有人可以解释这个Fragment Shader？它是一个色度键滤镜（绿屏效果） [英] Can someone please explain this Fragment Shader? It is a Chroma Key Filter (Green screen effect)

问题描述

我想了解这个色度键筛选器的运作方式。色度键，如果你不知道，是一个绿色屏幕效果。有人能够解释这些函数中的一些是如何工作的，以及他们在做什么吗？

I'm trying to understand how this chroma key filter works. Chroma Key, if you don't know, is a green screen effect. Would someone be able to explain how some of these functions work and what they are doing exactly?

float maskY = 0.2989 * colorToReplace.r + 0.5866 * colorToReplace.g + 0.1145 * colorToReplace.b;
 float maskCr = 0.7132 * (colorToReplace.r - maskY);
 float maskCb = 0.5647 * (colorToReplace.b - maskY);

 float Y = 0.2989 * textureColor.r + 0.5866 * textureColor.g + 0.1145 * textureColor.b;
 float Cr = 0.7132 * (textureColor.r - Y);
 float Cb = 0.5647 * (textureColor.b - Y);

 float blendValue = smoothstep(thresholdSensitivity, thresholdSensitivity + smoothing, distance(vec2(Cr, Cb), vec2(maskCr, maskCb)));


 gl_FragColor = vec4(textureColor.rgb * blendValue, 1.0 * blendValue);

我理解前6行（转换要替换的颜色，这可能是绿色的，纹理颜色到YCrCb颜色系统）。

I understand the first 6 lines (converting the color to replace, which is probably green, and the texture color to the YCrCb color system).

此片段着色器有两个输入浮点值：thresholdSensitivity和Smoothing。

This fragment shader has two input float values: thresholdSensitivity and Smoothing.

• 阈值灵敏度控制类似像素需要着色的替换方式。


• 平滑控制逐渐相似的颜色在图像中被替换。

我不明白这些值如何在blendValue行中使用。 blendValue计算是什么？

I don't understand how those values are used in the blendValue line. What does blendValue compute? How does the blendValue line and the gl_FragColor line actually create the green screen effect?

推荐答案

smoothstep 函数在一个间隔（由前两个参数指定）中评估一个平滑的三次曲线。与GLSL的 mix 函数相比，它将其参数线性混合为：

The smoothstep function in GLSL evaluates a smooth cubic curve over an interval (specified by the first two parameters). As compared to GLSL's mix function, which linearly blends its parameters as:

smoothstep 使用Hermite立方多项式来确定值

smoothstep uses a Hermite cubic polynomial to determine the value

在您的着色器中，blendValue

In your shader, blendValue is a smooth interpolation of your smoothing value based on the distance between the red and blue chrominance values.

最后， gl_FragColor c $ c>指定最终片段颜色（在混合之前，其发生在片段着色器完成之后）。在您的情况下，它是从输入图像读取的调制值，以及半透明的调制Alpha值。

Finally, gl_FragColor specifies the final fragment color (before blending, which occurs after completion of the fragment shader). In your case, it's the modulated value read from the input image, and a modulated alpha value for translucency.

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