Painting on alpha is a technique used at times to create fancy
selection masks, through which you can thereafter do additional
painting, without impacting the neighboring pixels (those that were not
There are various tools and techniques available for making good
selections. The Magic wand, the lasso tool, rectangle and oval tools
into alpha, and more. You can often combine multiple iterations or
invocations of these tools, with the Shift key to 'add' to the prior
selection. Using the left mouse button with Shift will general add to
the selection, while using the right button with let you remove from
Sometimes, it's best to just paint the selection right on there.
Perhaps you made a rough initial selection with a rectangle tool, and
want to splatter some rough edges along its edges. Perhaps the magic
wand is able to pick up some but not all intended regions. PErhaps a
color keying approach picked up or left out similar colors inside or
outside the desired region. For this and many similar scenarios, you
may need the ability to paint on alpha. Even more, you may want to be
able to take the existing image and transfer it into the alpha channel.
In PD Pro, there's an option to copy the image from the Swap buffer to
the alpha buffer. So if you first copy the image from the Main buffer
to the Swap buffer, you're ready to do this.
In PD Particles 1, there is no swap buffer. However, there still is a
way to transfer the image buffer's content to the Alpha channel.
This tutorial shows how to do this in PD Particles, and of course the
same can be done in PD Pro.