title: ImageMagick: Replace RGB channels with white, retain Alpha
body: <p>I have an image created in a Affinity Designer, which has the following channels:</p>
  
  <p>R, G and B:</p>
  
  <p><a href="https://i.stack.imgur.com/wtrWa.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wtrWa.png" alt="RGB channels"></a></p>
  
  <p>Alpha:</p>
  
  <p><a href="https://i.stack.imgur.com/U4jap.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U4jap.png" alt="Alpha channel"></a></p>
  
  <p>When I use this image in other places (Unreal Engine), I get small black artifacts on the edge of the shape due to the black part of the RGB channels. The only important information I need is the alpha channel.</p>
  
  <p>How can I use ImageMagick to replace the RGB channels with white, while retaining the alpha channel?</p>