Drawing on the retina display using CoreGraphics - Image pixelated

Question

In my iOS application, I am trying to draw curves using CoreGraphics. The drawing itself works fine, but on the retina display the image gets drawn using the same resolution, and does not get pixel doubled. The result is a pixelated image.

I am drawing using the following functions:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.canvasView];

    UIGraphicsBeginImageContext(self.canvasView.frame.size);
    [canvasView.image drawInRect:self.canvasView.frame];
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextSetShouldAntialias(ctx, YES);
    CGContextSetLineCap(ctx, kCGLineCapRound);
    CGContextSetLineWidth(ctx, 5.0);
    CGContextSetRGBStrokeColor(ctx, 1.0, 0.0, 0.0, 1.0);
    CGContextBeginPath(ctx);
    CGContextMoveToPoint(ctx, lastPoint.x, lastPoint.y);
    CGContextAddLineToPoint(ctx, currentPoint.x, currentPoint.y);
    CGContextStrokePath(ctx);
    canvasView.image =  UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    lastPoint = currentPoint;
    // some code omitted from this example
}

The advice I have found was to use the scaleFactor property, or the CGContextSetShouldAntialias() function, but neither of these helped so far. (Although I might have used them improperly.)

Any help would be greatly appreciated.

Answer 1

You need to replace UIGraphicsBeginImageContext with

if (UIGraphicsBeginImageContextWithOptions != NULL) {
  UIGraphicsBeginImageContextWithOptions(size, NO, 0.0);
} else {
  UIGraphicsBeginImageContext(size);
}

UIGraphicsBeginImageContextWithOptions was introduced in 4.x software. If you're going to run this code on 3.x devices, you need to weak link UIKit framework. If your deployment target is 4.x or higher, you can just use UIGraphicsBeginImageContextWithOptions without any additional checking.