How to Re draw an erased image with touches moved in iphone - iphone

Question

I am erasing an image with touch using the blend mode destination out. I did this successfully.
Actually I am reducing the alpha with touch so I am able to set the strength too.
Now my problem is about redrawing the erased part of image with touch (i.e I want to draw back the image with strength or want to set the alpha darker). For that I am having a backup of original image then I cropped the part of touch and merged it with image. But the problem is that it is drawing more than it should.
Note that Redraw procedure just darkens the image more than original when drawing overlaps (need to set a upper bound). So how can I avoid the redrawing at the point on which I have already drawn the image in order to avoid darkening of the original image.
I have also attached the code.
// Code to erase an image
UIGraphicsBeginImageContext(self._overlayImage.image.size);
CGRect rect =CGRectMake(0, 0, self._overlayImage.image.size.width, self._overlayImage.image.size.height) ;
CGContextRef context = UIGraphicsGetCurrentContext();
CGImageRef imageRef=self._overlayImage.image.CGImage;
if (imageRef) {
// Restore the screen that was previously saved
CGContextTranslateCTM(context, 0, rect.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextDrawImage(context, rect, imageRef);
//CGImageRelease(imageRef);
CGContextTranslateCTM(context, 0, rect.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
}
// Erase the background -- raise the alpha to clear more away with eash swipe
// [[UIImage imageNamed:#"eraser222.png"] drawAtPoint:point blendMode:kCGBlendModeDestinationOut alpha:.2];
[ [UIImage imageNamed:#"eraser222.png"] drawInRect:CGRectMake(newPoint.x-self.imgOrignal.size.width*2*radius/self._overlayImage.bounds.size.width, newPoint.y-self.imgOrignal.size.height*2*radius/self._overlayImage.bounds.size.height, self.imgOrignal.size.width*2*radius/self._overlayImage.bounds.size.width, self.imgOrignal.size.height*2*radius/self._overlayImage.bounds.size.height) blendMode:kCGBlendModeDestinationOut alpha:strength/3];
self._overlayImage.image=UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// code to draw an image
UIImage *cropped = [self imageByCropping:self.imgOrignal toRect:CGRectMake(newPoint.x-self.imgOrignal.size.width*2*radius/self._overlayImage.bounds.size.width, newPoint.y-self.imgOrignal.size.height*2*radius/self._overlayImage.bounds.size.height, self.imgOrignal.size.width*2*radius/self._overlayImage.bounds.size.width, self.imgOrignal.size.height*2*radius/self._overlayImage.bounds.size.height)];
UIGraphicsBeginImageContext(self._overlayImage.image.size);
CGRect rect =CGRectMake(0, 0, self._overlayImage.image.size.width, self._overlayImage.image.size.height) ;
CGContextRef context = UIGraphicsGetCurrentContext();
CGImageRef imageRef=self._overlayImage.image.CGImage;
if (imageRef) {
// Restore the screen that was previously saved
CGContextTranslateCTM(context, 0, rect.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextDrawImage(context, rect, imageRef);
//CGImageRelease(imageRef);
CGContextTranslateCTM(context, 0, rect.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
}
[ cropped drawInRect:CGRectMake(newPoint.x-self.imgOrignal.size.width*2*radius/self._overlayImage.bounds.size.width, newPoint.y-self.imgOrignal.size.height*2*radius/self._overlayImage.bounds.size.height, self.imgOrignal.size.width*2*radius/self._overlayImage.bounds.size.width, self.imgOrignal.size.height*2*radius/self._overlayImage.bounds.size.height) blendMode:kCGBlendModeNormal alpha:strength];
cropped= [UIImage imageWithData:UIImagePNGRepresentation(cropped)];
UIImage *finalimage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
self._overlayImage.image=finalimage;

Answer 1

I know it too late to ans here but I have reached one solution and I will like to share here as it can help someone in need. It was not possible to delete and redraw the same image. So I worked on two images. One that was original Image and second image was nil. Second image was used to draw path where user touches screen. Thenceforth created a new context , drew original image, then drew the path image with kCGBlendModeDestinationOut blend mode. It is kCGBlendModeDestinationOut the main hero of this method. Main task was accomplished by using the kCGBlendModeDestinationOut blend mode. Hence getting the required effect. Check my blog here.