Photoshop. Not free.

However I don't see why PSP should not be able to handle the job. As long as it supports editing 24bit with transparency, then you should be able to manually get what you want. The transparency is defined with an 8bit alpha channel, whether or not it shows you that channel from its interface. And whether or not it saves compatible BMP files.

The two methods I see: 1 manual pixel pushing, making two images (as you have already touched upon) and 2, creating one image with transparency, saving it in a format that preserves that transparency, and then finding a converter on the net, or making your own to rework the headers nd byte order into your BMP format. Photoshop files, TIFF, TARGA and PNG should be suitable formats for the initial save.

Method #1 is what I use (with Photoshop) to make Icons with an alpha channel for Mac OS X. I don't have the time to describe the complete process, but basically I work on an image in layers, all of which have transparency, some of which have only 100% opaque pixels and some which have variable transparency. I always keep my source file layered - this is good in case you need to come back later and make edits you may not be able to make once flattening or merging some portions.

Using Photoshop it's very easy to select only the portions of the canvas containing pixels (control (or is it alt?) clicking the desired layer in the layer palette will do this). There should be something similar is PSP. Once you have that election (which itself knows about the transparency values of the pixels it contains), you can (with Photoshop) save the selection into a channel. This can be your mask or may need to be inverted to create your mask (the icon software I use treats black as "show" and white as "hide" with values in between being translucent of course). Now you'll also need to save the image that will be masked. I do this by creating a new black background behind it and then saving it flat. The black background is there because the mask will be used so that portions of the image taht are translucent will darken the background they're placed on, not lighten it. This way a shadow will be invisible when placed on a black background, instead of showing up as greys and white (this is what would happe if the file had been saved composited onto a white background). However, if your item is a highlight and not a shadow, you'd want the reverse to be true.

I add both the imge and the mask into the icon program which support locations for each. Either that or they're used in custom software and dealt with by the programmer and Mac OS's support for masks.

I hope the info helps, even if not specific to your intended usage. Again, the process may need to change depending on how your target will use its files.


Bruno