Metal每日分享，调整灰度系数gamma滤镜效果

// 灰度系数滤镜
let filter = C7Gamma.init(gamma: 3.0)
// 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

/// 灰度系数
public struct C7Gamma: C7FilterProtocol {
    public static let range: ParameterRange<Float, Self> = .init(min: 0.0, max: 3.0, value: 1.0)
    /// The adjusted gamma, from 0 to 3.0, with a default of 1.0
    public var gamma: Float = range.value
    public var modifier: Modifier {
        return .compute(kernel: "C7Gamma")
    }
    public var factors: [Float] {
        return [gamma]
    }
    public init(gamma: Float = range.value) {
        self.gamma = gamma
    }
}

kernel void C7Gamma(texture2d<half, access::write> outputTexture [[texture(0)]],
                    texture2d<half, access::read> inputTexture [[texture(1)]],
                    constant float *gamma [[buffer(0)]],
                    uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    const half4 outColor = half4(pow(inColor.rgb, half3(*gamma)), inColor.a);
    outputTexture.write(outColor, grid);
}