Transparency

Alpha 

// baseColor has already been premultiplied
vec4 shadeSurface(vec4 baseColor) {
    float alpha = baseColor.a;

    vec3 diffuseColor = evaluateDiffuseLighting();
    vec3 specularColor = evaluateSpecularLighting();    

    return vec4(diffuseColor + specularColor, alpha);
}
Alpha Blend

  • With Z-Buffer Rasterization, the order matters.

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    • If renderer back to front, it works fine.

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  • With A-Buffer Rasterization, the objects get sorted based on the alpha, so we don't have this problem.

    • GPUs use Z-Buffer, so we are stuck with it.

Alpha Testing

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  • If the alpha is below a certain threshold, discard.

  • Alpha Testing with mipmapping can have problems:

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    • The alpha ends up converging to a value below the threshold

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      • At far away, the character loses its beard.

Hashed Alpha Testing

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  • Test randomly.

  • The discard is made in software, not in hardware.

  • It's really noise, as it looks like dithering.

Alpha Distribution

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  •   - Alpha Distribution + Alpha to Coverage.

  • Uses dithering first.

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Alpha to Coverage

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  • You get smoother pixels than with Alpha Testing.

  • It adds different values of alpha, instead of 0 or 1.

    • With 4x, you can get 4 different values of alpha: 0.0, 0.25, 0.5, 0.75.

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    • Alpha to Coverage (left), Alpha Testing (right).

Order-Independent Transparency

Depth Peeling

  • Layer 1:

    • Render as full opaque.

  • Layer 2:

    • Use the depth buffer from Layer 1 to perform additional depth test while rendering Layer 2.

  • Etc, etc.

  • The amount of layers depend on the object.

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  • For every layer, you have to render the scene again; 4 layers = 4 times the cost.

"Software A-Buffer"

  • To avoid the costs of Depth Peeling, we soft-implement A-Buffer.

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  • "Order-Independent Transparency in OpenGL 4.X - Nvidia".

Refraction

  • You need to know the light that is coming from behind the surface.

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  • The front and back need to be considered.

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    • The rays refract in and out.

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  • A normal render gives you the depth and normals of the front.

  • We render the back as a pre-pass, to get the depth and normals of the back.

  • The difference between the depth value from the back and the front will give you the thickness of the object.

  • This is an approximation for how long the ray will travel inside that object.

  • "If I were to travel this much, where will I end up in the Back texture?".

  • This gives you the direction of the exiting ray, so you can sample an environment map.

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  • To see other objects, besides the environment map, the objects need to be rendered first, etc; a process similar to parallax mapping is used to solve this.

  • Super fast, cool.