Is there a way of selecting an area of the image and adjusting ONLY the alpha channel to make it partially transparent?
The gradient tool can do a transparency gradient, but it seems odd that I can't just set it to all the same level of transparency.
Actually what I would really like is the ability to translate saturation into transparency levels - i.e. anything white should be fully transparent (alpha = 0), whereas as the color increases in saturation, the alpha value is increased. This is useful for taking images that are designed with a solid background color that use shadowing effects and converting them to transparent-background images.