How do I turn an image into a transparency?

Question

Hi,
There's a question I've seen asked dozens or hundreds of times, and I thought I found an answer a few years ago. I've since forgotten how to do it:
Turn an image into a transparency.
Now, the obvious solution is to set the layer to "multiply." Yes, this is the effect I want. However, I want the layer to be transparent instead.
Another solution is to draw an image on a transparent layer. This works great, but only applies if I create the image from scratch. I want to convert an existing image.
Another solution is multiple steps:
1) Copy image
2) Click "edit in quick mask mode."
3) Paste image
4) Exit quick mask mode.
5) Invert selection.
6) Ctrl+backspace to fill selection with black, on a blank transparent layer.
Result? The image is now transparent! However, this converts the image to greyscale! Great for linework and text, terrible for photos or art!
In an older version of Photoshop 7, I believe I got the following to work:
A) Follow the previous steps 1-6, creating a layer with transparency
B) Copy the original picture to a layer above the transparency
C) Group the layer with the transparency layer below. (Note, grouping doesn't work the same any more.)
D) The top layer provides the colors, but the bottom layer provides the transparency.
E) It is too light, so you take the top layer, and maximize "saturation" 100%.
F) Merge the two layers. It retains the transparency of the bottom layer, with the hue of the top layer.
This no longer works, because I don't know how grouping layers works anymore.
So, I need white pixels to be transparent. Black pixels to have zero transparency. Red pixels to have zero transparency. Etc.
Meanwhile, grey pixels are solid black, but partly transparent. Pink pixels are solid red, but partly transparent.
All pixels are 100% saturated, and the "lightness" is determined by how transparent they are.
So, an analogy would be printing a photo on a transparency. I need to convert an image to a transparency.
If the image/layer were overlaid on white, it would look exactly like the original photo.
Does anyone know how to accomplish this? Mathematically, it's easy. But I don't know about any filter, process, or method to make the conversion using CS5.
Thanks,
Ben

Answer 1

Hello!
I hope that I understand what you need.
(One could just put the image on a layer, and lower its opacity, but you seem to be looking for an effect in wich the tranparency is not the same for all pixels.)
Try this:
1) Turn your image image as a layer (double-click it if it is a background layer) [In order to add a mask]
2) Select all, Copy (CTRL+a, CTRL+C)
3) add a layer mask (click the rectangle with a circle in the bottom of the layers panel) [to be able to change transparency]
4) target the mask [so that you can past an image in it] ALT+click the mask, paste the image.
5) Invert the colors of the mask (CTRL+I) [in order for the white to be transparent and the black opaque].
You now have a layer whose transparency is based on the lightness of the pixels.
Hope that's what you are after!
Pierre