cropping an area CGRect from UIGraphicsImageContext - ios

Question

I have a CGPath shaped like an arrow that I am drawing in the CGContext of my current view. I would like to generate a miniature version (thumbnail) of the arrow to add it as an Image to a UITableView showing all selected arrows.
I am succeeding to downscale a picture of the full context which leaves the arrow smaller than it should be. Ideally I would like to crop the image of the full context to the bounds of the arrow. However, I was not yet successful. Any leads? Thanks for the help!
Here are a picture of the full view containing an arrow and another picture of the thumbnail I am generating.
Ideally the thumbnail above would be cropped to contain the arrow only - not the full context.
The code I use is the follwoing:
- (UIImage*) imageForObject:(id<GraphicalObject>) object
inRect:(CGRect)rect {
UIImage *image = [UIImage new];
CGRect objectBounds = [object objectBounds];
UIGraphicsBeginImageContext(self.view.frame.size);//objectBounds.size);
CGContextRef context =UIGraphicsGetCurrentContext();
[object drawInContext:context];
//doesn't work
CGContextClipToRect(context, objectBounds);
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
}

Answer 1

The CGRect called objectBounds has two components, an origin and a size. In order the draw the object correctly as a thumbnail, the code needs to scale the image (to get the size right) and translate the image (to move the origin to {0,0}). So the code looks like this
- (UIImage *)getThumbnailOfSize:(CGSize)size forObject:(UIBezierPath *)object
{
// to maintain the aspect ratio, we need to compute the scale
// factors for x and y, and then use the smaller of the two
CGFloat xscale = size.width / object.bounds.size.width;
CGFloat yscale = size.height / object.bounds.size.height;
CGFloat scale = (xscale < yscale) ? xscale : yscale;
// start a graphics context with the thumbnail size
UIGraphicsBeginImageContext( size );
CGContextRef context = UIGraphicsGetCurrentContext();
// here's where we scale and translate to make the image fit
CGContextScaleCTM( context, scale, scale );
CGContextTranslateCTM( context, -object.bounds.origin.x, -object.bounds.origin.y );
// draw the object and get the resulting image
[object stroke];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
}