如何使用CoreImage创建平滑的阈值过滤器？

Question

是否存在可用作平滑阈值的现有过滤器？我的意思是像素值应该在两个值之间进行阈值处理,但中间值是内插的.

基本上,我正在尝试实现这个:

https://www.filterforge.com/wiki/index.php/Thresholds#Interpolation_.28.22Blending.22.29

Answer 1

您需要使用Core Image Kernel Language和a编写自己的语言CIColorKernel.

我会使用a smoothstep并传入两个边值以及当前像素的亮度.CIKL看起来像这样:

kernel vec4 color(__sample pixel, float inputEdgeO, float inputEdge1)
{
    float luma = dot(pixel.rgb, vec3(0.2126, 0.7152, 0.0722));
    float threshold = smoothstep(inputEdgeO, inputEdge1, luma);
    return vec4(threshold, threshold, threshold, 1.0);
}

您可以CIFilter使用以下代码将其包装为:

class SmoothThreshold: CIFilter
{
    var inputImage : CIImage?
    var inputEdgeO: CGFloat = 0.25
    var inputEdge1: CGFloat = 0.75

    var colorKernel = CIColorKernel(string:
        "kernel vec4 color(__sample pixel, float inputEdgeO, float inputEdge1)" +
        "{" +
        "    float luma = dot(pixel.rgb, vec3(0.2126, 0.7152, 0.0722));" +
        "    float threshold = smoothstep(inputEdgeO, inputEdge1, luma);" +
        "    return vec4(threshold, threshold, threshold, 1.0);" +
        "}"
    )

    override var outputImage: CIImage!
    {
        guard let inputImage = inputImage,
            colorKernel = colorKernel else
        {
            return nil
        }

        let extent = inputImage.extent
        let arguments = [inputImage,
                         inputEdgeO,
                         inputEdge1]

        return colorKernel.applyWithExtent(extent,
                                           arguments: arguments)
    }
}

请注意,您需要确保inputEdge0小于inputEdge1,否则smoothstep会有点疯狂.你可能也想要saturate它们,以确保它们在0和1之间.

看看CIFilter.registerFilterName哪个允许你创建一个SmoothThreshold熟悉语法的实例,虽然这样可以正常工作:

let image = CIImage(image: UIImage(named: "monalisa.jpg")!)!

let filter = SmoothThreshold()
filter.inputImage = image

let final = filter.outputImage

干杯!

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