Adding an Image to a Current UIGraphics Context

Question

I've got a slideshow that allows users to annotate slides with a simple drawing tool. Just allows you to draw on the screen with your finger and then 'save'. The save feature uses UIImagePNGRepresentation and works rather well. What I need to work out is how to 'continue' existing annotations so when a save happens it also takes into account what is already on the slide.

It works using UIImageContext and saving that image context into a file. When an image is saved it opens onto an overlay UIImageView, so if you are 'continuing' then your drawing onto an existing png file.

Is there a way I can add an existing image to the UIImageContext? Here I control the addition of the lines upon movement:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    if(drawToggle){
        UITouch *touch = [touches anyObject];   
        CGPoint currentPoint = [touch locationInView:self.view];
        currentPoint.y -= 40;

        //Define Properties
        [drawView.image drawInRect:CGRectMake(0, 0, drawView.frame.size.width, drawView.frame.size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineJoin(UIGraphicsGetCurrentContext(), kCGLineJoinBevel);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0);
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0, 0.0, 0.0, 1.0);
        //Start Path
        CGContextBeginPath(UIGraphicsGetCurrentContext());
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        //Save Path to Image
        drawView.image = UIGraphicsGetImageFromCurrentImageContext();

        lastPoint = currentPoint;
    }
}

Here is the magic saving line:

NSData *saveDrawData = UIImagePNGRepresentation(UIGraphicsGetImageFromCurrentImageContext());
NSError *error = nil;
[saveDrawData writeToFile:dataFilePath options:NSDataWritingAtomic error:&error];

Thanks for any help you can offer.

UPDATE:

Oops I forgot to add, when an annotation is 'saved' the Image Context is ended, so I can't use any Get Current Image Context style methods.

Answer 1

I achieved this by adding this between my Start and End lines:

UIImage *image = [[UIImage alloc] initWithContentsOfFile:saveFilePath];
CGRect imageRect = CGRectMake(0, 0, image.size.width, image.size.height);       
CGContextTranslateCTM(UIGraphicsGetCurrentContext(), 0, image.size.height);
CGContextScaleCTM(UIGraphicsGetCurrentContext(), 1.0, -1.0);
CGContextDrawImage(UIGraphicsGetCurrentContext(), imageRect, image.CGImage);

The Context Translate and Scales are necessary because converting a UIImage to a CGImage flips the image - it does this because a CGImage draws from a bottom left origin and the UIImage draws from a top left origin, the same co-ordinates on a flipped scale results in a flipped image.

Because I drew the existing image into the UIGraphicsGetCurrentContext() when I save the file it'll take this into account.