using color table to replace colors with different levels of transparency?

**codewizstw** · 01-10-2005, 01:01 PM

I'm using Photoshop CS and I want to take an image I converted to indexed color mode and use the color table to replace colors with a gradient from black to transparent. So 100% black would still be black. Middle gray would become 50% transparent black. White would become totally transparent. The color table only seems to be able to set one color transparent and won't let me select transparent when choosing either color of a gradient. Can it be done another way?

The Purpose: I want to use the dark areas of an image as a "shadow layer" on top of a new image. I could go through with the eraser tool at low opacity and take out all the white's and gray's by hand but it wouldn't be nearly as accurate as using the color table to replace them uniformly. (I'm superimposing a mural on the side of a building and I don't want to make the mural layer very transparent to keep detail on the building like shadows.)

**pappasaa** · 01-11-2005, 05:26 PM

I would just use an eraser brush at 50% or whatever best fits your needs

**0beron** · 01-12-2005, 02:30 PM

I've not used photoshop, but can you copy the image, desaturate it so it is in greyscale, if not already, and then use this image as a mask? Masks usually encode transparency in the opposite way to what you describe - black = transparent, white = solid. If you invert the image then you could possibly use this to control the alpha component of a black rectangle and achieve the effect that way.

I've never used photoshop, so I don't know if it supports this method, or whether they call it something else.

**codewizstw** · 01-14-2005, 08:41 AM

thank you both... what I did in the end was take my image in greyscale and used a layer blending mode called "multiply" in photoshop which did basically the same thing I was looking to do. So anyone else with this question for photoshop can have an answer now