I'm having troubles to manage and understand how to do to get acceleration and deceleration with a shape following a path using the speed graph.
I understand what a curve means when it's going up or down.
I think a 2d roller coaster is the best exercise to learn the use of speed variations. So I drew a path simulating a 2d roller coaster with the whole possibilities that a roller coaster could have without rotations (just hills, loopings downs... )
If I just try a loop part isolated, is easy to get the right speed at the beginning, in the middle and at the end of the path. BUT if I try to make the speed modification having the whole roller coaster circuit together, I'm not able to adjust a correct speed variation for each stage.
There are several ways of doing this, but my favorite one is to animate at constant speed, pre-compose and then apply Time Remap on the pre-comped layer. Now you have the ability to set keyframes where you want the speed changes to occur and shift those around to get the desired effect. It takes a few tries at first untilo you get used to the way Time Remap works but after that it's a breeze.
Pre-compositing the initial animation and applying Time Remap to that allows for AE to render real frames instead of blending existing frames ( as in the case of Time Remap on a rendered layer).