Answers.Body: <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.sstatic.net/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>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-03T01:21:47.630Z
Answers.Text: Masks transform with their layers, so that's not the reason it didn't work.
Answers.UserDisplayName: stib
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-03T09:19:36.513Z
Answers.Text: See my comment above on your answer - the mirror is needed to maintain mirrored movement - I should have made it clearer in my answer
Answers.UserDisplayName: tomh
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2018-12-06T22:09:51.630Z
Answers.Text: This did not mirror the movement.
Answers.UserDisplayName: Michael Rader
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-02T19:12:31.917Z
Answers.LastActivityDate: 2015-01-02T19:12:31.917Z
Answers.UserDisplayName: tomh
Answers.Body: <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.sstatic.net/yJt01.png" alt="linked and unlinked scale"></p>
  
  <p>What you described should work, masks are transformed with their layers.
  <img src="https://i.sstatic.net/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>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-03T09:18:41.617Z
Answers.Text: This doesn't solve the problem of the movement - the original question says the line moves up and to the left, so the mirrored object should move up and right.  If you mirror using horizontal scale of -100%, any movement keyframes will still move up and left.  Same if you used Transform -> Flip Horizontal.  The quickest/simplest way to get mirrored movement was by pre-comping and using a Mirror effect.
Answers.UserDisplayName: tomh
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-03T11:43:46.143Z
Answers.Text: I was assuming that the OP had already precomped - since he said: "I now want to create a copy of that composition".
Answers.UserDisplayName: stib
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-03T01:21:13.800Z
Answers.LastActivityDate: 2015-01-03T01:21:13.800Z
Answers.UserDisplayName: stib
Body: <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>
  
Tags: <after-effects>
Title: How to mirror a composition?
UserDisplayName: membersound
ThreadText: # How to mirror a composition?
  
  (Asked by: username_0 on 2015-01-02)
  
  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.
  
  **Tags:** after-effects
  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_1 on 2015-01-02. Score: 1 | Accepted answer
  
  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.![enter image description here](https://i.sstatic.net/85RaD.png)
  
  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.
  
  > **Comments:**
  > **username_2**: Masks transform with their layers, so that's not the reason it didn't work.  
  > **username_1**: See my comment above on your answer - the mirror is needed to maintain mirrored movement - I should have made it clearer in my answer  
  > **username_3**: This did not mirror the movement.  
  
  ---
  
  ### Answer by username_2 on 2015-01-03. Score: 1
  
  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.
  
  ![linked and unlinked scale](https://i.sstatic.net/yJt01.png)
  
  What you described should work, masks are transformed with their layers.
  ![mirrored layer](https://i.sstatic.net/zaBG3.png)
  
  There's a more convenient way of flipping a layer - by going to the **Layer** menu and choosing `Layer>Transform>Flip Horizontal`
  
  > **Comments:**
  > **username_1**: This doesn't solve the problem of the movement - the original question says the line moves up and to the left, so the mirrored object should move up and right.  If you mirror using horizontal scale of -100%, any movement keyframes will still move up and left.  Same if you used Transform -> Flip Horizontal.  The quickest/simplest way to get mirrored movement was by pre-comping and using a Mirror effect.  
  > **username_2**: I was assuming that the OP had already precomped - since he said: "I now want to create a copy of that composition".  
  
  ---
  
  