Movement Shadow

There are at least three ways this can be done.

1) Instead of clearing the screen, just paint a half-opaque black quad over it. This leaves "traces" of the old image on the screen. However, to make the ground not draw over the old image, you can't clear the Z buffer, which means that the cube must move towards you to keep rendering correctly.

2) Copy the screen, after it is finished, to an accumulation buffer or offscreen texture. After you have rendered the screen (before you copy it the next time), draw that texture/accumulation buffer to the screen with a faint alpha blend. This leaves trails that slowly fade away.

3) Remember the N previous positions of the cube, and draw them using an alpha blend mode, in addition to drawing the actual cube opaquely. You might want to use stencil testing (or destination alpha) to avoid drawing the same pixel twice while drawing the trail.

(Actually, there are more ways, like calculating the geometry of the movement path, etc, but these should suffice).

Quote:

Original post by hplus0603 1) Instead of clearing the screen, just paint a half-opaque black quad over it. This leaves "traces" of the old image on the screen. However, to make the ground not draw over the old image, you can't clear the Z buffer, which means that the cube must move towards you to keep rendering correctly.

Just a question: I don't quite understand why one must clear the depth buffer - I thought you'd want to draw the ground afresh? If you just clear the depth buffer and not clear the color buffer, it would almost be the same as having an offscreen texture of last frame's render (only cheaper). Most probably I'm missing something obvious, but I would be happy if someone could point this out for me. [smile]

edit: I just realised that this wonder-method won't work if you're doing anything that requires more than 1 pass. [sad]

Thank you hplus0603 for your quick answer.

I guess I should opt for your third solution, because the 2 firsts do not seem to offer a ghost effect for one single object among other objects in a scene. As a matter of fact, I would like to use this effect for say, only one object in the middle of others that would not have this effect while moving.

Do you see any other way of doing it ? Which way do you think is the best option in term of complexity/performance ?
Thank you in advance
StratBoy61