Metal每日分享，基于色温调整白平衡滤镜效果

// 白平衡滤镜
let filter = C7WhiteBalance.init(temperature: 4000, tint: -200)
// 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

/// 白平衡
public struct C7WhiteBalance: C7FilterProtocol {
    public static let range: ParameterRange<Float, Self> = .init(min: 4000, max: 7000, value: 5000)
    /// The tint to adjust the image by. A value of -200 is very green and 200 is very pink.
    public var tint: Float = 0
    /// The temperature to adjust the image by, in ºK. A value of 4000 is very cool and 7000 very warm.
    /// Note that the scale between 4000 and 5000 is nearly as visually significant as that between 5000 and 7000.
    public var temperature: Float = range.value
    public var modifier: Modifier {
        return .compute(kernel: "C7WhiteBalance")
    }
    public var factors: [Float] {
        return [temperature, tint]
    }
    public init(temperature: Float = range.value, tint: Float = 0) {
        self.temperature = temperature
        self.tint = tint
    }
}

kernel void C7WhiteBalance(texture2d<half, access::write> outputTexture [[texture(0)]],
                           texture2d<half, access::read> inputTexture [[texture(1)]],
                           constant float *temperature_ [[buffer(0)]],
                           constant float *tint [[buffer(1)]],
                           uint2 grid [[thread_position_in_grid]]) {
    const half4 inColor = inputTexture.read(grid);
    const half3x3 RGBtoYIQ = half3x3({0.299, 0.587, 0.114}, {0.596, -0.274, -0.322}, {0.212, -0.523, 0.311});
    const half3x3 YIQtoRGB = half3x3({1.000, 0.956, 0.621}, {1.000, -0.272, -0.647}, {1.000, -1.105, 1.702});
    half3 yiq = RGBtoYIQ * inColor.rgb;
    yiq.b = clamp(yiq.b + half(*tint/100) * 0.5226 * 0.1, -0.5226, 0.5226);
    const half3 rgb = YIQtoRGB * yiq;
    const half3 warm = half3(0.93, 0.54, 0.0);
    const half r = rgb.r < 0.5 ? (2.0 * rgb.r * warm.r) : (1.0 - 2.0 * (1.0 - rgb.r) * (1.0 - warm.r));
    const half g = rgb.g < 0.5 ? (2.0 * rgb.g * warm.g) : (1.0 - 2.0 * (1.0 - rgb.g) * (1.0 - warm.g));
    const half b = rgb.b < 0.5 ? (2.0 * rgb.b * warm.b) : (1.0 - 2.0 * (1.0 - rgb.b) * (1.0 - warm.b));
    half temperature = half(*temperature_);
    temperature = temperature < 5000 ? 0.0004 * (temperature - 5000) : 0.00006 * (temperature - 5000);
    const half4 outColor = half4(mix(rgb, half3(r, g, b), temperature), inColor.a);
    outputTexture.write(outColor, grid);
}