Metal每日分享，调整对比度滤镜效果

// 对比度
let filter = C7Contrast.init(contrast: 2.0)
// 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

/// 对比度
public struct C7Contrast: C7FilterProtocol {
    public static let range: ParameterRange<Float, Self> = .init(min: 0, max: 2.0, value: 1.0)
    /// The adjusted contrast, from 0 to 2.0, with a default of 1.0 being the original picture.
    public var contrast: Float = range.value
    public var modifier: Modifier {
        return .compute(kernel: "C7Contrast")
    }
    public var factors: [Float] {
        return [contrast]
    }
    public init(contrast: Float = range.value) {
        self.contrast = contrast
    }
}

kernel void C7Contrast(texture2d<half, access::write> outputTexture [[texture(0)]],
                       texture2d<half, access::read> inputTexture [[texture(1)]],
                       constant float *contrast [[buffer(0)]],
                       uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    const half3 zero = half3(0.5h);// 平均亮度
    const half4 outColor((zero + (inColor.rgb - zero) * half3(*contrast)), inColor.a);
    outputTexture.write(outColor, grid);
}