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

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  1. POHow do I render a view which contains Core Animation layers to a bitmap?
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    <p>I am using an <code>NSView</code> to host several Core Animation <code>CALayer</code> objects. What I want to be able to do is grab a snapshot of the view's current state as a bitmap image.</p> <p>This is relatively simple with a normal <code>NSView</code> using something like this:</p> <pre><code>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 </code></pre> <p>However, when I use this code, the Core Animation layers are not rendered.</p> <p>I have investigated <code>CARenderer</code>, as it appears to do what I need, however I cannot get it to render my existing layer tree. I tried the following:</p> <pre><code>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]]; </code></pre> <p>However, when I do this I get an exception:</p> <p><code>CAContextInvalidLayer -- layer &lt;CALayer: 0x1092ea0&gt; is already attached to a context</code></p> <p>I'm guessing that this must be because the layer tree is hosted in my <code>NSView</code> and therefore attached to its context. I don't understand how I can detach the layer tree from the <code>NSView</code> in order to render it to a bitmap, and it's non-trivial in this case to create a duplicate layer tree.</p> <p>Is there some other way to get the <code>CALayer</code>s 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 <code>CARenderer</code> at all.</p>
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