Touchpoint coordinates in iOS are not mapped properly - ios

Question

I'm trying create an application that allows the user to move a frame over an image, so that I can apply some effects on a selected region.
I need to allow the user to precisely drag and scale the masked-frame on the image. I need this to be exact, just like any other photo app does.
My strategy is to get the touch points of the user, on a touch-moved event, and scale my frame accordingly. That was pretty intuitive. I coded the following stuff for handling the touch moved event :
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
CGPoint touchPoint = [touch locationInView:[self view]];
float currX = touchPoint.x;
float currY = touchPoint.y;
/*proceed with other operations on currX and currY,
which is coming out quite well*/
}
But the only problem is, the coordinates of the currX and currY variables are not quite where they are supposed to be. There is a parallax error, which keeps shifting from device to device. I also think the x and y coordinates gets swapped in the case of an iPad.
Could you please help me to figure out how to get the exact touch coordinates?
My background image is in one view (imageBG) and the masked frame is in a separate one (maskBG). I have tried out :
CGPoint touchPoint = [touch locationInView:[maskBG view]];
and
CGPoint touchPoint = [touch locationInView:[imageBG view]];
...but the same problem persists. I have also noticed the error on touch being worse on an iPad than on an iPhone or iPod.

Answer 1

image.center = [[[event allTouches] anyObject] locationInView:self.view];

Answer 2

Hi your issue is the image and the iPhone screen are not necessarily in same aspect ratio.Your touch point might not translate correctly to your actual image.
- (UIImage*) getCroppedImage {
CGRect rect = self.movingView.frame;
CGPoint a;
a.x=rect.origin.x-self.imageView.frame.origin.x;
a.y=rect.origin.y-self.imageView.frame.origin.y;
a.x=a.x*(self.imageView.image.size.width/self.imageView.frame.size.width);
a.y=a.y*(self.imageView.image.size.height/self.imageView.frame.size.height);
rect.origin=a;
rect.size.width=rect.size.width*(self.imageView.image.size.width/self.imageView.frame.size.width);
rect.size.height=rect.size.height*(self.imageView.image.size.height/self.imageView.frame.size.height);
UIGraphicsBeginImageContext(rect.size);
CGContextRef context = UIGraphicsGetCurrentContext();
// translated rectangle for drawing sub image
CGRect drawRect = CGRectMake(-rect.origin.x, -rect.origin.y, self.imageView.image.size.width, self.imageView.image.size.height);
// clip to the bounds of the image context
// not strictly necessary as it will get clipped anyway?
CGContextClipToRect(context, CGRectMake(0, 0, rect.size.width, rect.size.height));
// draw image
[self.imageView.image drawInRect:drawRect];
// grab image
UIImage* croppedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return croppedImage;
}
This is what i did to crop my moving view is the rect which i pass for cropping see how its being translated to reflect correctly on image.Make sure the image view on which the user sees image is aspectfit content mode.
Note:- I make the rect of image view fit the aspectFit image
use this to do it
- (CGSize)makeSize:(CGSize)originalSize fitInSize:(CGSize)boxSize
{
widthScale = 0;
heightScale = 0;
widthScale = boxSize.width/originalSize.width;
heightScale = boxSize.height/originalSize.height;
float scale = MIN(widthScale, heightScale);
CGSize newSize = CGSizeMake(originalSize.width * scale, originalSize.height * scale);
return newSize;
}

Answer 3

have you tried these:
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
CGPoint touchPoint = [touch locationInView:selectedImageView];
float currX = (touchPoint.x)/selectedImageView.frame.size.width;
float currY = (touchPoint.y)/selectedImageView.frame.size.height;
/*proceed with other operations on currX and currY,
which is coming out quite well*/
}
or you can also use UIPanGestureRecognizer..