Example output and documentation appear below:

Here's the alpha channel in action (rendered over solid noise):

Here's the result of setting the variation to spherical, and then layering many flames over a white background:

Here are screen-shots of the UI. The main window:

And the shape edit window:

The colormap can be set to the current gradient, one of several presets, or a set of colors from an image in another window. The Smooth Palette plug-in (below) is good for creating such images. The "Sample Density" widget controls how accurate the rendering process is (and hence how long it takes to compute). It does not effect the preview (which is always computed at low sample density).

The center image in the edit window shows the current shape. When you click "ok, this shape is returned to the main window. The eight surrounding images indicate possible ways of changing the current shape. Clicking on one replaces the current shape with that image. Clicking on the center image picks eight new random directions. Clickon on "randomize" replaces the current shape with a new random shape. The "variation" menu controls the overall character of the shapes.