iOS核心图形:仅绘制CGPath的阴影 [英] iOS Core Graphics: Draw ONLY shadows of a CGPath

问题描述

我正在使用Core Graphics在iOS 5中绘制一条简单的路径:

I am drawing a simple path in iOS 5 with Core Graphics:

CGMutablePathRef path = CGPathCreateMutable();
CGPathMoveToPoint(   path, NULL, center.x   , topMargin   );
CGPathAddLineToPoint(path, NULL, center.x+20, topMargin+50);
CGPathAddLineToPoint(path, NULL, center.x   , topMargin+40);
CGPathAddLineToPoint(path, NULL, center.x-20, topMargin+50);
CGPathAddLineToPoint(path, NULL, center.x   , topMargin   );

现在，我想像这样在叠加模式下填充它:

Now i want to fill it in Overlay mode like so:

[[UIColor colorWithRed:0 green:0 blue:0 alpha:0.4] setFill];
CGContextAddPath(context, path);
CGContextSetBlendMode (context, kCGBlendModeOverlay);
CGContextFillPath(context);

这给了我确切的预期结果.但是接下来，我想创建一个浮雕效果.我想到了使用白色和黑色的阴影来实现这种效果，就像这样:

Which gives me exactly the expected result. But next, i want to create an embossed effect. I thought of using a white and a black drop shadow in order to achieve this effect like so:

[[UIColor colorWithRed:0 green:0 blue:0 alpha:0] setFill];
CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(1, 1), 1.0, highlightColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);

[[UIColor colorWithRed:0 green:0 blue:0 alpha:0] setFill];
CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(-1, -1), 1.0, shadowColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);

问题是，当alpha设置为0时，没有绘制阴影.
现在的问题是:是否有一种方法可以只绘制 阴影而不填充颜色，但以完整的alpha绘制?我能以某种方式阻止绘制路径的内部吗?还是有一种更简单的方法为一条路径绘制两个阴影?

The problem is, the shadows are not drawn when alpha is set to 0.
Now the question: Is there a way to draw only the shadows without the fill color but in full alpha? Can I somehow prevent the inside of my path from being drawn? Or is there perhaps a simpler way of drawing two shadows for one path?

推荐答案

我建议您将上下文的剪切路径设置为形状路径的倒数，配置阴影，并以完全不透明的方式正常填充形状.剪切路径将掩盖填充颜色，仅阴影将保留.

I suggest you set the context's clipping path to the inverse of the shape's path, configure the shadow, and fill the shape normally, with full opacity. The clipping path will mask out the fill color, and only the shadow would remain.

CGContextSaveGState(context);
CGRect boundingRect = CGContextGetClipBoundingBox(context);
CGContextAddRect(context, boundingRect);
CGContextAddPath(context, path);
CGContextEOClip(context);

[[UIColor blackColor] setFill];
CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(1, 1), 1.0, highlightColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);

CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(-1, -1), 1.0, shadowColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);

CGContextRestoreGState(context);

诀窍是使用CGContextEOClip和附加的矩形子路径将裁剪区域设置为原始路径未覆盖的任何区域.这将适用于任何非自相交的路径.

The trick is using CGContextEOClip and an additional rectangle subpath to set the clipping area to whatever is not covered by the original path. This will work for any path that is not self-intersecting.

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