如何使用CAGradientLayer绘制渐变色轮？

Question

我从这些链接中得到了一些参考-

What is the Algorithm behind a color wheel?
Math behind the Colour Wheel
Basic color schemes

我把"HSV colour space"的概念讲完了。但是我想在 CAGradientLayer.

的帮助下使用 RGB 绘制色轮

这是使用简单的 RGB 颜色数组和 UIBezierPath -

制作色轮的代码片段

func drawColorWheel()
{
    context?.saveGState()

    range = CGFloat(100.00 / CGFloat(colorArray.count))

    for k in 0 ..< colorArray.count
    {

        drawSlice(startPercent: CGFloat(k) * range, endPercent: CGFloat(CGFloat(k + 1) * range), color: colorArray.object(at: k) as! UIColor)
    }

    context?.restoreGState()
}

func drawSlice(startPercent: CGFloat, endPercent: CGFloat, color: UIColor)
{        
    let startAngle = getAngleAt(percentage: startPercent)
    let endAngle = getAngleAt(percentage: endPercent)

    let path =  getArcPath(startAngle: startAngle, endAngle: endAngle)
    color.setFill()
    path.fill()
}

其中getAngleAt()和getArcPath()是绘制带角度路径的私有函数

这是我的代码的最终输出 -

现在，我的问题是如何为这些颜色赋予渐变效果，以便每种颜色与渐变色层混合？

Answer 1

一种方法是构建图像并手动操作像素缓冲区：

创建特定大小和特定类型的 CGContext；
通过 data 属性;
将其重新绑定到可以轻松操作该缓冲区的东西（我使用 Pixel、struct 表示像素的 32 位）；
逐一循环像素，将其转换为此图像中圆的 angle 和 radius；和
如果在圆圈内，则创建一个具有适当颜色的像素；如果不是，则将其设为零阿尔法像素。

所以在 Swift 3:

func buildHueCircle(in rect: CGRect, radius: CGFloat, scale: CGFloat = UIScreen.main.scale) -> UIImage? {
    let width = Int(rect.size.width * scale)
    let height = Int(rect.size.height * scale)
    let center = CGPoint(x: width / 2, y: height / 2)

    let space = CGColorSpaceCreateDeviceRGB()
    let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: 8, bytesPerRow: width * 4, space: space, bitmapInfo: Pixel.bitmapInfo)!

    let buffer = context.data!

    let pixels = buffer.bindMemory(to: Pixel.self, capacity: width * height)
    var pixel: Pixel
    for y in 0 ..< height {
        for x in 0 ..< width {
            let angle = fmod(atan2(CGFloat(x) - center.x, CGFloat(y) - center.y) + 2 * .pi, 2 * .pi)
            let distance = hypot(CGFloat(x) - center.x, CGFloat(y) - center.y)

            let value = UIColor(hue: angle / 2 / .pi, saturation: 1, brightness: 1, alpha: 1)

            var red: CGFloat = 0
            var green: CGFloat = 0
            var blue: CGFloat = 0
            var alpha: CGFloat = 0
            value.getRed(&red, green: &green, blue: &blue, alpha: &alpha)

            if distance <= (radius * scale) {
                pixel = Pixel(red:   UInt8(red * 255),
                              green: UInt8(green * 255),
                              blue:  UInt8(blue * 255),
                              alpha: UInt8(alpha * 255))
            } else {
                pixel = Pixel(red: 255, green: 255, blue: 255, alpha: 0)
            }
            pixels[y * width + x] = pixel
        }
    }

    let cgImage = context.makeImage()!
    return UIImage(cgImage: cgImage, scale: scale, orientation: .up)
}

在哪里

struct Pixel: Equatable {
    private var rgba: UInt32

    var red: UInt8 {
        return UInt8((rgba >> 24) & 255)
    }

    var green: UInt8 {
        return UInt8((rgba >> 16) & 255)
    }

    var blue: UInt8 {
        return UInt8((rgba >> 8) & 255)
    }

    var alpha: UInt8 {
        return UInt8((rgba >> 0) & 255)
    }

    init(red: UInt8, green: UInt8, blue: UInt8, alpha: UInt8) {
        rgba = (UInt32(red) << 24) | (UInt32(green) << 16) | (UInt32(blue) << 8) | (UInt32(alpha) << 0)
    }

    static let bitmapInfo = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Little.rawValue

    static func ==(lhs: Pixel, rhs: Pixel) -> Bool {
        return lhs.rgba == rhs.rgba
    }
}

产生：

或者您可以根据半径调整亮度或饱和度：

如何使用CAGradientLayer绘制渐变色轮？

How to draw a gradient color wheel using CAGradientLayer?

core-graphics

ios

uibezierpath

cagradientlayer

swift3