How do I render a view which contains Core Animation layers to a bitmap?

Question

I am using an NSView to host several Core Animation CALayer objects. What I want to be able to do is grab a snapshot of the view's current state as a bitmap image.

This is relatively simple with a normal NSView using something like this:

void ClearBitmapImageRep(NSBitmapImageRep* bitmap) {
    unsigned char* bitmapData = [bitmap bitmapData];
    if (bitmapData != NULL)
        bzero(bitmapData, [bitmap bytesPerRow] * [bitmap pixelsHigh]);
}

@implementation NSView (Additions)
- (NSBitmapImageRep*)bitmapImageRepInRect:(NSRect)rect
{
    NSBitmapImageRep* imageRep = [self bitmapImageRepForCachingDisplayInRect:rect];
    ClearBitmapImageRep(imageRep);
    [self cacheDisplayInRect:rect toBitmapImageRep:imageRep];
    return imageRep;
}
@end

However, when I use this code, the Core Animation layers are not rendered.

I have investigated CARenderer, as it appears to do what I need, however I cannot get it to render my existing layer tree. I tried the following:

NSOpenGLPixelFormatAttribute att[] = 
{
    NSOpenGLPFAWindow,
    NSOpenGLPFADoubleBuffer,
    NSOpenGLPFAColorSize, 24,
    NSOpenGLPFAAlphaSize, 8,
    NSOpenGLPFADepthSize, 24,
    NSOpenGLPFANoRecovery,
    NSOpenGLPFAAccelerated,
    0
};

NSOpenGLPixelFormat *pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:att];
NSOpenGLView* openGLView = [[NSOpenGLView alloc] initWithFrame:[self frame] pixelFormat:pixelFormat];
NSOpenGLContext* oglctx = [openGLView openGLContext];

CARenderer* renderer = [CARenderer rendererWithCGLContext:[oglctx CGLContextObj] options:nil];
renderer.layer = myContentLayer;
[renderer render];
NSBitmapImageRep* bitmap = [oglView bitmapImageRepInRect:[oglView bounds]];

However, when I do this I get an exception:

CAContextInvalidLayer -- layer <CALayer: 0x1092ea0> is already attached to a context

I'm guessing that this must be because the layer tree is hosted in my NSView and therefore attached to its context. I don't understand how I can detach the layer tree from the NSView in order to render it to a bitmap, and it's non-trivial in this case to create a duplicate layer tree.

Is there some other way to get the CALayers to render to a bitmap? I can't find any sample code anywhere for doing this, in fact I can't find any sample code for CARenderer at all.

Answer 1

There is a great post on "Cocoa is my girlfriend" about recording Core Animations. The author captures the whole animation into a movie, but you could use the part where he grabs a single frame.
Jump to the "Obtaining the Current Frame" section in this article:
http://www.cimgf.com/2009/02/03/record-your-core-animation-animation/

The basic idea is:

• Create a CGContext
• Use CALayer's renderInContext:
• Create a NSBitmapImageRep from the context (using CGBitmapContextCreateImage and NSBitmapImageRep's initWithCGImage)

Update:
I just read, that the renderInContext: method does not support all kind of layers in Mac OS X 10.5. It does not work for the following layers classes:

• QCCompositionLayer
• CAOpenGLLayer
• QTMovieLayer

Answer 2

If you want sample code for how to render a CALayer hierarchy to an NSImage (or UIImage for the iPhone), you can look at the Core Plot framework's CPLayer and its -imageOfLayer method. We actually created a rendering pathway that is independent of the normal -renderInContext: process used by CALayer, because the normal way does not preserve vector elements when generating PDF representations of layers. That's why you'll see the -recursivelyRenderInContext: method in this code.

However, this won't help you if you are trying to capture from any of the layer types mentioned by weichsel (QCCompositionLayer, CAOpenGLLayer, or QTMovieLayer).