ios - 如何使用 CoreGraphics 绘制一条边缘逐渐变化的线？
<p><p>我想完成一个功能，就像一个画笔。手指滑动区域变为透明，边框逐渐变化。</p>
<p> <a href="/image/FGcwX.jpg" rel="noreferrer noopener nofollow"><img src="/image/FGcwX.jpg" alt="enter image description here"/></a> <br/>
我现在只能使用以下代码将颜色更改为晶莹剔透:</p>
<pre><code>-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
if(self.eraser) return;
CGFloat scale = self.transform.a;
if (scale < 1) scale = 1;
CGPoint p = [ locationInView: self];
CGPoint q = [ previousLocationInView: self];
UIImage* image;
image = self.image;
CGSizesize = self.frame.size;
UIGraphicsBeginImageContext(size);
CGRectrect;
rect.origin = CGPointZero;
rect.size = size;
;
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineCap(context, kCGLineCapRound);
CGContextBeginPath(context);
CGContextSaveGState( context );
CGContextSetLineWidth(context, (10.0 / scale) + 1);
CGContextSetBlendMode(context, kCGBlendModeClear);
CGContextMoveToPoint(context, q.x, q.y);
CGContextAddLineToPoint(context, p.x, p.y);
CGContextStrokePath(context);
CGContextRestoreGState( context );
UIImage* editedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
;
;
</code></pre>
<p>}</p>
<p>我怎样才能通过逐渐变化获得优势？提前致谢。</p></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><p>您可以通过在用户通过的每个点以 <code>kCGBlendModeDestinationIn</code> 模式绘制具有可变 alpha 的径向渐变来实现此效果。</p>
<p>此混合模式的效果是仅将图层的 Alpha 应用于下面的图层。通过我们渐变的可变 alpha，我们可以实现这个效果。</p>
<pre><code>const CGFloat kBrushSize = 10.f;
CGContextSaveGState(context);
// Make a radial gradient that goes from transparent black on the inside
// to opaque back on the outside.
size_t num_locations = 2;
CGFloat locations = { 0.0, 1.0 };
CGFloat components = { 1.0, 1.0, 1.0, 0.0,
1.0, 1.0, 1.0, 1.0 };
CGColorSpaceRef myColorspace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
CGGradientRef myGradient = CGGradientCreateWithColorComponents (myColorspace, components,
locations, num_locations);
CGColorSpaceRelease(myColorspace);
// Draw the gradient at the point using kCGBlendModeDestinationIn
// This mode only applies the new layer's alpha to the lower layer.
CGContextSetBlendMode(context, kCGBlendModeDestinationIn);
CGContextDrawRadialGradient(context, myGradient, p, 0.f, p, (kBrushSize / scale) + 1, kCGGradientDrawsAfterEndLocation);
CGGradientRelease(myGradient);
CGContextRestoreGState(context);
</code></pre>
<p>下面是这段代码的截图:</p>
<p> <a href="/image/u1rrV.png" rel="noreferrer noopener nofollow"><img src="/image/u1rrV.png" alt="CGBrush Scribble"/></a> </p>
<p>注意:使用这种技术，如果用户快速移动他/她的手指，您可能会看到离散的画笔点可见的间距效果。这是某些绘图软件的功能，但如果您不希望这样做，您可以添加代码以在当前和上一个之间插入点以绘制更多画笔点，从而创建更连续的笔触。</p>
<p>此外，您应该能够调整渐变色标以实现您喜欢的任何类型的笔刷柔和度。</p>
<p>来源:<a href="https://developer.apple.com/library/mac/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_shadings/dq_shadings.html" rel="noreferrer noopener nofollow">https://developer.apple.com/library/mac/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_shadings/dq_shadings.html</a> </p></p>
<p style="font-size: 20px;">关于ios - 如何使用 CoreGraphics 绘制一条边缘逐渐变化的线？，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/36591545/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/36591545/
</a>
</p>
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