Just to be clear, the +/- button in the calculator is the negation key, so following the keypresses in the tutorial means that you entered the value -100%.
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This is not easy to explain with just text but let's try....
- to my understanding:
An Acceleration of 0% represents a linear transition to the next step, the exit vector, has the same angle as the direction to the next stepvalue.
An Acceleration of -100% represents a ease-out towards the next stepvalue, the exit vector is horizontal.
A Deceleration of 0% represents a linear transition from previous stepvalue, the entry vector has same angle as direction from the previous stepvalue.
A Deceleration of -100% represents an ease-in from previous stepvalue, the entry vector, is is horizontal.
When entry and exit vector both are horizontal the transition is smooth, as a sinewave
Acc/Dec with even lower value than -100% represents direction vectors further away than horizontal, in other words in the wrong direction of the target.
Acc/Dec with positive values represents the equivalent vectors on the other side of the vector of the linear transition, representing a more aggressive entry/exit
for testing try the following with some fixtures selected:
At 75
Next Step
At 25
Step 1 + 2
Accel -100
Decel -100
Accel -400
Decel 200
Accel 100
Decel -300
When the sum of Accel and Decel = -200, the entry and exit vectors are parallel
When entry and exit vector are parallel, the transition is stepless/smooth
The math behind is bezier curves, the valueranges are normalized for easier interaction.