如何在CGBitmapContext中实现软橡皮擦笔划 [英] How do I implement a soft eraser stroke in a CGBitmapContext

问题描述

我想在iOS中使用我的触控功能擦除图片。通过设置混合模式为kCGBlendModeClear我可以做到这一点 - 但只有硬边缘。我可以绘制我的笔画与不同的线宽和alphas，但它似乎CGContextSetAlpha没有影响与kCGBlendModeClear。

如何解决这个问题？

解决方案

透明层与 kCGBlendModeDestinationOut （ Da *（1-Sa），Dc *（1-Sa）类似这样的：

  CGPathRef pathToErase = ...; //你想擦除的路径
 //也可以是一个图像或（几乎）任何其他的
 //可以在位图上下文中绘制
 
 CGContextSetBlendMode（ctx， kCGBlendModeDestinationOut）; 
 CGContextBeginTransparencyLayer（ctx，NULL）; 
 {
 CGContextSetGrayFillColor（ctx，0.0，1.0）; // solid black 
 
 CGContextAddPath（ctx，pathToErase）; 
 CGContextFillPath（ctx）; 
 //上面两行可以改为CGContextDrawImage（）
 //或任何其他你用来清除
} 
 CGContextEndTransparencyLayer（ctx）;请注意，您还应该保存和恢复gstate（ CGContextSaveGState（）
 / code> /  /  CGContextRestoreGState（）），以确保混合模式和任何其他gstate更改不会持久。 b 
 $ b 
 注意：这是大脑编译的，透明层可能不会在所有混合模式下播放。如果是，请尝试将路径/图片绘制到第二个位图上下文中，然后使用上述混合模式绘制该上下文的内容。
 
 
 
您也可以尝试其他混合不同效果的模式。
 
I am trying to erase an image with my touch in iOS.  By setting the blend mode to kCGBlendModeClear I am able to do this - but only with hard edges.  I could draw my stroke with varying line widths and alphas, but it appears that CGContextSetAlpha does not have an effect with kCGBlendModeClear.  

How do I go about this?
解决方案
I would use a transparency layer compositing with kCGBlendModeDestinationOut (Da * (1 - Sa), Dc * (1 - Sa).) Something like this:
CGPathRef pathToErase = ...; // The path you want erased
// could also be an image or (nearly) anything else
// that can be drawn in a bitmap context

CGContextSetBlendMode(ctx, kCGBlendModeDestinationOut);
CGContextBeginTransparencyLayer(ctx, NULL);
{
    CGContextSetGrayFillColor(ctx, 0.0, 1.0); // solid black

    CGContextAddPath(ctx, pathToErase);
    CGContextFillPath(ctx);
    // the above two lines could instead be CGContextDrawImage()
    // or whatever else you're using to clear
}
CGContextEndTransparencyLayer(ctx);
Note that you should also save and restore the gstate (CGContextSaveGState()/CGContextRestoreGState()) before/after the transparency layer to ensure that the blend mode and any other gstate changes do not persist.

Note: This is brain-compiled and it's possible that transparency layers don't play nice with all blend modes. If so, try drawing the path/image into a second bitmap context, then drawing the contents of that context with the above blend mode.

You can also experiment with other blend modes for different effects.

                        
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