Gear Animation Exercise - Circular Movement, Cast Shadows, and Afterimages

Been meaning to try something like this in Photoshop for a while, but finally made time to do it the other day. I was thinking about animating a rotating gear. 

What I had been wondering was if animating a quarter of the rotation would be enough to animate the whole thing--instead of trying to animate the full 360 movement. Tried it out and only needed 8 frames total.

But it was a pretty quick exercise so I figured I may as well go a bit further. How would a cast shadow look on the ground if light were hitting this thing?

I took the 8 frame layers, duplicated them, and distorted each of these "shadow layers" in the same way:

Recolored to dark gray.
Resized horizontally (15%).
Skewed horizontally (40%).
Repositioned southwest of the actual gear.

Voila, cast shadow.

Something else I had been meaning to try was to replicate an after image effect, the kind your eye perceives when objects move very quickly across your field of vision. So what did I was, on each frame that the gear spins, I made a duplicate of the previous frame and set it to the Overlay blending mode in Photoshop.

So on frame 8, there's a faint looking frame 7 underneath set to Overlay. Likewise, on frame 1 there's a duplicate of frame 8 set to Overlay. So at this point I have 3 layers for each frame:

The actual gear.
A corresponding shadow layer.
A faint Overlay of the previous layer to produce an 'afterimage'.

The difference between the 2nd and 3rd versions might be difficult to see, but if you look carefully you'll notice that the above version leaves a light blue 'trail' when it moves.

This was basically a way for me to test out a bunch of ideas I had been meaning to try so I'm glad to see them in action. I'll definitely look for more ways to use that afterimage effect, and I hope anyone reading this got some ideas of their own as well for future animating. :D If you have any questions about what I've explained here (or anything at all) I can try to clarify--just shoot me an e-mail at

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