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1) Leg is always in the middle of the body
2) Foot is never level
3) Leg is always in an arc
4) Size of leg is always the same
In a walk, one foot is always on the ground.
In a run, one frame where no foot is on the ground.
Whatever your animation is on (ones, twos, etc), move the background on the same exposure.
Run: shorter the amount of drawings the funnier the run.