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POHow to use CIContext drawImage:inRect:fromRect: with a CADisplayLink
 

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  1. POHow to use CIContext drawImage:inRect:fromRect: with a CADisplayLink
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    <p><a href="http://developer.apple.com/library/ios/DOCUMENTATION/GraphicsImaging/Reference/QuartzCoreFramework/Classes/CIContext_Class/Reference/Reference.html#//apple_ref/occ/instm/CIContext/drawImage%3ainRect%3afromRect%3a" rel="nofollow">From the docs on <code>CIContext drawImage:inRect:fromRect:</code></a>:</p> <blockquote> <p>.. On iOS 6, this method is asynchronous ..</p> </blockquote> <p>So if I am using it in a <a href="http://developer.apple.com/library/ios/#documentation/QuartzCore/Reference/CADisplayLink_ClassRef/Reference/Reference.html" rel="nofollow">CADisplayLink</a> it runs into a problem since it will keep firing off asynchronous draws at 60fps while the actual drawing might not be able to keep up. </p> <pre><code>- (void) displayLinkDidFire:(CADisplayLink *)displatLink; { CFTimeInterval duration = [displatLink duration]; CGFloat fps = round (1.0 / duration); NSLog(@"%f fps", fps); // Always logs 60 fps since drawImage is async // This method is fast since a CIImage is just a 'recipe' for an image CIImage * result = [Helper generateCIImage]; // This drawing is unable to keep up with the calls to the displayLinkDidFire method [self.ciContext drawImage:result inRect:self.destFrame fromRect:self.targetFrame]; } </code></pre> <p>How do I get around this issue?</p> <hr> <p><strong>Edit - more info</strong></p> <p>I am using CoreImage with an <code>EAGLContext</code> (for better drawing performance according to WWDCs).</p> <pre><code>self.eaglContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2]; self.ciContext = [CIContext contextWithEAGLContext:self.eaglContext options: @{kCIContextWorkingColorSpace:[NSNull null]} ]; GLKView *view = (GLKView *)self.view; view.context = self.eaglContext; view.drawableDepthFormat = GLKViewDrawableDepthFormat24; NSURL * testImageURL = [[NSBundle mainBundle] URLForResource:@"image" withExtension:@"jpg"]; NSAssert(nil != testImageURL, @"Image not found"); self.image = [CIImage imageWithContentsOfURL:testImageURL options:@{ kCIImageColorSpace:[NSNull null] }]; [EAGLContext setCurrentContext:self.eaglContext]; self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(displayLinkDidFire:)]; glEnable(GL_BLEND); glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA); </code></pre>
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    1. COGenerally a display link will do what it needs and then call `setNeedsDisplay` to tell the view to refresh. This doesn't result in any asynchronous timing issues.
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    2. CO@Rob I don't think that the display link invokes `-setNeedsDisplay`. It works independently of the standard `-drawRect:` view update cycle.
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    3. CO@Rob - Thanks for your comment. I am getting about 0.5 visible fps on my device whereas the displayLink was firing at 60 fps, so I assumed the async was the issue. Are you saying that it should not matter that drawImage is async or that I should be using something other than `CADisplayLink` to trigger drawing?
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