Drawing on a zoomable view - ios

Question

I'm working on a small drawing application, which has a basic requirement of supporting zoom-in/out. I have two main issues:
Drawing doesn't appear crisp and clear, when view is zoomed/transformed. Is there a better approach, or is there a way to improve the drawing when the view is zoomed?
The drawing performance is slow, when drawing on 1200 x 1200 pts canvas (on iPhone). Any chance I can improve it for large canvas sizes?
Zooming Code:
- (void)scale:(UIPinchGestureRecognizer *)gestureRecognizer {
[self adjustAnchorPointForGestureRecognizer:gestureRecognizer];
UIView *canvas = [gestureRecognizer view];
if ([gestureRecognizer state] == UIGestureRecognizerStateBegan ||
[gestureRecognizer state] == UIGestureRecognizerStateChanged) {
// Calculate the drawing view's size
CGSize drawingViewSize = ...;
// Calculate the minimum allowed tranform size
// Developer's Note: I actually wanted to use the size 1/4th of the view
// but self.view.frame.size doesn't return the correct (actual) width and height
// It returns these values inverted i.e. width as height, and vice verse.
// The reason is that the view appears to be transformed (90 degrees).
// Since there's no work-around this, so for now, I'm just using fixed values.
CGSize minTranformSize = CGSizeMake(100.0f, 100.0f);
if ((minTranformSize.width > drawingViewSize.width) && (minTranformSize.height > drawingViewSize.height)) {
minTranformSize = drawingViewSize;
}
// Transform the view, provided
// 1. It won't scale more than the original size of the background image
// 2. It won't scale less than the minimum possible transform
CGSize transformedSize = CGSizeMake(canvas.frame.size.width * [gestureRecognizer scale],
canvas.frame.size.height * [gestureRecognizer scale]);
if ((transformedSize.width <= drawingViewSize.width) && (transformedSize.height <= drawingViewSize.height) &&
(transformedSize.width >= minTranformSize.width) && (transformedSize.height >= minTranformSize.height)) {
canvas.transform = CGAffineTransformScale([canvas transform],
[gestureRecognizer scale],
[gestureRecognizer scale]);
}
[gestureRecognizer setScale:1.0];
} else if ([gestureRecognizer state] == UIGestureRecognizerStateEnded) {
// Recenter the container view, if required (piece is smaller than the view and it's not aligned)
CGSize viewSize = self.view.bounds.size;
if ((canvas.frame.size.width < viewSize.width) ||
(canvas.frame.size.height < viewSize.height)) {
canvas.center = CGPointMake(viewSize.width/2, viewSize.height/2);
}
// Adjust the x/y coordinates, if required (piece is larger than the view and it's moving outwards from the view)
if (((canvas.frame.origin.x > 0) || (canvas.frame.origin.y > 0)) &&
((canvas.frame.size.width >= viewSize.width) && (canvas.frame.size.height >= viewSize.height))) {
canvas.frame = CGRectMake(0.0,
0.0,
canvas.frame.size.width,
canvas.frame.size.height);
}
canvas.frame = CGRectIntegral(canvas.frame);
}
}
Drawing Code
- (void)draw {
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context);
if (self.fillColor) {
[self.fillColor setFill];
[self.path fill];
}
if ([self.strokeColor isEqual:[UIColor clearColor]]) {
[self.path strokeWithBlendMode:kCGBlendModeClear alpha:1.0];
} else if (self.strokeColor) {
[self.strokeColor setStroke];
[self.path stroke];
}
CGContextRestoreGState(context);
}

Answer 1

This is a pretty complicated problem, that I have struggled a lot with.
I ended up converting the drawings to vector.
draw all lines in one layer, draw all fills in another.
Convert the line drawings to Vector, using potrace (http://potrace.sourceforge.net/)
draw the vector using SVGKit
(https://github.com/SVGKit/SVGKit) and hide the layer drawn in 1)
It is working pretty well and fairly fast, but it requires a lot of work. We have an app in our company that applies this technique:
https://itunes.apple.com/us/app/ideal-paint-hd-mormor/id569450492?mt=8.
If your only problem is performance, try taking a look at CATiledLayer. (also used in app mentioned above). It will increase performance tremendously (You can find a pretty good tutorial here http://www.cimgf.com/2011/03/01/subduing-catiledlayer/).
Good luck! :)

Answer 2

First of all You are Transforming View this is not better way for zooming .Transforming may use only for increase or decrease size of UIVew For zooming you can d this.
1) get get pic of screen by using this code
UIGraphicsBeginImageContext(self.drawingView.bounds.size);
CGContextRef context = UIGraphicsGetCurrentContext();
//[self.view.layer renderInContext:context];
[self.layerContainerView.layer renderInContext:context];
UIImage *screenShot = UIGraphicsGetImageFromCurrentImageContext();
[scaleLabel setHidden:FALSE];
return screenShot;
2) then put it on some UIImageView and then perform Zomming on this Image
scrollView.userInteractionEnabled=TRUE;
self.scrollView.minimumZoomScale = 1.000;
self.scrollView.maximumZoomScale = 30.0f;
[self centerScrollViewContents];
CGFloat newZoomScale = self.scrollView.zoomScale / 1.5f;
newZoomScale = MAX(newZoomScale, self.scrollView.minimumZoomScale);
[self.scrollView setZoomScale:newZoomScale animated:YES];
3) then again Set zoomed image on uiView's background
this works perfect for me hope this will also works for you