Technical Q&A QA1550

QuickTime Visual Context - Setting the kQTVisualContextOutputColorSpaceKey Attribute

Q: How do I set the output color space attribute when creating a QuickTime OpenGL Texture Visual Context?

A: How do I set the output color space attribute when creating a QuickTime OpenGL Texture Visual Context?

Both QuickTime Visual Context creation APIs, QTOpenGLTextureContextCreate and QTPixelBufferContextCreate take a dictionary of key-value paired attributes that influence aspects of the created context. Single attributes may also be set after creation by calling QTVisualContextSetAttribute. A full list of visual context attributes can be found in ImageCompression.h.

Two of these attributes are the working color space (kQTVisualContextWorkingColorSpaceKey) and the output color space (kQTVisualContextOutputColorSpaceKey). Setting one or both of these color space attributes allows developers to influence how QuickTime interprets color information during rendering.

Key: kQTVisualContextWorkingColorSpaceKey

Value: CGColorSpaceRef

Description:

The color space in which QuickTime will perform image processing. If this attribute is not set, images will be
processed in the output color space.

Key: kQTVisualContextOutputColorSpaceKey

Value: CGColorSpaceRef

Description:

The color space of images produced by this visual context. If this attribute is not set, images may be in any
color space.

To set the output color space attribute, first create an attributes dictionary containing a CGColorSpaceRef paired with the kQTVisualContextOutputColorSpaceKey key, then pass this attributes dictionary to QTOpenGLTextureContextCreate as shown in listing 1. Listing 2 demonstrates how to create a CGColorSpaceRef from the display profile.

Listing 1 Visual Context creation with attributes.

QTVisualContextRef  theVisualContext;

...

NSDictionary *attributes = [NSDictionary dictionaryWithObjectsAndKeys:
                                         [self displayColorSpace], kQTVisualContextOutputColorSpaceKey, nil];

error = QTOpenGLTextureContextCreate(kCFAllocatorDefault, (CGLContextObj)[[self openGLContext] CGLContextObj],
                                    (CGLPixelFormatObj)[[self pixelFormat] CGLPixelFormatObj],
                                    (CFDictionaryRef)attributes,
                                    &theVisualContext);
if (noErr == error) {
    [self setVisualContext:theVisualContext];
} ...

...

Listing 2 Creating a display color space.

...

// return currently set display color space
- (CGColorSpaceRef)displayColorSpace
{
    return mDisplayColorSpace;
}

// set the display color space, this function in null safe
- (void)setDisplayColorSpace:(CGColorSpaceRef)inDisplayColorSpace
{
    CGColorSpaceRetain(inDisplayColorSpace);
    CGColorSpaceRelease(mDisplayColorSpace);

    mDisplayColorSpace = inDisplayColorSpace;
}

// update the display color space
- (void)updateDisplayColorSpace:(CGDirectDisplayID)inDisplayID
{
    CMProfileRef profile;

    if (noErr == CMGetProfileByAVID((CMDisplayIDType)inDisplayID, &profile)) {
        CGColorSpaceRef theDisplayColorSpace = CGColorSpaceCreateWithPlatformColorSpace(profile);

        [self setDisplayColorSpace:theDisplayColorSpace];

        CGColorSpaceRelease(theDisplayColorSpace);

        CMCloseProfile(profile);
    } else {
        [self setDisplayColorSpace:NULL];
    }

    if (NULL != [self visualContext]) {
        QTVisualContextSetAttribute(qtVisualContext,
                                    kQTVisualContextOutputColorSpaceKey,
                                    [self displayColorSpace]);
    }
}

...

References

Document Revision History

Date	Notes
2007-09-28	New document that describes how to configure a QuickTime OpenGL Texture Visual Context with the kQTVisualContextOutputColorSpaceKey attribute.