How to loop a linear function

[default6ju6ffz027h0]

I have an image of a car tire rim. The lighting is different on all the "spokes". Ultimately, I want the tire to look like the lighting is the same as it rotates. I am trying to set it up so that when I rotate the tire, the opacity of a duplicated layer, that is rotated at -72degrees (there are 5 rims, 5/360=72), changes from 0 to 100 and then loops as the rotation of the layer continues. I am not sure if this is possible or if I am just not using the right combination of expressions in the correct order. This is what I have >

src=transform.rotation;
loopOut("cycle");
linear(src, -72, 0, 0, 100)

I hope this makes sense and that I provided as much information as I could. Thanks in advance!!!

[Roland Kahlenberg · Accepted solution]

The algorithm is awkward and won't work. You need to split up the problem into two areas; the rotation and the opacity.

For rotation, start with the loopOut method -

loopOut("cycle");

Ensure your keyframes allow for seamless looping. A first keyframe of -72 and the second keyframe with a value of 288 (for a clockwise rotation) will suffice.

On the Opacity Property, an Expression such as this should work -
src=transform.rotation;
linear(src, -72, 288, 0, 100)

The issue now, is whether the results are what you were expecting. Regardless, the structure is valid and robust and I am hopeful you will be able to tweak the values to suit your needs. HTH

[default6ju6ffz027h0 · Accepted solution]

Hi Roland! Thank you for the reply!! This helped a ton! I didn't even think about seperating the expressions in Rotation and Opacity. I added this expression to the rotation, which "resets" the value back to -72 if it is over 0 and now it's working perfectly!

src=thisComp.layer("Rot").transform.rotation;
d = Math.floor(src / 72) + 1;
num = (72 * d);
src - num

Opacity:

src=transform.rotation;
linear(src, -72, 0, 0, 100)

Now in my car rig, I have control over the changing the position of the car on the ground, the wheels auto-rotate and the lighting of the rims looks like it's always the same. 

[Rick Gerard]

 I have not seen your comp but if it is anything like what I think it is I would use MiniMax to even out the lighting on the wheel that rotates, create a solid or a shape layer to use for a gradient overlay to bring back the lighting, use Set Matte or Track matte to knock out the spokes on the wheel and simply add 2 keyframes, on and 0º and one at 72º and LoopOut ().

I did this with two shape layers:

I don't know how you are going to rotate the wheel and not have the lighting change and bounce back. I would probably mask out the whole wheel and just set keyframes at 0º and 360º, then use loopOut().