL == [self visualContextRef]) {
        QTVisualContextRef aQTOpenGLTextureContext;

        OSStatus status = QTOpenGLTextureContextCreate(kCFAllocatorDefault,
                                         [[self openGLContext] CGLContextObj],
                                         (CGLPixelFormatObj)[[self pixelFormat] CGLPixelFormatObj],
                                         NULL, &aQTOpenGLTextureContext);
        if (noErr == status) {
            [self setVisualContextRef:aQTOpenGLTextureContext];

            ...

        } else {
            [self setVisualContextRef:NULL];
        }
    }

    ...

}
    ...

- (QTVisualContextRef)visualContext
{
    return mQTOpenGLTextureVisualContext;
}

- (void)setVisualContextRef:(QTVisualContextRef)inVisualContext
{
    mQTOpenGLTextureVisualContext = inVisualContext;
}
    ...

References

Back to Top

Document Revision History


DateNotes
2007-09-18

New document that discusses how to create a display mask and avoid a kQTVisualContextNotAllowedErr when using a non-accelerated graphics card.




Copyright © 2007 Apple Inc. All Rights Reserved. Terms of Use | Privacy Policy | Updated: 2007-09-18