Changing transparency in Paint.net

I have come up with these proposed methods, I don't know if there are better ones.

Increase transparency of selected area

1. Select the area
2. Cut
3. Paste as new layer
4. Select Layer properties and drag the Opacity slider
5. Merge Layer Down

Decrease transparency of selected area

6. Select the area
7. Copy
8. Paste as new layer
9. Choose an appropriate color and use the Fill tool to fill the section completely
10. Select Layer properties and drag the Opacity slider
11. Merge Layer Down

My use case is simpler, I want to set all but an outlined object to transparent.

• Copy/move object to another layer.
• List item
• Select outside of object - Tool > Magic Wand works great for me.
• Delete
• Select inside of object - Magic Wand again
• Delete

A variant of this is to use the "paint-bucket" fill tool (in Paint.Net)

• Select a primary or secondary color:
  • F8 to open the color tool
  • More >> to see Opacity - Alpha (lower right corner)
  • Set opacity to 0 for transparent
• Paint-bucket to fill (make it transparent or semi-transparent as you choose)

Effects > Replace colors is another way. It is a bit more complicated to use, but especially useful for complex shapes with lots of holes, as compared to a simple filled rectangle.

Note: Replace colors uses transparency, where 0 = opaque, 255= transparent. This is inverted from the color menu.

Since Paint.Net is a pixel editor (not a vector editor), new users struggle until they learn to use layers like objects. Once on a separate layer, duplicating, rotating etc of a single area of interest "object" is easy to control.

A paint.net hero named Boltbait made a plugin to do that.

Update There's an update for paint.net v4.1+ that includes an installer. There are a lot of things that come with the installer, make sure you only check the options you want. Also, make sure you download the plugin version that corresponds with your paint.net version. (it's pretty obvious from the forum post)

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Old answer

You'll have to follow the links provided on the page depending on which version of paint.net you have. After following the appropriate link, download the zip.

There are several plugins in the zip but to answer this question:

1. extract the "transparency.dll"
2. drop it into your "Program Files\paint.net\Effects" folder
3. (Re)start paint.net and check under "Adjustments" (not "Effects") for "Transparency"