This question is a follow-up from another SO question from a while back: zooming using pinch but not using transform methods. I got the zooming to work pretty near perfectly, however it will still make the drawn image ‘jump around’ as I’ve been calling it. The most important parts of the code are the following:
When a pinch event is observed this method is called:
[code snippet 1]
- (IBAction)handlePinchGesture:(UIGestureRecognizer *)sender {
float previousZoomLevel = zoomLevel;
UIPinchGestureRecognizer *pinch = (UIPinchGestureRecognizer *)sender;
float zoomIncreaseFactor = [self getZoomFactor:[pinch scale]]; //= 13 or 7 or 10
float estimatedDelta = (previousZoomLevel * zoomIncreaseFactor) / 150;
if ([pinch scale] > previousScale) {
zoomLevel += estimatedDelta;
}
if ([pinch scale] < previousScale) {
zoomLevel -= estimatedDelta;
}
previousScale = [pinch scale];
if (zoomLevel < MIN_ZOOM_LEVEL) {
zoomLevel = MIN_ZOOM_LEVEL;
}
if (zoomLevel > MAX_ZOOM_LEVEL) {
zoomLevel = MAX_ZOOM_LEVEL;
}
CGPoint loc = [pinch locationInView:self];
NSLog(@"loc: {%.1f,%.1f}, lastPinchLoc: {%.1f,%.1f}", loc.x,loc.y,lastPinchLocation.x,lastPinchLocation.y);
//uncomment the below part to always zoom from the first zoom location.
//if (lastPinchLocation.x != 0 && lastPinchLocation.y != 0) {
//loc.x += lastPinchLocation.x - loc.x;
//loc.y += lastPinchLocation.y - loc.y;
//}
//NSLog(@"newLastPinchLocation: {%.1f,%.1f}", loc.x,loc.y);
lastPinchLocation = loc;
hasBeenMoved = YES;
if (zoomLevel != previousZoomLevel) {
[self setNeedsDisplay];
}
}
The above NSLog gives the following prints when i pinch all over the screen:
2012-10-16 10:15:19.357 App[2609:907] loc: {147.0,232.0}, lastPinchLoc: {0.0,0.0}
2012-10-16 10:15:19.391 App[2609:907] loc: {147.0,231.0}, lastPinchLoc: {147.0,232.0}
2012-10-16 10:15:19.436 App[2609:907] loc: {148.0,232.0}, lastPinchLoc: {147.0,231.0}
2012-10-16 10:15:19.457 App[2609:907] loc: {148.0,232.0}, lastPinchLoc: {148.0,232.0}
2012-10-16 10:15:19.474 App[2609:907] loc: {148.0,233.0}, lastPinchLoc: {148.0,232.0}
2012-10-16 10:15:19.507 App[2609:907] loc: {147.0,233.0}, lastPinchLoc: {148.0,233.0}
2012-10-16 10:15:19.573 App[2609:907] loc: {103.0,355.0}, lastPinchLoc: {147.0,233.0}
2012-10-16 10:15:20.324 App[2609:907] loc: {585.0,710.0}, lastPinchLoc: {103.0,355.0}
2012-10-16 10:15:20.340 App[2609:907] loc: {584.0,709.0}, lastPinchLoc: {585.0,710.0}
2012-10-16 10:15:20.356 App[2609:907] loc: {584.0,710.0}, lastPinchLoc: {584.0,709.0}
2012-10-16 10:15:20.374 App[2609:907] loc: {584.0,710.0}, lastPinchLoc: {584.0,710.0}
2012-10-16 10:15:20.407 App[2609:907] loc: {583.0,710.0}, lastPinchLoc: {584.0,710.0}
2012-10-16 10:15:20.456 App[2609:907] loc: {512.0,811.0}, lastPinchLoc: {583.0,710.0}
2012-10-16 10:15:21.056 App[2609:907] loc: {645.0,163.0}, lastPinchLoc: {512.0,811.0}
2012-10-16 10:15:21.090 App[2609:907] loc: {647.0,164.0}, lastPinchLoc: {645.0,163.0}
2012-10-16 10:15:21.107 App[2609:907] loc: {647.0,165.0}, lastPinchLoc: {647.0,164.0}
2012-10-16 10:15:21.156 App[2609:907] loc: {602.0,249.0}, lastPinchLoc: {647.0,165.0}
2012-10-16 10:15:21.839 App[2609:907] loc: {153.0,702.0}, lastPinchLoc: {602.0,249.0}
2012-10-16 10:15:21.856 App[2609:907] loc: {157.0,703.0}, lastPinchLoc: {153.0,702.0}
2012-10-16 10:15:21.874 App[2609:907] loc: {159.0,704.0}, lastPinchLoc: {157.0,703.0}
2012-10-16 10:15:21.890 App[2609:907] loc: {160.0,704.0}, lastPinchLoc: {159.0,704.0}
2012-10-16 10:15:21.906 App[2609:907] loc: {123.0,796.0}, lastPinchLoc: {160.0,704.0}
2012-10-16 10:15:21.923 App[2609:907] loc: {125.0,794.0}, lastPinchLoc: {123.0,796.0}
2012-10-16 10:15:22.706 App[2609:907] loc: {368.0,303.0}, lastPinchLoc: {125.0,794.0}
2012-10-16 10:15:22.723 App[2609:907] loc: {369.0,307.0}, lastPinchLoc: {368.0,303.0}
2012-10-16 10:15:22.725 App[2609:907] loc: {369.0,307.0}, lastPinchLoc: {369.0,307.0}
2012-10-16 10:15:22.739 App[2609:907] loc: {371.0,312.0}, lastPinchLoc: {369.0,307.0}
2012-10-16 10:15:22.756 App[2609:907] loc: {374.0,312.0}, lastPinchLoc: {371.0,312.0}
2012-10-16 10:15:22.773 App[2609:907] loc: {338.0,392.0}, lastPinchLoc: {374.0,312.0}
Just before drawing an item on the CGContext the x and y are put into this method and i draw the item on the returned location. The class variable ‘zoomTouchLocation’ has the same value as ‘lastPinchLocation’ in the above snippet.
[code snippet 2]
- (CGPoint)handleCoordinatesX:(float)x y:(float)y {
x -= zoomTouchLocation.x;
y -= zoomTouchLocation.y;
x = x * zoomLevel;
y = y * zoomLevel;
x += zoomTouchLocation.x;
y += zoomTouchLocation.y;
return CGPointMake(x, y);
}
If i pinch in roughly the same location each time it will work as intended, but when i pinch all over the screen the image jumps around like crazy.
Now basically my question is if anyone can help me with this ‘jumping around’ problem and get the image to scale from the location i zoomed at without jumping around. If you need any more information just ask in the comments below.
(and i do know it’s quite normal that the jumping occurs with my code as it is, i just can’t find the way to stop it from doing that)
It’s hard to help without the whole code but you should try to set the view.layer.anchorPoint to your pinchLocation (if it’s not already done).
Like this, when the transformation is done, this point doesn’t move. Really useful for rotation or scaling.