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I overlaid a rectangle atop an existing PNG image in Preview:

Rectangle over picture

After enabling the Markup toolbar we can select Instant Alpha

Instant Alpha Menu Icon

Now what? I tried to get a selection tool to mark the rectangle but no selecting action occurs. The instructions I was looking at are here https://discussions.apple.com/thread/6774112

Instant Alpha Sample Pic

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  • I just tried this on your same image and I am able to select. Are you clicking and holding while dragging the mouse? – Allan Apr 5 '20 at 3:07
  • In this case the "holding" does not result in any cropping ant bars at all like one gets for cropping an area to make a smaller image. I don't know why its not doing that – WestCoastProjects Apr 5 '20 at 3:42
  • You should see red highlighting first, then the crawling ants. – Allan Apr 5 '20 at 3:44
  • I don't understand what it is doing. I uploaded a youtube video youtu.be/JYCRORUVBPY – WestCoastProjects Apr 5 '20 at 5:25
  • Ah! I see what your doing! Let me write up an answer and make a short vid... – Allan Apr 5 '20 at 5:30
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The way Instant Alpha works is that it selects color that are similar to it. It's not as nice as Photoshop where you can select multiple areas and get really granular with the selection criteria so you'll have to do multiple steps.

  1. Select the Instant Alpha tool
  2. Using your mouse, select an area you'd like to remove
  3. Slowly drag your mouse down and select an area you'd like to remove. If you select too much, back off until you've selected as much as you can
  4. Hit the ⌫ Delete key
  5. Repeat until you're removed the background completely.

enter image description here

(I may have gone too far on the first background selection, but you'll get the idea)

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