So I have an animation that I'm coding in javascript and HTML5 (no libraries, no plugins, no nothing and I'd like it to stay that way). The animation uses physics (basically a bunch of unusual springs attached to masses) to simulate a simple liquid. The output from this part of the program is a grid (2d-array) of objects, each with a z value. This works quite nicely. My problem arises when drawing the data to an HTML5 Canvas.
That's what it looks like. Trust me, it's better when animated.
For each data point, the program draws one circle with a color determined by the z value. Just drawing these points, however, the grid pattern is painfully obvious and it is difficult to see the fluid that it represents. To solve this, I made the circles larger and more transparent so that they overlapped each other and the colors blended, creating a simple convolution blur. The result was both fast and beautiful, but for one small flaw:
As the circles are drawn in order, their color values don't stack equally, and so later-drawn circles obscure the earlier-drawn ones. Mathematically, the renderer is taking repeated weighted averages of the color-values of the circles. This works fine for two circles, giving each a value of 0.5*alpha_n, but for three circles, the renderer takes the average of the newest circle with the average of the other two, giving the newest circle a value of 0.5*alpha_n, but the earlier circles each a value of 0.25*alpha_n. As more circles overlap, the process continues, creating a bias toward newer circles and against older ones. What I want, instead, is for each of three or more circles to get a value of 0.33*alpha_n, so that earlier circles are not obscured.
Here's an image of alpha-blending in action. Notice that the later blue circle obscures earlier drawn red and green ones: