仅有效地检测对 UIImageView 的非透明像素的触摸 [英] Detect touches only on non-transparent pixels of UIImageView, efficiently

问题描述

如何仅有效地检测 UIImageView 的非透明像素上的触摸?

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

考虑如下图，用 UIImageView 显示.目标是让手势识别器仅在触摸发生在图像的非透明(本例中为黑色)区域时做出响应.

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.

• 覆盖 hitTest:withEvent: 或 pointInside:withEvent:，尽管这种方法可能非常糟糕效率低下，因为这些方法在触摸事件期间被多次调用.
• 检查单个像素是否透明可能会产生意想不到的结果，因为手指大于一个像素.检查命中点周围的圆形像素区域，或尝试找到通向边缘的透明路径可能会更好.

• 最好区分图像的外部和内部透明像素.在示例中，零内的透明像素也应视为有效.
• 如果图像有变换会发生什么?
• 图像处理可以硬件加速吗?

推荐答案

这是我的快速实现:(基于 获取 UIImage 的像素 alpha 值)

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;
}

这假设图像与 point 位于相同的坐标空间中.如果继续缩放，您可能需要在检查像素数据之前转换 point.

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.

对我来说似乎工作得很快.我正在测量大约.此方法调用需要 0.1-0.4 毫秒.它不做内部空间，并且可能不是最佳的.

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.

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