CODEWITHSUNDEEP
iphoneiosimage-processinguiimageviewhittest
hpique
hpique

Reputation: 120324

Detect touches only on non-transparent pixels of UIImageView, efficiently

How would you detect touches only on non-transparent pixels of a UIImageView, efficiently?

Consider an image like the one below, displayed with UIImageView. The goal is be to make the gesture recognisers respond only when the touch happens in the non-transparent (black in this case) area of the image.

enter image description here

Ideas

Bonus

Upvotes: 28

Views: 10396

Answers (3)

Adrian Demetrescu
Adrian Demetrescu

Reputation: 168

Well, if you need to do it really fast, you need to precalculate the mask.

Here's how to extract it:

UIImage *image = [UIImage imageNamed:@"some_image.png"];
NSData *data = (NSData *) CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage));
unsigned char *pixels = (unsigned char *)[data bytes];
BOOL *mask = (BOOL *)malloc(data.length);
for (int i = 0; i < data.length; i += 4) {
  mask[i >> 2] = pixels[i + 3] == 0xFF; // alpha, I hope
}
// TODO: save mask somewhere

Or you could use the 1x1 bitmap context solution to precalculate the mask. Having a mask means you can check any point with the cost of one indexed memory access.

As for checking a bigger area than one pixel - I would check pixels on a circle with the center in the touch point. About 16 points on the circle should be enough.

Detecting also inner pixels: another precalculation step - you need to find the convex hull of the mask. You can do that using the "Graham scan" algorithm http://softsurfer.com/Archive/algorithm_0109/algorithm_0109.htm Then either fill that area in the mask, or save the polygon and use a point-in-polygon test instead.

And finally, if the image has a transform, you need to convert the point coordinates from screen space to image space, and then you can just check the precalculated mask.

Upvotes: 3

OC Rickard
OC Rickard

Reputation: 1077

Here's my quick implementation: (based on Retrieving a pixel alpha value for a UIImage)

- (BOOL)pointInside:(CGPoint)point withEvent:(UIEvent *)event {
    //Using code from https://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage

    unsigned char pixel[1] = {0};
    CGContextRef context = CGBitmapContextCreate(pixel,
                                                 1, 1, 8, 1, NULL,
                                                 kCGImageAlphaOnly);
    UIGraphicsPushContext(context);
    [image drawAtPoint:CGPointMake(-point.x, -point.y)];
    UIGraphicsPopContext();
    CGContextRelease(context);
    CGFloat alpha = pixel[0]/255.0f;
    BOOL transparent = alpha < 0.01f;

    return !transparent;
}

This assumes that the image is in the same coordinate space as the point. If scaling goes on, you may have to convert the point before checking the pixel data.

Appears to work pretty quickly to me. I was measuring approx. 0.1-0.4 ms for this method call. It doesn't do the interior space, and is probably not optimal.

Upvotes: 20

sergio
sergio

Reputation: 69027

On github, you can find a project by Ole Begemann which extends UIButton so that it only detects touches where the button's image is not transparent.

Since UIButton is a subclass of UIView, adapting it to UIImageView should be straightforward.

Hope this helps.

Upvotes: 6

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