A potential solution is to use a visualization technique involving the rendering of pathlines, streamlines and streaklines. You're interested in rendering pathlines.
Unfortunately, I can't show you a picture of how to achieve this, but I'll explain it shortly:
- you need to keep track of the trajectory your sword or swinging object leaves behind. You should record an array of positions (you usually can have a pre-recorded data collection from your animation system):
- for each segment of your path,
pathPositions[k] -> pathPositions[k+1], you must build a quad that's best facing your camera. You must best align this quad by alowing it to rotate against the axis defined by that segment mentioned above.
- Now apply a texture onto each quad. The texture should be artistically tweaked to have fading borders so that its middle is solid, highlighting your sword's trajectory.
Of course, you can achieve better results using either spline interpolation or using a higher number of samples.
This way of drawing trails is known in the gaming community as ribbon trails, but the guys in the visualization realm usually refer to it as stream ribbons .