Metal每日分享，自然饱和度滤镜效果

// 自然饱和度滤镜
let filter = C7Vibrance.init(vibrance: -1.2)
// 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

/// 自然饱和度
public struct C7Vibrance: C7FilterProtocol {
    public static let range: ParameterRange<Float, Self> = .init(min: -1.2, max: 1.2, value: 0.0)
    /// Change the vibrance of an image, from -1.2 to 1.2, with a default of 0.0
    public var vibrance: Float = range.value
    public var modifier: Modifier {
        return .compute(kernel: "C7Vibrance")
    }
    public var factors: [Float] {
        return [vibrance]
    }
    public init(vibrance: Float = range.value) {
        self.vibrance = vibrance
    }
}

kernel void C7Vibrance(texture2d<half, access::write> outputTexture [[texture(0)]],
                       texture2d<half, access::read> inputTexture [[texture(1)]],
                       constant float *vibrance [[buffer(0)]],
                       uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    const half average = (inColor.r + inColor.g + inColor.b) / 3.0h;
    const half mx = max(inColor.r, max(inColor.g, inColor.b));
    const half amt = (mx - average) * (-half(*vibrance) * 3.0h);
    const half4 outColor = half4(mix(inColor.rgb, half3(mx), amt), inColor.a);
    outputTexture.write(outColor, grid);
}