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I want to replace the whiteness in pixels with transparency in the same way as described in this question - i.e. for lighter pixels to be made proportionally as transparent. All solutions found via google are based on magic wand or other methods that do not work for my needs. I'm sure there used to be a way to do this in earlier version of photoshop (a simple functions to replace a colour with transparency), but I can't it in CS6. 'Replace Colour' has no alpha options. Grateful for any pointers.

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Why is the question you linked to insufficient? Have you tried Select > Color Range... to create a selection then mask or remove it? –  Scott Oct 3 '13 at 14:55
    
Because I suspect the Imagemagick solution in that link won't work in CS6 ;) The select method is surely also flawed because it constrains the selection. I want the lightness value in all pixels to convert to alpha value. –  geotheory Oct 3 '13 at 15:01
    
Not a full answer, but... maybe it's worth trying to make a b&w copy of the image and pasting that into a layer mask? –  Bakabaka Oct 3 '13 at 15:06
    
Note that imagemagick is free and open source. You can use the tool to make intermediary images which you can then further manipulate in PS6 or any other image editing software. There is no reason to limit yourself to any proprietary tool. The linked answer however does not sound like the right tool for you in this specific instance because it relies on having the blue gradient background used in the development of the original item as a unique image for the masking process. –  horatio Oct 3 '13 at 16:41
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2 Answers

up vote 4 down vote accepted

One method is to use the original image/layer as its own layer mask. You'd create a duplicate of the layer, desaturate and invert it, pasting the greayscale result into the original layers layermask.

This thread may be useful for a full step by step instruction.

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Thanks John (and @Vegard) this is it –  geotheory Oct 4 '13 at 15:40
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You can use blending to add transparency:

  1. Right click the layer you want to add transparency to, pick the "Blending Options..."
  2. Locate the scale under the "This layer:" that is under the "Blend if" box
  3. Press and hold the alt key while dragging on the white knob, the further you drag it to the left the more transparency is added based on the whiteness of the layer. See the image of how the two white knobs are separated.

"Blend if" box

If you also want the layers below it to have the same transparency as the current layer you can set the "Knockout" under "Advanced Blending" to "Deep".

I have only tested this in CS5.5, but I think it should be very similar in CS6.

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