Premultiply and Unpremultiply - a definition

[rcallicotte]

Terragen uses these terms and so do many graphics programs.  I'm trying to understand these in the easiest and simplest way possible.  Would anyone be willing to clarify what these are and how they differ and maybe even give an example how these work in real world 3D / 2D artwork?

[jo]

Hi,

It relates to alpha channels. Do a search for "premultiplied alpha" and you'll get lots of information. The first three Google results cover the basics, although in a wordy way. I would suggest having a browse until you find something which makes sense to you.

Here's a video to check out:

http://vimeo.com/11064139

I didn't watch the whole thing but he seems to cover the basics well.

Unpremultiplying basically takes a premultiplied image and converts it to an image with a straight alpha channel. You would only want to do that if you know the image is premultiplied.

Regards,

Jo

[rcallicotte]

Thanks Jo.  This was a good (and simple) one - http://www.gimp.org/docs/plug-in/appendix-alpha.html