OpenGL Layer - Core Animation - page 133

Layer Timing
Notice in both -canDrawInCGLContext and -drawInCGLContext, two fields relate to time:

.

forLayerTime , which is a CFTimeInterval

.

displayTime , which is a CVTimeStamp

We won't focus on forLayerTime because we won't be using it with CAOpenGLLayer ;
however, displayTime is important for this exercise.

Playing back movies correctly with a display's refresh rate and in-sync with the movie's
sound can be tricky. However, Apple has achieved stable movie playback in the Mac OS
using what they refer to as a display link . Here is how Apple defines a display link in the
Core Video Programming Guide : 1

To simplify synchronization of video with a display's refresh rate, Core Video
provides a special timer called a display link. The display link runs as a separate
high priority thread, which is not affected by interactions within your applica-
tion process.

In the past, synchronizing your video frames with the display's refresh rate was
often a problem, especially if you also had audio. You could make only simple
guesses for when to output a frame (by using a timer, for example), which didn't
take into account possible latency from user interactions, CPU loading, window
compositing, and so on. The Core Video display link can make intelligent esti-
mates for when a frame needs to be output, based on display type and latencies.

In essence this means that you must set up a callback that will get called periodically, at
which point you can check to see if a new frame is available for display at the time speci-
fied in the callback's inOutputTime CVTimeStamp field. The display link callback signature
is in Listing 8-2.

LISTING 8-2

Display Link Callback Signature

CVReturn MyDisplayLinkCallback (
CVDisplayLinkRef displayLink,
const CVTimeStamp *inNow,
const CVTimeStamp *inOutputTime,
CVOptionFlags flagsIn,
CVOptionFlags *flagsOut,
void *displayLinkContext);

1

Apple's Core Video Programming Guide (p. 10): http://developer.apple.com/mac/library/
documentation/GraphicsImaging/Conceptual/CoreVideo/CoreVideo.pdf.