Why does my WebGL alpha-transparency look wrong?

When using WebGL and creating a partially transparent canvas, you may have noticed that the transparency looks wrong. Perhaps you see harsh edges between areas with alpha = 0 and areas with non-zero alpha. In short, the solution to your problem is to write:

const gl = canvas.getContext("webgl", { premultipliedAlpha: false });

But what is this doing, and what is “premultiplied alpha”? First, let’s see how normal alpha-compositing works. The following is a <div id="outer"> containing a <div id="inner">, with this CSS:

#outer { background-color: black; }
#inner { background-color: red; opacity: 0.5; }

What color is the inner square? Due to its opacity: 0.5, it is half way between its own red color and the black color behind it. Formally, the alpha-compositor performs this calculation:

composite = opacity*foreground + (1-opacity)*background
          = 0.5*red + 0.5*black
          = 0.5*rgb(255,0,0) + 0.5*rgb(0,0,0)
          = rgb(127,0,0) + rgb(0,0,0)
          = rgb(127,0,0)

We can verify this by observing that this looks the same:

#outer2 { background-color: black; }
#inner2 { background-color: rgb(127,0,0); }

This is normal alpha-compositing, as performed by CSS and most other graphics systems. I imagine normal alpha-compositing as modelling a “spray paint”. Your spray paint covers up the previous color of the surface, and replaces it with the color of the spray paint.

But with premultiplied alpha, the compositor instead performs this calculation:

composite = foreground + (1-opacity)*background
          = red + 0.5*black
          = rgb(255,0,0) + 0.5*rgb(0,0,0)
          = rgb(255,0,0) + rgb(0,0,0)
          = rgb(255,0,0)

Notice that the foreground color is not multiplied by the opacity in the calculation; it is assumed to be “pre-multiplied” by the producer of the foreground.

This is the compositing equation used by default when the foreground is a WebGL canvas. What is the point of this alternative alpha-compositing equation? One advantage is efficiency; the compositor saves a multiplication. (Apparently there are also other advantages of correctness.)

Here’s one way to premultiply alpha in a WebGL fragment shader:

void main(void) {
  // ... do everything you were doing before, then ...

  gl_FragColor.rgb *= gl_FragColor.a;  // premultiply alpha
}

If your WebGL does not premultiply alpha, you’ll get weird results. Note that colors can be outside the normal range. For example, if we were to place a red square on top of a white square, we’d get:

composite = foreground + (1-opacity)*background
          = red + 0.5*white
          = rgb(255,0,0) + 0.5*rgb(255,255,255)
          = rgb(255,0,0) + rgb(127,127,127)
          = rgb(382,127,127)

I’m not sure what the compositor is supposed to do with the red value 382. Chrome’s compositor seems to cap the values, i.e. the resulting pixel would be rgb(255,127,127).

Another weird result of not pre-multiplying alpha is due to an optimization. If alpha == 0, Chrome’s compositor does not calculate foreground + (1-alpha)*background; instead, it just takes background. This is a safe optimization if the alpha has been premultiplied, because foreground will be zero. But if it hasn’t been premultiplied, this optimization means the foreground color is not added. As a result, you’ll see harsh transitions between pixels with alpha == 0 and those with non-zero alpha.

So one way to fix this is to premultiply your alpha, but another is to tell the browser to use the normal alpha-compositing function:

const gl = canvas.getContext("webgl", { premultipliedAlpha: false });
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Tagged #programming, #web, #webgl. All content copyright James Fisher 2020. This post is not associated with my employer. Found an error? Edit this page.