6 Replies Latest reply on Sep 6, 2012 1:40 PM by Jonathan Hoyt

# Stroboscopic Problem

In this video (password is "problem") the red dotted circle is actually rotating clockwise but appears to be rotating counter-clockwise.  How do I fix this?

• ###### 1. Re: Stroboscopic Problem

Change the rate of rotation.

Have you ever seen an old western movie where the stagecoach wheels appear to be rotating backwards?  It's the same phenomenon.

• ###### 2. Re: Stroboscopic Problem

I've tried slowly the rotation and it didn't help.  the frame rate for the comp is 23.976, is there some formula that will help me figure out the right speed?

• ###### 3. Re: Stroboscopic Problem

I don't know of any formula.  In Days of Yore, DP's had them & kept them as closely-guarded secrets.

However, if you can isolate the dotted line using the solo feature plus taking advantage of the AE Region Of Interest, you can get speedy RAM Previews and just play with the rotation speed until it's right.

• ###### 4. Re: Stroboscopic Problem

The rotation speed that changes the apparent speed of rotation depends entirely on the spacing of the dashes. Say the dash is 10 pixels wide with a 10 pixel space. If you move 20 pixels per frame then there is no apparent movement. So how many degrees is 20 pixels. That's a simple circumfrance calculation. 2 π r is the formula. It's also π D

Let's say your circle is 400 pixels in diameter. That means the circumfrance is  400 X π or 1256.6 pixels in diameter. That means that there are about 60 dashes in the circle. You can make the number of dashes exactly fit inside any circle with a little math. If you want 60 then the formula (because AE measures elipses in diameters) π D / nDashes / 2. Fortunately you can write that as an expression for Dash in an AE shape layer. It looks like this:

```D = content("Ellipse 1").content("Ellipse Path 1").size[0];
nDashes = 60;
Math.PI * D / nDashes / 2
```

A full rotation of a circle is 360º. Divide that by 60 and you get 3º  Rotate your circle anywhere between 0º and 2.99999º per frame and the apparent movment speed in a clockwise direction because each dash will be moving closer to the next dash in the line. At 3º per frame it's hard to tell exactly what direction is going in. From 3.09º to 6º per frame the movement will appear to be going counter clockwise because each dash will be moving farther from the next dash in the line. When you reach 6º per frame the apparent movement will stop because each dash moves exactly to the next position in the line.

Change the number of dashes and you change the critical rotation speed.

You can attach a simple expression and an effects slider to the rotation property of this shape layer and use the slider to adjust the amount of rotation per second. It looks like this. Then a few simple calculations By simply dividing the 360 by the number of dashes you want in your circle (or points on a star) or blades in a fan, you can accurately predict the movement of your circle.

Here's the expression for rotation.

```timeToFrames(time)*effect("Slider Control")("Slider")
```

Animating this slider produces odd results. When I get time to get to my pre-sets cache I'll give you the expression that you can animate. to speed up or slow down rotation.

By the way, when the gaps get real big, like with say 4 or 5 dashes, the apparent motion gets a little more confused because the image is drawn from the top line down. The same principals apply, it's just difficult to accurately predict the apparent movement when scan lines (all digital movie formats) are involved because the picture is drawn from the top left down a line at a time.

EDIT:

As promised here's a preset that gives the number of segments in a dashed line by slider and sets the speed of rotation by slider and the speed is keyframeable. All you have to do is to create a nwq Shape layer and apply this preset.

Here's the expression for rotation that allows keyframing. You can start at zero, speed up the the maximum speed of 360÷number of segments÷2 then slow down to zero again without a perceved change in direction. To apply this preset to any rotation property you must add an Effect>Expression Controls>Slider Control to the layer and name it Speed.

```spd = effect("Speed")("Slider");
angl = 0;
fr = timeToFrames(inPoint);
while (fr < timeToFrames()){
angl +=  spd.valueAtTime(framesToTime(fr));
fr++;
}
angl
```
• ###### 5. Re: Stroboscopic Problem

Jonathan,

Did you get a chance to try this solution???

• ###### 6. Re: Stroboscopic Problem

Yes, thank you. We slowed the rotation, and retimed the wipe.