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I have discovered Gimp's "Color to Alpha" feature, and man is it awesome! Basically, it will extract a particular color out of a selection and convert it to transparency. This gave me the ability to make a bottle taken on a flat background a truly transparent layer with highlights and shadows intact.
Does Adobe Photoshop have anything similar? Paid or otherwise? (Here's my attempt at steps coming to around 60% quality of the Gimp plugin)
There is a plugin for Photoshop that can do this, but it's a little ancient and only 32bit. It is, however, free and may do the trick for you.
I believe you can use the Select -> Color Range... tool to achieve this affect. The color range selection tool is a pretty handy feature that lets you click a color from a gray-scaled version of your image, then use a "fuzziness" slider to adjust the range. You can also preview the selection in the full image. This creates a normal photoshop mask when you choose OK.
Once you have your baseline selection, you can then further adjust the mask using all of the normal masking tools in Photoshop. This is probably the most ideal way to do what you need to do (based on your previous question, your trying to isolate a bottle?) The Color Range selection will give you an almost-ideal mask with transparency in tact...and you can further adjust that to fix areas that were selected that should not have been, or that were excluded and should have been selected. Once you have fine-tuned your mask, simply hit Delete, and watch the magic occur. Since the mask was a full alpha mask, the deletion will remove varying degrees of "opacity" based on how heavily an area matched the base color you selected initially.
Since your background is white, selecting the white color and adjusting fuzziness to appropriately "fill in" the parts of white that show through your bottles should give you a pretty effective mask in about 30 seconds flat. A little adjusting might take a couple minutes, but the end result should be better than the Gimp "Color to Alpha" plugin (at least, based on the few examples I've seen...it seems to be a bit over-zealous in its application of transparency.)
Generally speaking, a "full green" or "full blue" background might be better for this kind of thing. White is a blend of all colors, and that can make it more difficult to isolate specific objects from an "all-color" background. Thats usually why a lot of special effects sequences shot for movies are filmed against a green backgrop...its easier to filter out during post-process compositing (its a single "primary" color in the RGB model, rather than a blend of all three colors. This is called Chroma Key filtering.) It should be noted that you want to use a chroma key that is complementary to the color(s) of the object being shot. For example, if you are shooting a green bottle, it might actually be best to use a red background, as using a green background would make it very difficult to exclude a green background without losing the bottle along with it. If you have yellow, orange, red or brown bottles, a blue background may be best. The general idea, though, is to use a background that is complementary (think opposite color on the color wheel) to the foreground objects being shot.
I have not used the Color to Alpha feature in GIMP, but it looks similar to converting a channel to alpha. In the channels panel, control click (or command click if you are on a Mac) on the channel you want to convert. Then you can either create a mask from the selection or press delete.
If you want more control you can edit the mask using curves, levels, etc. Just create the mask as I mentioned above and select it in the channels panel and apply whatever adjustments you want.
The Color Range tool as suggested by jrista will also work, but I find the channels method a little easier to control.
2020 UPDATE: there is no way to do it in Adobe Photoshop. Color Range and masks can not do it. The image below explains, what Color to Alpha does.
The old image is fully opaque (transparency 100%). RGB values are between #ff0000 and #000000.
The new image has transparency between 0% and 100%. The RGB values are all #ff0000. So Color to Alpha modifies both the transparency and the RGB values.
Few years late, but I think what you're looking for is called Color Range Masking. Look it up or follow roughly these steps: 1. Create a mask for the layer containing that colour you want to get transparent and make sure the mask is selected. 2. Look up the Masks or Properties (since CS6) window (Window > Masks/Properties). 3. There you'll find a button called "Color range..." 4. In the new window you can sample the colour you want to mask, or even add multiple colours using the button with the sampler and the plus sign. I'd suggest setting the Selection Preview to Grayscale. Just play around with it. :)
In my personal experience, "Select Color Range" does a rather hard selection. You can compliment this, however, with the "Refine Edge..." tool after performing the initial selection. Choose your view as "Overlay", select "Smart Radius", and adjust the radius until the overlay covers all of the color you would like to remove. Choose to Output to Selection, click OK, then delete the selection. That should remove all of the bits that a simple "Select Color Range" leaves behind.
Colour to Transparency
Photoshop
GIMP
