Corel Photo Paint 8 has Alpha channels. Maybe you can get that cheap because it's very old. It's one of the best art packages too.
Read this because I do not know much about Alpha Channels, but this is in the Corel 8 Help Files.
Quote: "You can edit an object's transparency globally to change all of its pixels at once by a uniform amount, or locally to vary the transparency of its pixels. When you change the transparency of an object, you modify the grayscale value of its individual pixels. The grayscale uses 256 shades of gray to represent levels of transparency, ranging from black which has a value of 0 (transparent) to white which has a value of 255 (opaque). When you change an object's transparency with a clip mask, the grayscale value is indicated in the Paint or Fill color swatch on the Status Bar, depending on the tool you are using. The darker the display is when working in the grayscale, the more transparent the tool makes the object appear.
Editing transparency globally
Editing an object's global transparency evenly reveals the object or background that lies beneath the selected object, like a double exposure with a camera. You can change the global transparency of one or more objects at once using the Opacity slider in the Objects Docker window. You can also change or undo the effect of the Opacity slider at any time, even after saving it to the object. The Opacity slider operates on a percentage basis ranging from 1%, which makes objects fully transparent, to 100%, which makes them fully opaque.
When you create a clipping group, you can use the Opacity slider to make all pixels of a child object more transparent to reveal the object below it. You can adjust the global transparency of all objects in a clipping group so their pixels appear to blend evenly together within the shape of the parent object.
Editing transparency locally
When you edit an object's transparency locally, you change only some of its pixels to reveal the underlying object or background in varying degrees. This can be a sporadic effect where pixels of greater or lesser transparency stand out from surrounding pixels, like a canvas showing through an old painting, or a gradual effect where the transparency of the pixels changes progressively to create a gradient.
The clip mask
A very useful feature in Corel PHOTO-PAINT for changing an object's transparency locally is a clip mask, which covers the editable and non-editable areas of an object like an invisible sheet. With the clip mask, you can edit an object's transparency values without affecting any other attributes of the object. You can change the transparency values directly on the object and then add the clip mask, or add the clip mask before making the changes. When you change the transparency before adding the clip mask, the clip mask can be used to restore the pixels to their full opacity so long as their transparency has a grayscale value of at least one. You can undo transparency changes at any time with the clip maskeven after saving them in the imageand turn the clip mask on and off to view the object with or without the changes.
You can also use a clip mask on a clipping group to vary the transparency of pixels in a child object, so that parts of the object underneath show through with different intensities.
The clip mask and alpha channels
Just like a regular mask, the clip mask has its own alpha channel in the Channels Docker window. When you enable the Preview icon of the clip mask thumbnail in the Channels Docker window, the Image Window reveals changes to the clip mask as a red overlay that corresponds in depth to its transparency value. As with a regular mask, the clip mask thumbnail in both the Channels and the Objects Docker windows interprets any changes as shades of gray. With the clip mask thumbnail, however, white indicates areas that make the object appear fully opaque, while black indicates areas that make it look fully transparent. Shades of gray in the clip mask thumbnail are areas that make the object appear partly transparent. With the regular mask thumbnail, white signifies the editable areas and black signifies the nonnegotiable areas.
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