user8018: I have a moving solid mask composition, which forms a white line that moves vertically from the middle to the left and back.
I now want to create a copy of that composition, mirror it and link the position. So that the mirrored comp will move from middle to right and back.
How could I achieve this? Setting the width = -100% did not work.

user4426: Pre comp your layer, then apply the Mirror effect to the comp.  If your comps are 1920x1080, set the mirror centre to 960x540, and the reflection angle to 0.
I think your scaling to -100% didn't work because it's a masked layer.  You might have more luck using a shape layer to create your line, then you should be able to scale it as you suggested.

user1950: Did you unlink the scale x and y before you set it? if you don't the y will be set to -100 too. It's the little chain link next to the value.

What you described should work, masks are transformed with their layers.

There's a more convenient way of flipping a layer - by going to the Layer menu and choosing Layer>Transform>Flip Horizontal

8018: <p>I have a moving solid mask composition, which forms a white line that moves vertically from the middle to the left and back.</p>

<p>I now want to create a copy of that composition, mirror it and link the position. So that the mirrored comp will move from middle to right and back.</p>

<p>How could I achieve this? Setting the <code>width = -100%</code> did not work.</p>
 4426: <p>Pre comp your layer, then apply the Mirror effect to the comp.  If your comps are 1920x1080, set the mirror centre to 960x540, and the reflection angle to 0.<img src="https://i.stack.imgur.com/85RaD.png" alt="enter image description here"></p>

<p>I think your scaling to -100% didn't work because it's a masked layer.  You might have more luck using a shape layer to create your line, then you should be able to scale it as you suggested.</p>
 1950: <p>Did you unlink the scale x and y before you set it? if you don't the <strong>y</strong> will be set to <strong>-100</strong> too. It's the little chain link next to the value.</p>

<p><img src="https://i.stack.imgur.com/yJt01.png" alt="linked and unlinked scale"></p>

<p>What you described should work, masks are transformed with their layers.
<img src="https://i.stack.imgur.com/zaBG3.png" alt="mirrored layer"></p>

<p>There's a more convenient way of flipping a layer - by going to the <strong>Layer</strong> menu and choosing <code>Layer&gt;Transform&gt;Flip Horizontal</code></p>
