Set transparency of wood print [duplicate]

Question

I am trying to make initials found in old books transparent. E.g.

The paper should be completely transparent and the dark parts of the print fully opaque. But the parts of the print that are not fully saturated and allow seeing the color of the paper should become semitransparent. In the end I should be able to change the background color (on a different layer, or the background color on a webpage) without there being some yellowish artifacts remaining. Or in other words "print" it on a paper of a different color :-)

I did search and find some tutorials which seemed promising, but somehow they all leave out some crucial detail, which I seem unable to fill in.

I believe I understand what I need to do in abstract terms but can't find and/or use the appropriate tools (alpha channel, histogram and gradient tool seem most promising). I would think that I would start with a diagram with black and white as the extrems, substitute transparent for white and then move a slider to fine tune the transparency of the faded print and make sure all of the print-less paper is fully transparent. I might have to first convert the image to black and white.

Answer 1

Alright I'll make this with limited screenshots since I don't use Gimp but I would hope this all exists in Gimp:

1. Convert to Black and White

2. If Gimp has it, tweak particularly the Yellow when converting to Black and White to remove a lot of it. (Update: Hue/Saturation... Yellow before we convert to B/W via Takkat in comments)

3. Then adjust the levels to further darken blacks and lighten whites

4. Convert Image Mode from RGB/CMYK to Grayscale
5. Then Convert Image Mode to Bitmap
6. Now if you want to be really precise zoom in and clean up the points in spaces that should be pure white/transparent

Hopefully this helps, the more you clean up the spaces in between / outside the better quality the final product will be. Here's some screenshots:

Bitmap:

Zoom in its actually not gray, just black dots:

Clean pretty easily clean up those black dots:

Answer 2

First, let me begin by saying I'm less familiar with Gimp than I am with Photoshop, but the concept works the same way. This is the best method I've found to easily create detailed masks.

Step 1: Prepare Mask

Boost the contrast between lights and darks. Desaturate and adjust curves.

Step 2: Create Mask

Select all and paste layer into layer mask.

Step 3: Clean up

With the layer mask selected, using the Dodge/Burn Tool, Burn shadows and dodge highlights. Invert the mask to activate the mask.

Result

After inverting the mask and adding a background layer, this is the result. Using a layer mask, you preserve the original layer.

Answer 3

You can actually try just layer->transparency->color to alpha and pick a medium color from the paper tone. This will make just that tone transparent, and preserve the texture noise, so that you can add other colors behind and still "feel" like it is paper.

To further remove "whiteish" pixels, just repeat the process picking one of the remaining tones - if you want the transparent areas to be completly clear, you can proceed to the mask/alpha gimnastics described in the other answers.

• transfer alpha to mask
• working on the mask do:
  • select by color on opaque area
  • feather selection by large amount
  • sharpen selection
  • feather by small amount
  • invert selection
  • fill mask with black
• apply mask