user6173: Ok, so using the overlay filter, one can change the x/y location of a video over time by using the "t" variable. For example, overlay=x='if(gte(t,2), -w+(t-2)*20, NAN)':y=0 can be used to start sliding an overlayed element from the top left to top right, after 2 seconds has passed.
What I want to do is to use a similar principle for scaling elements. I would like to be able to change the scale of a video over time by using an expression (like, scale=w=iw*t, or something along that line).
Is this possible?
when I run that expression through -filter_complex, I get

Error when evaluating the expression 'iw*t'.
Maybe the expression for out_w:'iw*t' or for out_h:'-1' is self-referencing.

I'm feeling like there's a fair chance I'm going to need to patch FFMpeg if I want to get this functionality, but I thought I would ask first, to see if anyone would be in the know about this kind of thing!
Does anyone know how such a task might be accomplished?
Thanks for the help!

user18314: From what I understand 't' is evaluated once. Have you read this: https://ffmpeg.org/ffmpeg-filters.html#zoompan ?
Ffmpeg Docs:
9.170.1 Examples

Zoom-in up to 1.5 and pan at same time to some spot near center of picture:

zoompan=z='min(zoom+0.0015,1.5)':d=700:x='if(gte(zoom,1.5),x,x+1/a)':y='if(gte(zoom,1.5),y,y+1)':s=640x360

Zoom-in up to 1.5 and pan always at center of picture:

zoompan=z='min(zoom+0.0015,1.5)':d=700:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'

Same as above but without pausing:

zoompan=z='min(max(zoom,pzoom)+0.0015,1.5)':d=1:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'

6173: <p>Ok, so using the overlay filter, one can change the x/y location of a video over time by using the &quot;t&quot; variable. For example, <code>overlay=x='if(gte(t,2), -w+(t-2)*20, NAN)':y=0</code> can be used to start sliding an overlayed element from the top left to top right, after 2 seconds has passed.</p>
<p>What I want to do is to use a similar principle for scaling elements. I would like to be able to change the scale of a video over time by using an expression (like, <code>scale=w=iw*t</code>, or something along that line).</p>
<p>Is this possible?</p>
<p>when I run that expression through -filter_complex, I get</p>
<blockquote>
<p>Error when evaluating the expression 'iw*t'.</p>
<p>Maybe the expression for out_w:'iw*t' or for out_h:'-1' is self-referencing.</p>
</blockquote>
<p>I'm feeling like there's a fair chance I'm going to need to patch FFMpeg if I want to get this functionality, but I thought I would ask first, to see if anyone would be in the know about this kind of thing!</p>
<p>Does anyone know how such a task might be accomplished?</p>
<p>Thanks for the help!</p>
 18314: <p>From what I understand 't' is evaluated once. Have you read this: <a href="https://ffmpeg.org/ffmpeg-filters.html#zoompan" rel="nofollow noreferrer">https://ffmpeg.org/ffmpeg-filters.html#zoompan</a> ?</p>

<p>Ffmpeg Docs:</p>

<p>9.170.1 Examples</p>

<ul>
<li>Zoom-in up to 1.5 and pan at same time to some spot near center of picture:</li>
</ul>

<p>zoompan=z='min(zoom+0.0015,1.5)':d=700:x='if(gte(zoom,1.5),x,x+1/a)':y='if(gte(zoom,1.5),y,y+1)':s=640x360</p>

<ul>
<li>Zoom-in up to 1.5 and pan always at center of picture:</li>
</ul>

<p>zoompan=z='min(zoom+0.0015,1.5)':d=700:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'</p>

<ul>
<li>Same as above but without pausing:</li>
</ul>

<p>zoompan=z='min(max(zoom,pzoom)+0.0015,1.5)':d=1:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'</p>
