Zovirl Industries

Mark Ivey’s weblog

Alpha in WebGL and Canvas

I've been learning WebGL and have been confused how alpha blending works. Coming from working with the 2D <canvas> API, the WebGL behavior was was surprising to me.

(This post uses some recent HTML5 features which won't show up properly in older browsers or IE. I know Chome and Firefox will show it correctly).

The Problem

The demo below uses WebGL to draw white, gray, and black triangles in a range of alpha values, all with the standard gl.BlendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA). The results are weird. For example, why does the row of black triangles get lighter than the background as alpha decreases? If you change the background to be almost-but-not-quite white, why do the white triangles stop fading entirely?

For reference, here's a version done using the canvas API which behaves as expected and clearly shows the differences in alpha behavior between canvas and WebGL:

The Answer

As I learned from Gregg Tavares, the answer to the puzzle is that WebGL blends the alpha channel differently than canvas does. Using the standard gl.BlendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA), WebGL's blending equation for alpha is:

A = Asrc * Asrc + Adest * (1 - Asrc)

If you start with an opaque buffer and render a semi-transparent polygon, the result is that the buffer is now semi-transparent (A = .5 * .5 + 1 * (1 - .5) = .75).

This transparency allows the white background of the page to show through and blend with the WebGL colors. Putting text behind the WebGL canvas makes this obvious:

Gregg details several ways to address this, including using { alpha: false } when requesting the context: