Iphone 如何使用cgcontext绘制nil

我想画

nil

而不是颜色，因为最后我想让代码删除

UIImageView

的一部分，以便您可以看到

UIImageView

后面的内容

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event 
{
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.view];
    currentPoint.x = currentPoint.x;
    currentPoint.y = currentPoint.y;
    mouseSwiped = YES;
    UIGraphicsBeginImageContext(self.view.frame.size);
    [drawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
    CGContextSetShouldAntialias(UIGraphicsGetCurrentContext(), YES);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), brushSizeVal);
    CGContextSetAlpha(UIGraphicsGetCurrentContext(), drawingColorAlpha);
    CGContextSaveGState(UIGraphicsGetCurrentContext());    //Probably right here
    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 
            drawingColorRed, // I want to draw nil here instead of these CGFloats that I have made
            drawingColorGreen, // I want to draw nil here instead of these CGFloats that I have made
            drawingColorBlue, // I want to draw nil here instead of these CGFloats that I have made
            drawingColorAlpha); // I want to draw nil here instead of these CGFloats that I have made
    CGContextBeginPath(UIGraphicsGetCurrentContext());
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(),
                                          lastPoint.x,
                                          lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), 
                                           currentPoint.x,
                                           currentPoint.y);
    CGContextStrokePath(UIGraphicsGetCurrentContext());
    drawImage.image = UIGraphicsGetImageFromCurrentImageContext(); //drawImage is the UIImageView that I am drawing my context to
    NSLog(@"current point x: %d current point y: %d",currentPoint.x, currentPoint.y); //Used for my purposes
    lastPoint = currentPoint;
    mouseMoved++;

及

---

你想要的不是零，而是一种透明的颜色。如果UIImageView在IB中未标记为“不透明”，则它应在像素透明的任何区域显示。

您要做的是将

drawingColorAlpha

设置为0。这将创建一种透明颜色，相当于擦除。您可能还必须使用

CGContextSetBlendMode（）

设置混合模式<代码>kCGBlendModeNormal应该可以正常工作。

科林·吉斯拉森的答案不正确。如果将

drawingColorAlpha

设置为0，它只会在现有图像上绘制一种不可见的颜色

以下是有效的方法

//set this at the beginning
CGContextSetLineCap(ctx,kCGImageAlphaNone);
//after moving the point to your pen location:
CGContextClearRect (ctx, CGRectMake(lastPoint.x, lastPoint.y, 10, 10)); 

按如下方式设置混合模式： CGContextSetBlendMode（UIGraphicsGetCurrentContext（），kCGBlendModeClear）

---

希望它能帮助你！！：P

不，实际上我想让它删除UIImageView的一部分我想你没有理解我说的话。如果在图像视图中绘制透明像素，则其后面的任何内容都将显示出来。

//set this at the beginning
CGContextSetLineCap(ctx,kCGImageAlphaNone);
//after moving the point to your pen location:
CGContextClearRect (ctx, CGRectMake(lastPoint.x, lastPoint.y, 10, 10));