The presenter created a simple stage with a controllable character, a turtle and a small alcove. The alcove is one block big (same size as character). He wanted to be able to tweak the "bounce" physics after jumping on a turtle to get into this alcove perfectly. Instead of tweaking the variables randomly until it worked, he implemented a "rewind" feature in conjunction with an "onion layer" affect.
He would control the character, jump on the turtle forward toward the alcove... missing it...
He'd rewind and show this "onion layer" of ghost images of where his trajectory was. As he changed the "bounce" value from jumping on the turtle, the onion layer, in real time would change and show what would happen "in the future." As a result he can fine tune his platform jump.
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Read more: Bret Victor
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