Metal每日分享，图像阀值素描滤镜效果

// 用于图像阀值素描，形成有噪点的素描
let filter = C7ThresholdSketch.init(edgeStrength: 2.5, threshold: 0.25)
// 方案1:
let dest = BoxxIO.init(element: originImage, filter: filter)
ImageView.image = try? dest.output()
dest.filters.forEach {
    NSLog("%@", "\($0.parameterDescription)")
}
// 方案2:
ImageView.image = try? originImage.make(filter: filter)
// 方案3:
ImageView.image = originImage ->> filter

public struct C7ThresholdSketch: C7FilterProtocol {
    public static let range: ParameterRange<Float, Self> = .init(min: 0.0, max: 1.0, value: 0.25)
    /// Any edge above this threshold will be black, and anything below white. Ranges from 0.0 to 1.0
    @ZeroOneRange public var threshold: Float = range.value
    /// Adjusts the dynamic range of the filter.
    /// Higher values lead to stronger edges, but can saturate the intensity colorspace.
    public var edgeStrength: Float = 1
    public var modifier: Modifier {
        return .compute(kernel: "C7ThresholdSketch")
    }
    public var factors: [Float] {
        return [edgeStrength, threshold]
    }
    public init(edgeStrength: Float = 1, threshold: Float = range.value) {
        self.edgeStrength = edgeStrength
        self.threshold = threshold
    }
}

kernel void C7ThresholdSketch(texture2d<half, access::write> outputTexture [[texture(0)]],
                              texture2d<half, access::sample> inputTexture [[texture(1)]],
                              constant float *edgeStrength [[buffer(0)]],
                              constant float *threshold [[buffer(1)]],
                              uint2 grid [[thread_position_in_grid]]) {
    constexpr sampler quadSampler(mag_filter::linear, min_filter::linear);
    const float x = float(grid.x);
    const float y = float(grid.y);
    const float width = float(inputTexture.get_width());
    const float height = float(inputTexture.get_height());
    const float2 leftCoordinate = float2((x - 1) / width, y / height);
    const float2 rightCoordinate = float2((x + 1) / width, y / height);
    const float2 topCoordinate = float2(x / width, (y - 1) / height);
    const float2 bottomCoordinate = float2(x / width, (y + 1) / height);
    const float2 topLeftCoordinate = float2((x - 1) / width, (y - 1) / height);
    const float2 topRightCoordinate = float2((x + 1) / width, (y - 1) / height);
    const float2 bottomLeftCoordinate = float2((x - 1) / width, (y + 1) / height);
    const float2 bottomRightCoordinate = float2((x + 1) / width, (y + 1) / height);
    const half leftIntensity = inputTexture.sample(quadSampler, leftCoordinate).r;
    const half rightIntensity = inputTexture.sample(quadSampler, rightCoordinate).r;
    const half topIntensity = inputTexture.sample(quadSampler, topCoordinate).r;
    const half bottomIntensity = inputTexture.sample(quadSampler, bottomCoordinate).r;
    const half topLeftIntensity = inputTexture.sample(quadSampler, topLeftCoordinate).r;
    const half topRightIntensity = inputTexture.sample(quadSampler, topRightCoordinate).r;
    const half bottomLeftIntensity = inputTexture.sample(quadSampler, bottomLeftCoordinate).r;
    const half bottomRightIntensity = inputTexture.sample(quadSampler, bottomRightCoordinate).r;
    half h = -topLeftIntensity - 2.0h * topIntensity - topRightIntensity + bottomLeftIntensity + 2.0h * bottomIntensity + bottomRightIntensity;
    h = max(0.0h, h);
    half v = -bottomLeftIntensity - 2.0h * leftIntensity - topLeftIntensity + bottomRightIntensity + 2.0h * rightIntensity + topRightIntensity;
    v = max(0.0h, v);
    half mag = length(half2(h, v)) * half(*edgeStrength);
    mag = 1.0h - step(half(*threshold), mag);
    const half4 outColor = half4(half3(mag), 1.0h);
    outputTexture.write(outColor, grid);
}