Before jumping into creating our first Composition Panel controller, I wanted to present this 5½ minute video about the linear( ) expression because the concept behind it is critical to understanding what’s going on in many of the expressions used in this series. Even if you understand what interpolation is, you might want to view the video to see how the expression works if you are unfamiliar with it.
For more about interpolation expressions in After Effects, check out this Word Of The Moment (WOTM) link: linear( ). The expression example used in the video below is explained in that link if you’re interested in having a closer look at it. There is also a link to linear( )’s related expressions ease( ), easeIn( ), and easeOut( ) on the bottom of that page which will take you to Adobe’s Expression Language Reference page.