Poor looking Quartz on Retina Display

brian515 · 08-10-2010, 11:02 PM

Hey everyone,

I'm trying to update an app I have which lets the user draw with their finger. I do this in Quartz. However, when I test on the iPhone 4 (well, the simulator, actually, as I don't have an iPhone 4), the Quartz drawing looks pixelated.

They way I'm drawing is as follows:
In my UIView subclass, in the initWithFrame method, I have this:

Code:

		CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
		
		_myContext = CGBitmapContextCreate(NULL, self.frame.size.width, self.frame.size.height, 8, 
										   4*self.frame.size.width, colorSpace, kCGImageAlphaPremultipliedLast);
		
		drawingLayer = CGLayerCreateWithContext(_myContext, self.frame.size, NULL);
		CGContextRef layerContext = CGLayerGetContext(drawingLayer);
		CGContextSetLineWidth(layerContext, 10.0);
		CGContextSetLineCap(layerContext, kCGLineCapRound);
		drawingColor = [UIColor redColor];
		CGContextSetStrokeColorWithColor(layerContext, drawingColor.CGColor);

Then, in the drawRect method, I have this:

Code:

ctx = UIGraphicsGetCurrentContext();
	
	CGImageRef image = CGBitmapContextCreateImage(_myContext);
	CGContextDrawImage(ctx, self.bounds, image);
	CGImageRelease(image);
	
	CGContextDrawLayerInRect(ctx, self.bounds, drawingLayer);

And finally, in touchesMoved, I have this:

Code:

CGContextSetLineWidth(layerContext, pointSize);
				CGPoint pastLocation = [touch previousLocationInView:self];
			
				CGContextBeginPath(layerContext);
				CGContextMoveToPoint(layerContext, pastLocation.x, pastLocation.y);
				CGContextAddLineToPoint(layerContext, location.x, location.y);
				CGContextStrokePath(layerContext);

I believe the error is in this line

Code:

_myContext = CGBitmapContextCreate(NULL, self.frame.size.width, self.frame.size.height, 8, 
										   4*self.frame.size.width, colorSpace, kCGImageAlphaPremultipliedLast);

because if I change it to

Code:

_myContext = CGBitmapContextCreate(NULL, 640, 960, 8, 
										   4*self.frame.size.width, colorSpace, kCGImageAlphaPremultipliedLast);

everything looks sharp. However, when I do that, I get the

Code:

<Error>: CGBitmapContextCreateImage: invalid context 0x0

message.

Any help or suggestions would be greatly appreciated. Thanks!