How to obtain contrail visual effect?

Question

I have objects moving in OpenGL ES1 application. I want some of them to leave behind contrail – like if they would emit some radiation or something.

EDIT: the kind of contrail is:

• light contrail – not smoke contrail,
• the same effect occurs on LCD monitors with long pixel-switch time (it is called response time AFAIK).

HERE IS THE EXAMPLE IMAGE: https://i.sstatic.net/w3dHZ.jpg

Answer 1

The example image makes it much clearer what you want. "Contrail" probably isn't the right word for this; at my company we call it a "swoosh" but other people probably call it something like "sword trail", "sword slash" or similar.

Anyway, the way to do this is basically to generate dynamic geometry, by recording the position of two points on the sword (one at the hilt, one at the tip) each frame and keeping a history for the last n frames. Then draw a triangle strip that connects these points, with a translucent or additive shader.

To make it look better, you'll probably want to fade out the trailing edge of the swoosh, e.g. using vertex alpha based on the age of each vertex. You can also generate UVs for each vertex so you can put a texture on the swoosh. Finally, if sampling the sword once per frame doesn't give you smooth enough geometry, you can interpolate additional vertices using Hermite splines (these require calculating the velocites of the two sample points each frame, which are used as tangent vectors for the splines).