Chroma key Fragment Shader fails to find the color

问题

I'm trying to write a fragment-shader that functions as a chroma-key filter for a specific color (for example make all pixels with a specific green transparent).

The shader I'm writing is for use in WebGL trough PIXI.js.

JSFiddle: https://jsfiddle.net/IbeVanmeenen/hexec6eg/14/

So far, I wrote this code for the shader, based on the shader I've found here.

varying vec2 vTextureCoord;

uniform float thresholdSensitivity;
uniform float smoothing;
uniform vec3 colorToReplace;
uniform sampler2D uSampler;

void main() {
    vec4 textureColor = texture2D(uSampler, vTextureCoord);

    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, textureColor.a * blendValue);
}

Now, when I define and test this, nothing happens. The problem lies with the shader, because the other filters I tried work.

The color I use for the test is rgb(85, 249, 44).

The Full code for the shader with PIXI is below:

function ChromaFilter() {
    const vertexShader = null;
    const fragmentShader = [
        "varying vec2 vTextureCoord;",

        "uniform float thresholdSensitivity;",
        "uniform float smoothing;",
        "uniform vec3 colorToReplace;",
        "uniform sampler2D uSampler;",

        "void main() {",
            "vec4 textureColor = texture2D(uSampler, vTextureCoord);",

            "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, textureColor.a * blendValue);",
        "}"
    ].join('\n');

    let uniforms = {};

    PIXI.Filter.call(this,
        vertexShader,
        fragmentShader,
        uniforms
    );

    this.uniforms.thresholdSensitivity = 0.4;
    this.uniforms.smoothing = 0.1;
    this.uniforms.colorToReplace = [0.33, 0.97, 0.17];

    this.glShaderKey = 'chromakey';
}

ChromaFilter.prototype = Object.create(PIXI.Filter.prototype);
ChromaFilter.prototype.constructor = ChromaFilter;

This is applied to the video-sprite like this:

videoBase = new PIXI.VideoBaseTexture(videoLoaderVid);
videoBase.on('loaded', () => {
    video = videoBase.source;
    video.volume = 0;
    video.pause();
    video.currentTime = 0;

    videoTexture = new PIXI.Texture(videoBase);
    videoSprite = new PIXI.Sprite(videoTexture);

    const filter = new ChromaFilter();
    videoSprite.filters = [filter];

    resolve();
});

And PIXI is set up like this:

stage = new PIXI.Container();

renderer = PIXI.autoDetectRenderer(720, 720, {
    preserveDrawingBuffer: true,
    clearBeforeRender: true
});

canvasContainer.appendChild(renderer.view);

The video-sprite sits on it's own DisplayObjectContainer and is displayed above another DisplayObjectContainer (hence the need for a chroma-filter)

UPDATE:

The fixed shader can be found here:
https://gist.github.com/IbeVanmeenen/d4f5225ad7d2fa54fabcc38d740ba30e

And a fixed demo can be found here:
https://jsfiddle.net/IbeVanmeenen/hexec6eg/17/

回答1:

The shader is fine, the problem is that uniforms (colorToReplace, thresholdSensitivity and smoothing) aren't passed, they're all set to 0s. By blind luck I've found that to fix that you need to remove third parameter you're passing to PIXI.Filter constructor:

/* ... */
PIXI.Filter.call(this, vertexShader, fragmentShader) // no uniforms param here
/* ... */

PS. You haven't answer in chat, so I'm posting my findings here.

来源：https://stackoverflow.com/questions/44043782/chroma-key-fragment-shader-fails-to-find-the-color

标签

javascript

WebGL

shader

fragment-shader

pixi.js