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POCGContextDrawImage draws large images very blurry
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2013-12-11T02:04:34.757
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Body
I'm trying to make an object that can draw a portion of a large (say 2048 x 1537) image using CGContextDrawImage(…). It works just fine, except that it is extremely blurry. I'm using a "drawingController" which overrides drawLayer:inContext: like this <pre><code>-(void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx{ CGImageRef drawImage = CGImageRetain(imageToDraw); if(drawImage) { CGRect drawRect = CGRectMake(0, 0, 1024, 748);//Just hard coded to my window size for now. CGContextSetInterpolationQuality(ctx, kCGInterpolationHigh); CGContextDrawImage(ctx, drawRect, drawImage); } CGImageRelease(drawImage); } </code></pre> And this is how I set up my custom image hosting view: <pre><code>- (id)initWithFrame:(NSRect)frame { self = [super initWithFrame:frame]; if (self) { // Initialization code here. NSString *filePath = [[NSBundle mainBundle] pathForResource:@"large" ofType:@"jpg"]; CGImageRef imageRef = [self createCGImageFromFilePath:filePath]; drawingController = [[DrawingLayerController alloc] init]; drawingController.imageToDraw = imageRef; [self setWantsLayer:YES]; imageLayer = [[CALayer alloc] init]; imageLayer.frame = self.bounds; imageLayer.drawsAsynchronously = YES; imageLayer.delegate = drawingController; [self.layer addSublayer:imageLayer]; imageLayer.contentsRect = CGRectMake(0.5, 0.265452179, 0.5, 0.486662329); //Just an example contents rect. imageLayer.drawsAsynchronously = YES; [imageLayer setNeedsDisplay]; [imageLayer displayIfNeeded]; } return self; } </code></pre> Here is a portion of the image produced along side the source image at a similar zoom scale opened in Preview, so you can see just how blurry the image is as compared to the source image. <img src="https://i.stack.imgur.com/EZkHz.png" alt="Hopefully the image doesn't become blurry when I upload it haha"> As you can see, the image produced by my code looks HORRIBLE. What is up with that? Also, just for completeness, I have tried using a CATiledLayer for my imageLayer. The resulting image looks just as awful. One final note: The code needs to be compatible with iOS AND OSX, but I am doing all my testing on OSX. EDIT: Thanks to @Andrea, I've added the following above CGContextDrawImage(…) <pre><code>CGFloat scale = MAX(1,MAX(CGImageGetWidth(drawImage)/drawRect.size.width,CGImageGetHeight(drawImage)/drawRect.size.height)); layer.contentsScale = scale; </code></pre>
Tags
<ios><objective-c><image><macos><core-graphics>
Title
CGContextDrawImage draws large images very blurry
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