如何在iPhone上绘制透明笔划（或任何清晰的图像部分）

Question

我有一个小应用程序，允许用户用手指在屏幕上绘图。 我有一个 UIImageView ，用户通过创建 CGContextRef 和各种 CG draw 函数来绘制。我主要用函数 CGContextAddLineToPoint

绘制笔画/线条

现在我的问题是： 用户可以绘制各种颜色的线条。我想让他能够使用“橡胶”工具用手指删除到目前为止绘制的图像的某些部分。我最初通过使用白色作为笔划（使用 CGContextSetRGBStrokeColor 函数设置）来做到这一点，但它没有用完...因为我后来发现 UIImage 在 UIImageView 上实际上有一个透明的背景，而不是白色...所以我最终会得到一个带有白线的透明图像！

无论如何设置＆quot; transparent＆quot; 描边颜色，还是有任何其他方法可以清除用户手指下的 CGContextRef 的内容，当他移动时它？ 感谢

Answer 1

这样可以解决问题：

CGContextSetBlendMode(context, kCGBlendModeClear)

Answer 2

我最终使用了Bresenham的线算法（当我不得不编写自己的图形例程时，回到过去的日子）......

- (void) contextEraseLine:(CGContextRef) ctx from:(CGPoint)startPoint to:(CGPoint) endPoint withThickness:(int)thickness {
    int x, cx, deltax, xstep,
    y, cy, deltay, ystep,
    error, st, dupe;

    int x0, y0, x1, y1;

    x0 = startPoint.x;
    y0 = startPoint.y;
    x1 = endPoint.x;
    y1 = endPoint.y;

    // find largest delta for pixel steps
    st = (abs(y1 - y0) > abs(x1 - x0));

    // if deltay > deltax then swap x,y
    if (st) {
        (x0 ^= y0); (y0 ^= x0); (x0 ^= y0); // swap(x0, y0);
        (x1 ^= y1); (y1 ^= x1); (x1 ^= y1); // swap(x1, y1);
    }

    deltax = abs(x1 - x0);
    deltay = abs(y1 - y0);
    error  = (deltax / 2);
    y = y0;

    if (x0 > x1) { xstep = -1; }
    else         { xstep =  1; }

    if (y0 > y1) { ystep = -1; }
    else         { ystep =  1; }

    for ((x = x0); (x != (x1 + xstep)); (x += xstep))
    {
        (cx = x); (cy = y); // copy of x, copy of y

        // if x,y swapped above, swap them back now
        if (st) { (cx ^= cy); (cy ^= cx); (cx ^= cy); }

        (dupe = 0); // initialize no dupe

        if(!dupe) { // if not a dupe, write it out
            //NSLog(@"(%2d, %2d)", cx, cy);

            CGContextClearRect(ctx, CGRectMake(cx, cy, thickness, thickness));

    }

        (error -= deltay); // converge toward end of line

        if (error < 0) { // not done yet
            (y += ystep);
            (error += deltax);
        }
    }
}

唷！要创建（有点）笨重的橡皮擦线还有很长的路要走。

要使用它，请执行以下操作：

- (void)eraseStart {
    // erase lines
    UIGraphicsBeginImageContext(drawingBoard.size);
    ctx = UIGraphicsGetCurrentContext();
    CGContextDrawImage(ctx,CGRectMake(0,0,drawingBoard.size.width, drawingBoard.size.height),[drawingBoard CGImage]); 
}

- (void)eraseEnd {
    drawingBoard = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    [drawingView removeFromSuperview];
    [drawingView release];

    drawingView = [[UIImageView alloc] initWithImage:drawingBoard];
    drawingView.frame = CGRectMake(intEtchX, intEtchY, intEtchWidth, intEtchHeight);

    [self.view addSubview:drawingView];
}

这假设您已经创建了一个drawingView（UIImageView）和drawingBoard（UIImage）。

然后，要删除一行，只需执行以下操作：

CGContextRef ctx = UIGraphicsGetCurrentContext();
[self eraseStart];
[self contextEraseLine:ctx from:CGPointMake (x1, y1) to:CGPointMake (x2, y2) withThickness:10];
[self eraseEnd];

（用适当的值替换x1，y1，x2和y2）......

Answer 3

我尝试过使用：

CGContextSetStrokeColorWithColor (myContext, [[UIColor clearColor] CGColor]);

但这不起作用，因为它似乎是“绘图”。在上下文的顶部有一个不可见的颜色（和不可见的颜色+它绘制的任何颜色=它绘制的颜色）。

我找到的唯一解决方案（不是最优的）是：

CGContextClearRect (myContext, CGRectMake(x, y, width, height));

不幸的是，这意味着您必须追踪一系列的行并自己生成行...

Answer 4

如果你想用触摸擦除图像，这是我的代码......不建议使用的是去除部分，但它有效但稍慢。

func addNewPathToImage(){
    print("Start erasing or not erasing")

    UIGraphicsBeginImageContextWithOptions(TopImageUIImageView.bounds.size, false, UIScreen.main.scale) // or 0 for the scale

//My image is aspect fit so I need to get rect

    let aspect = TopImageUIImageView.image!.size.width / TopImageUIImageView.image!.size.height
    let viewRatio = TopImageUIImageView.bounds.size.width / TopImageUIImageView.bounds.size.height


    if aspect < viewRatio {
        let scale = TopImageUIImageView.bounds.size.height / TopImageUIImageView.image!.size.height
        let width = scale * TopImageUIImageView.image!.size.width
        let topLeftX = (TopImageUIImageView.bounds.size.width - width) * 0.5
        rect = CGRect(x: topLeftX, y: 0, width: width, height: TopImageUIImageView.bounds.size.height)
    }
    else {
        let scale = TopImageUIImageView.bounds.size.width / TopImageUIImageView.image!.size.width
        let height = scale * TopImageUIImageView.image!.size.height
        let topLeftY = (TopImageUIImageView.bounds.size.height - height) * 0.5
        rect = CGRect(x: 0.0, y: topLeftY, width: TopImageUIImageView.bounds.size.width, height: height)
    }
    ////
    context = UIGraphicsGetCurrentContext()
    TopImageUIImageView.image?.draw(in: rect)
    context?.setLineCap(.round)
    context?.setLineWidth(brushSize)



    if isErasing == true {

        context?.setShadow(offset: CGSize(width: 0, height: 0), blur: blurNumber)
        context?.setStrokeColor(UIColor.white.cgColor)
        context?.setBlendMode(.clear)


    }else{

        print("test the unerase image .... ?")
        context?.setStrokeColor(UIColor.init(patternImage: topImage.af_imageAspectScaled(toFit: CGSize(width: TopImageUIImageView.bounds.size.width, height: TopImageUIImageView.bounds.size.height))).cgColor)

    }


   // I am using these because I am using touch to define what to erase 
        context?.move(to: CGPoint(x: lastTouch.x, y: lastTouch.y))
        context?.addLine(to: CGPoint(x: currentTouch.x, y: currentTouch.y))
        context?.strokePath()
        context?.closePath() // when add this line or the "context?.beginPath" get ERROR CGContextClosePath: no current point.
        print("close the path")




        //get your image 
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        TopImageUIImageView.image = image

}

Answer 5

//erase part
if(mouseSwiped)
{

//**************Working Code*************// 


UIGraphicsBeginImageContext(frontImage.frame.size);
[frontImage.image drawInRect:CGRectMake(0, 0, frontImage.frame.size.width, frontImage.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(),kCGImageAlphaNone); //kCGImageAlphaPremultipliedLast);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 10);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1, 0, 0, 10);
CGContextBeginPath(UIGraphicsGetCurrentContext());
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextClearRect (UIGraphicsGetCurrentContext(), CGRectMake(lastPoint.x, lastPoint.y, 10, 10));
CGContextStrokePath(UIGraphicsGetCurrentContext());
frontImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();


lastPoint = currentPoint;

mouseMoved++;

if (mouseMoved == 10) 
{
    mouseMoved = 0;
}
}

Answer 6

UIColor *clearColor = [UIColor clearColor];