Graphics Reference
In-Depth Information
All the power of OpenGL is at your fingertips in a
QCCompositionLayer
as it inherits
directly from
CAOpenGLLayer
, which we discuss in detail in Chapter 8, “OpenGL Layer.”
What we learned in that chapter is that we first determine whether we should draw using
-canDrawInCGLContext
and then draw in the context using
-drawInCGLContext
. We can
override both of these methods if we create our own
QCCompositionLayer
-derived class
and add additional functionality.
This gives you the power to do any additional rendering you might want to do before
displaying the current frame in the composition. Remember that Quartz Compositions
are playing, which is to say that they are rendering to the screen on a regular basis. The
frequency is not something that you control because it is related to your hardware and
the video refresh rate. What you do know, however, is that if
-drawInCGLContext
has
been called, the next frame is ready to be rendered, and
-drawInCGLContext
has provided
you a
CGLContextObj
object into which you can render your own drawing. Listing 9-4
demonstrates what the default implementation of a
QCQuartzCompositionLayer
derived
class looks like.
LISTING 9-4
Default QCCompositionLayer Derived Class Implementation
@interface
QuartzLayer : QCCompositionLayer {
}
@end
@implementation
QuartzLayer
- (
BOOL
)canDrawInCGLContext:(CGLContextObj)glContext
pixelFormat:(CGLPixelFormatObj)pixelFormat
forLayerTime:(
CFTimeInterval
)timeInterval
displayTime:(
const
CVTimeStamp *)timeStamp;
{
return
[
super
canDrawInCGLContext:glContext
pixelFormat:pixelFormat
forLayerTime:timeInterval
displayTime:timeStamp];
}
- (
void
)drawInCGLContext:(CGLContextObj)glContext
pixelFormat:(CGLPixelFormatObj)pixelFormat
forLayerTime:(
CFTimeInterval
)timeInterval
displayTime:(
const
CVTimeStamp *)timeStamp;
{
