setNeedsDisplayInRect: paints a white rectangle only - iphone

Question

I'm still a little fresh to CoreGraphics programming, so please bear with me. I'm trying to write an application, which allows the user to rub stuff off an image with the finger. I have the basic functionality nailed down, but the result is sluggish since the screen is redrawn completely every time a touch is rendered. I did some research and found out that I can refresh only a portion of the screen using UIView's setNeedsDisplayInRect: method.
This does call drawRect: as expected, however everything I draw in the drawRect: following the setNeedsDisplayInRect: is ignored. Instead, the area in the rect parameter is simply filled with white. No matter what I draw inside, all I end up with is a white rectangle.
In essence, this is what I do:
1) when user touches screen, this touch is rendered into a mask
2) when the drawRect: is called, the image is masked with that mask
There must be something simple I'm overlooking, surely?

Answer 1

I found a solution, however it still escapes me how exactly this works. Here's the code:
This method flips the given rectangle in the same manner, in which the coordinate transformation in the context flips the context coordinate system:
- (CGRect) flippedRect:(CGRect)rect
{
CGRect flippedRect = rect;
flippedRect.origin.y = self.bounds.size.height - rect.origin.y - rect.size.height;
return CGRectIntersection( self.bounds, flippedRect );
}
This calculates the rectangle to be updated from the touch location. Note that the rectangle gets flipped:
- (CGRect) updateRectFromTouch:(UITouch *)touch
{
CGPoint location = [touch locationInView:self];
int d = RubbingSize;
CGRect touchRect = [self flippedRect:CGRectMake( location.x - d, location.y - d, 2*d, 2*d )];
return CGRectIntersection( self.frame, touchRect );
}
In render touch the 'flipped' update rectangles are coerced:
- (void) renderTouch:(UITouch *)touch
{
//
// Code to render into the mask here
//
if ( m_updateRect.size.width == 0 )
{
m_updateRect = [self updateRectFromTouch:touch];
}
else
{
m_updateRect = CGRectUnion( m_updateRect, [self updateRectFromTouch:touch] );
}
}
The whole view is refreshed with about 20Hz during the fingerpainting process. The following method is called every 1/20th second and submits the rectangle for rendering:
- (void) refreshScreen
{
if ( m_updateRect.size.width > 0 )
{
[self setNeedsDisplayInRect:[self flippedRect:m_updateRect]];
}
}
Here's a helper method to compare to rectangles:
BOOL rectIsEqualTo(CGRect a, CGRect b)
{
return a.origin.x == b.origin.x && a.origin.y == b.origin.y && a.size.width == b.size.width && a.size.height == b.size.height;
}
In the drawRect: method, the update rectangle is used to draw only the portion that needs updating.
- (void)drawRect:(CGRect)rect
{
BOOL drawFullScreen = rectIsEqualTo( rect, self.frame );
// Drawing code
CGContextRef context = UIGraphicsGetCurrentContext();
// Turn coordinate system around
CGContextTranslateCTM( context, 0.0, self.frame.size.height );
CGContextScaleCTM( context, 1.0, -1.0 );
if ( drawFullScreen )
{
// draw the full thing
CGContextDrawImage( context, self.frame, self.image );
}
else
{
CGImageRef partialImage = CGImageCreateWithImageInRect( self.image, [self flippedRect:m_updateRect] );
CGContextDrawImage( context, m_updateRect, partialPhoto );
CGImageRelease( partialImage );
}
...
// Reset update box
m_updateRect = CGRectZero;
}
If someone can explain to me why the flipping works, I'd appreciate it.

Answer 2

The flipping issue is most likely because UIKit has a 'flipped' coordinate compared to Core Graphics.
Apple documentation here