自适应阈值CIKernel/CIFilter iOS [英] Adaptive Threshold CIKernel/CIFilter iOS

问题描述

我已经进行了全面研究，以找到可以在iOS上执行自适应阈值处理的内核.不幸的是，我不了解内核语言或其背后的逻辑.在下面，我发现了一个执行阈值设置的例程( https://gist.github.com/xhruso00/a3f8a9c8ae7e33b8b23d )

I have researched all over in order to find a kernel that performs adaptive thresholding on iOS. Unfortunately I do not understand the kernel language or the logic behind it. Below, I have found a routine that performs thresholding (https://gist.github.com/xhruso00/a3f8a9c8ae7e33b8b23d)

static NSString * const kKernelSource = @"kernel vec4 thresholdKernel(sampler image)\n"
"{\n"
"  float inputThreshold = 0.05;\n"
"  float pass = 1.0;\n"
"  float fail = 0.0;\n"
"  const vec4   vec_Y = vec4( 0.299, 0.587, 0.114, 0.0 );\n"
"  vec4  src = unpremultiply( sample(image, samplerCoord(image)) );\n"
"  float Y = dot( src, vec_Y );\n"
"  src.rgb = vec3( compare( Y - inputThreshold, fail, pass));\n"
"  return premultiply(src);\n"
"}";

是否可以将其重写为自适应阈值内核?我提供给它的图像已变成黑白，并且已经模糊.您有什么资源可以指向我吗?我想坚持使用CoreImage，因为我的整个堆栈都是围绕它构建的.

Is it possible to rewrite this into an adaptive thresholding kernel? The image I am supplying to it has been turned into B&W and has already been blurred. Are there any resources you could point me to? I would like to stick with CoreImage as my whole stack is built around it.

我试图实现的最佳示例/参考已在GPUImage的GPUImageAdaptiveThresholdFilter中实现-

The best example / reference from what I am trying to achieve has been implemented in GPUImage's GPUImageAdaptiveThresholdFilter - https://github.com/BradLarson/GPUImage/blob/c5f0914152419437869c35e29858773b1a06083c/framework/Source/GPUImageAdaptiveThresholdFilter.m

推荐答案

西蒙滤镜是实现所需效果的正确方法，但是，您需要做一些修改.

Simon's Filter is the right approach to achieve the desired effect, however, you have to modify a couple of things.

首先，切换imageLuma和thresholdLuma的顺序，因为我们希望黑色字母保持黑色而不是相反.另外，您应该添加一个常数(我选择了0.01)以消除噪声.

First of all, switch the order of imageLuma and thresholdLuma, since we want black letters to remain black and not the other way around. Also, you should add a constant (I chose 0.01) to remove noise.

    var thresholdKernel =  CIColorKernel(string:
    "kernel vec4 thresholdFilter(__sample image, __sample threshold)" +
        "{" +
        "   float imageLuma = dot(image.rgb, vec3(0.2126, 0.7152, 0.0722));" +
        "   float thresholdLuma = dot(threshold.rgb, vec3(0.2126, 0.7152, 0.0722));" +
        "   return vec4(vec3(step(thresholdLuma, imageLuma+0.001)), 1);"     
    "}"

override var outputImage: CIImage! {
    guard let inputImage = inputImage,
        let thresholdKernel = thresholdKernel else {
        return nil
    }
    let blurred = inputImage.applyingFilter("CIBoxBlur", withInputParameters: [kCIInputRadiusKey: 5]) // block size
    let extent = inputImage.extent
    let arguments = [inputImage, blurred]
    return thresholdKernel.apply(withExtent: extent, arguments: arguments)
}

这就是您仅使用Apple的Core Image所获得的，而无需安装任何外部库:)

And this is, what you get Only using Apple's Core Image, without having to install any external libraries :)

当然，您可以使用常量和块大小的值来玩些游戏.

Of course, you can play around a little with the values of constant and block size.

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