Metal每日分享，海报画滤镜效果

// 海报画滤镜
let filter = C7Posterize.init(colorLevels: 2.3)
// 方案1:
ImageView.image = try? BoxxIO(element: originImage, filters: [filter, filter2, filter3]).output()
// 方案2:
ImageView.image = originImage.filtering(filter, filter2, filter3)
// 方案3:
ImageView.image = originImage ->> filter ->> filter2 ->> filter3

public struct C7Posterize: C7FilterProtocol {
    public static let range: ParameterRange<Float, Self> = .init(min: 1.0, max: 255.0, value: 10.0)
    /// The number of color levels to reduce the image space to.
    /// This ranges from 1 to 256, with a default of 10
    public var colorLevels: Float = range.value
    public var modifier: Modifier {
        return .compute(kernel: "C7Posterize")
    }
    public var factors: [Float] {
        return [colorLevels]
    }
    public init(colorLevels: Float = range.value) {
        self.colorLevels = colorLevels
    }
}

kernel void C7Posterize(texture2d<half, access::write> outputTexture [[texture(0)]],
                        texture2d<half, access::read> inputTexture [[texture(1)]],
                        constant float *colorLevelsPointer [[buffer(0)]],
                        uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    const half colorLevels = half(*colorLevelsPointer);
    const half4 outColor = floor((inColor * colorLevels) + half4(0.5h)) / colorLevels;
    outputTexture.write(outColor, grid);
}