iOS 核心图形：仅绘制 CGPath 的阴影

发布于 2024-12-16 17:28:19 字数 1373 浏览 5 评论 0原文

我正在 iOS 5 中使用 Core Graphics 绘制一条简单的路径：

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   );

现在我想以叠加模式填充它，如下所示：

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

这给了我完全预期的结果。但接下来，我想创建浮雕效果。我想到使用白色和黑色阴影来实现这种效果，如下所示：

[[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，不会绘制阴影。
现在的问题是：有没有一种方法可以仅绘制没有填充颜色但完整 Alpha 的阴影？我可以以某种方式阻止我的路径内部被绘制吗？或者是否有一种更简单的方法为一条路径绘制两个阴影？

原文

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   );

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);

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);

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?

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安人多梦 2024-12-23 17:28:19

我建议您将上下文的剪切路径设置为形状路径的逆向，配置阴影，并以完全不透明度正常填充形状。剪切路径将遮盖填充颜色，仅保留阴影。

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 和一个附加的矩形子路径将剪切区域设置为原始路径未覆盖的区域。这适用于任何不自相交的路径。

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);

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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