iOS paint optimization - ios

Question

Hi I am currently working on an app that contains taking notes by drawing. I followed ray wenderlich tutorials and as far I've got, I ended up with this code:
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
mouseSwiped = NO;
UITouch *touch = [touches anyObject];
lastPoint = [touch locationInView:self];
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
CGFloat red,green,blue,alpha;
mouseSwiped = YES;
UITouch *touch = [touches anyObject];
CGPoint currentPoint = [touch locationInView:self];
UIGraphicsBeginImageContext(self.mainImage.frame.size);
[self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.mainImage.frame.size.width, self.mainImage.frame.size.height)];
CGContextSetBlendMode(UIGraphicsGetCurrentContext(),[self getBlendMode]);
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x , lastPoint.y );
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x , currentPoint.y );
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), [self getBrushSize] );
[[self getPaintColor] getRed:&red green:&green blue:&blue alpha:&alpha];
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), red, green, blue, 1);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(),[self getBlendMode]);
CGContextStrokePath(UIGraphicsGetCurrentContext());
self.tempDrawImage.image = UIGraphicsGetImageFromCurrentImageContext();
[self.tempDrawImage setAlpha:[self getPaintAlpha]];
UIGraphicsEndImageContext();
lastPoint = currentPoint;
}
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
CGFloat red,green,blue;
if(!mouseSwiped) {
UIGraphicsBeginImageContext(self.mainImage.frame.size);
[self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.mainImage.frame.size.width, self.mainImage.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), [self getBrushSize]);
[[self getPaintColor] getRed:&red green:&green blue:&blue alpha:nil];
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), red, green, blue, [self getPaintAlpha]);
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y - self.mainImage.frame.origin.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y - self.mainImage.frame.origin.y );
CGContextStrokePath(UIGraphicsGetCurrentContext());
CGContextFlush(UIGraphicsGetCurrentContext());
self.tempDrawImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}
UIGraphicsBeginImageContext(self.mainImage.frame.size);
[self.mainImage.image drawInRect:CGRectMake(0, 0, self.mainImage.frame.size.width, self.mainImage.frame.size.height) blendMode:[self getBlendMode] alpha:1.0];
[self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.mainImage.frame.size.width, self.mainImage.frame.size.height) blendMode:[self getBlendMode] alpha:[self getPaintAlpha]];
if(self.drawMode != DrawEraser)
{
self.mainImage.image = UIGraphicsGetImageFromCurrentImageContext();
self.tempDrawImage.image = nil;
}
UIGraphicsEndImageContext();
mouseSwiped = NO;
}
This code was working just fine with a small frame but when I increased the frame as 2 times bigger than before, unfortunately performance is not so good. So I was thinking about optimizing the code. I especially focused on touchesMoved method. As far as I understand, it draws the whole image on the context and changes it some and assigns the context to image. Drawing whole image seemed to be overload. So I was wondering, if I can draw some parts of the image to the context and do some changes and then draw this part of the context to the image.

Answer 1

you're right - redrawing the whole image every time in touchesMoved is a bad idea. I think you should create and keep a reference to the context at the beginning. In touches moved, you should draw on to that and create an image from the context. You can use CGBitmapContextCreate() to create the context instead of UIGraphicsBeginImageContext() and CGBitmapContextCreateImage() to create an image from the context instead of UIGraphicsGetImageFromCurrentImageContext(). Here is the documentation on how to use those (https://developer.apple.com/library/mac/#documentation/graphicsimaging/reference/CGBitmapContext/Reference/reference.html).