No motion blur on scale attribute calculated with expression

[videowizard.io]

I have a footage layer with the scale calculated using an expression. As it stands the expression makes for a 200% zoom over just 12 frames. Motion blur is set to ON for both the layer and global settings, however the scale-up results in no motion blur at all. 

For resting purposes, removing the expression, and manually baking-in a 200% zoom over 12 frames, corrects the problem. All of a sudden there is motion blur. 

Would love to get this working with the expression. 

Any one have any ideas or advice?

[Dan Ebberts]

What's the expression?

[videowizard.io]

Code here: 

currentFrame = thisComp.layer("vars").effect("Current Frame")("Slider");
totalFrames = thisComp.layer("vars").effect("Zoom Duration (Frames)")("Slider");
t = currentFrame/totalFrames; // time

A = 100; // starting scale
finalScale = A + thisComp.layer("vars").effect("Zoom Amount")("Slider");
B = finalScale; // ending scale
C = finalScale; // ending scale

s = (1-t)*(1-t)*A + (2*(1-t)*t*B) + (t*t*C); // hard coded easy ease math

if (currentFrame >= totalFrames) {
	result = [finalScale,finalScale];
} else {
	result = [s,s];
}
result;

[Dan Ebberts · Accepted solution]

My guess is that your Current Frame slider is giving you a stepped result, like you would get if it was driven by an expression like this:

timeToFrames(time)

If you instead drive it with continuous values (like keyframing from 0 to 12) you should see the motion blur.

[videowizard.io]

You're right! And I got motion blur working now.

I think the issue is that my expression did not use the built-in time variable at all, which I imagine motion blur needs in order to render. 

Curious to learn more on "stepped results" if you have a resource. Bascially how AE makes that distinction. 

Thanks for your help.

[Dan Ebberts]

No resource in particular, but you can look at various expressions applied to a slider in the graph editor. timeToFrames(time), for example, will show a very distinct stair-step result, where time shows a smooth line.

[Mylenium]

Yeah, we need to see the code. Sounds like you are simply quantizing the value steps somehow, in which case of course there would be no motion blur.

Mylenium

[videowizard.io]

Sure here's the code:

currentFrame = thisComp.layer("vars").effect("Current Frame")("Slider");
totalFrames = thisComp.layer("vars").effect("Zoom Duration (Frames)")("Slider");
t = currentFrame/totalFrames; // time

A = 100; // starting scale
finalScale = A + thisComp.layer("vars").effect("Zoom Amount")("Slider");
B = finalScale; // ending scale
C = finalScale; // ending scale

s = (1-t)*(1-t)*A + (2*(1-t)*t*B) + (t*t*C); // hard coded easy ease math

if (currentFrame >= totalFrames) {
	result = [finalScale,finalScale];
} else {
	result = [s,s];
}
result;