zooming using pinch but not using transform methods - objective-c

Question

I'm trying to zoom the drawing on the context based on pinching in and out. This is working perfectly, but it will always zoom from the origin (top left). This doesn't give the nice effect you get in other applications and is really annoying. The old zooming method is as follows:
- (IBAction)handlePinchGesture:(UIGestureRecognizer *)sender {
UIPinchGestureRecognizer *pinch = (UIPinchGestureRecognizer *)sender;
if ([pinch scale] > zoomLevel + MAX_ZOOM_PER_CALL) {
[pinch setScale: zoomLevel + MAX_ZOOM_PER_CALL];
}
if ([pinch scale] < zoomLevel - MAX_ZOOM_PER_CALL) {
[pinch setScale:zoomLevel - MAX_ZOOM_PER_CALL];
}
float prevZoomLevel = zoomLevel;
CGFloat factor = [pinch scale];
zoomLevel = factor;
if (zoomLevel < MIN_ZOOM_LEVEL) {
zoomLevel = MIN_ZOOM_LEVEL;
}
if (zoomLevel > MAX_ZOOM_LEVEL) {
zoomLevel = MAX_ZOOM_LEVEL;
}
//The next piece of code is added here
[self setNeedsDisplay];
}
The actual zooming is done in the drawing methods of the parts that are shown on the context. I was thinking of changing the origin of the screen (that is the relative origin to the items that are to be drawn) so it would simulate this. But it is not working at all, this is what i have right now:
float dZoomLevel = zoomLevel - prevZoomLevel;
if (dZoomLevel != 0) {
CGPoint touchLocation = [pinch locationInView:self];
CGPoint zoomedTouchLocation = CGPointMake(touchLocation.x * prevZoomLevel, touchLocation.y * prevZoomLevel);
CGPoint newLocation = CGPointMake(touchLocation.x * zoomLevel, touchLocation.y * zoomLevel);
float dX = ( newLocation.x - zoomedTouchLocation.x );
float dY = ( newLocation.y - zoomedTouchLocation.y );
_originX += roundf(dX);
_originY += roundf(dY);
}
The origin is taken from the xml (the smallest x and y of all the elements). It will usually be something between 12000 and 20000 but could in theory be between 0 and 100000.
I'm not sure if I've made myself clear, but if you need any information or don't understand something just ask me and i'll try to reply ASAP. Thanks in advance!

Answer 1

I found the answer myself. The way one needs to zoom in my case is by zooming from the point of the touch. So when the x and y coordinates get zoomed do this:
x -= pinchLocation.x;
y -= pinchLocation.y;
x *= zoomLevel;
y *= zoomLevel;
x += pinchLocation.x;
y += pinchLocation.y;
or in short:
x = (x - pinchLocation.x) * zoomLocation + pinchLocation.x;
y = (y - pinchLocation.y) * zoomLocation + pinchLocation.y;
now x and y are on the correct location!!