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Old 08-18-2021, 05:14 PM   #1
csr
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Improving images in ebooks

I am trying to produce some reasonably well-formatted, attractive ebooks from scanned public domain works from Internet Archive, mostly for my own pleasure, but I would like to get good enough to share them. I open the IA epub in Sigil, and use the PDF as a reference to clean up and organize the text. But the images, which were generally b & w sketches, have a very yellowed background. What is the simplest way to get rid of the yellowing? I have played around a bit, trying to edit them in Windows photos, using Paint, or using Gimp. I have no real experience in working with graphics. I actually succeeded once with Gimp totally by accident, and have been unable to figure out what I did! What is the simplest way to do this?
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Old 08-19-2021, 04:14 AM   #2
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Image > Mode > Grayscale

Set foreground color to black. Add layer > Foreground color (i.e., add a black layer)

Set layer mode to "Burn" (I think it's this one, it should make everything look black, except for completely white stuff on the bottom layer).

Select the bottom layer. Color > Levels. Move left slider up to where the curve starts to rise. Move right slider to where the background looks almost all white, maybe with some speckles. (Make sure you have preview enabled.)

Create a transparent layer between the two.

Set foreground color to white, paint on the middle layer to get rid of the speckles, especially on the borders of the image.

Disable the top (black layer). Your image should now look fine.
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Old 08-19-2021, 07:06 AM   #3
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Or in The GIMP:
Import at desired resolution
Images -> Mode - > Grey scale
Go to menu Colours -> Levels and drag ^ for black level, gamma and white level. You'll see image change.
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Old 08-19-2021, 12:35 PM   #4
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archive.org grayscale PDFs from scans compose pages from 2 PBM files and a PGM file. One of the PBM files is a mask which allows the background of the page to be made pure white. Details in the post at:

https://www.mobileread.com/forums/sh...94&postcount=3
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Old 08-19-2021, 12:39 PM   #5
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In Gimp, here is what I do and it usually gets results easily good enough for eink readers. (I always use the CBR/CBZ version for OCR, useless to convert their pdfs, they are terrible, and the CBR/CBZ images are nice big originals):

1. Crop the images if necessary (rectangle select, then right-click and Crop to Selection) to get shape the way I want it and get rid of big margins. (Using a CBR/CBZ you are opening the whole page as an image).

2. Right-click and Scale Image. IA images are usually huge. For a full page image I use 1200px as the max height. Resolution at 100px/in or better. If a map, diagram, or something I may want to zoom in on, 300px/in or more. It gets maps you can actually read on eink.

3a. Right-click and use Colours-->Curves. Drag the right side of the curve up to lighten, drag the left side down to darken. If the image isn't too brown or yellow, this is often enough. Experiment!

OR

3b. Right-click and Colors-->Desaturate-->Colours to Grey. Then use Colours-->Curves or Colours-->Brightness-Contrast to get a sharp contrast that will show up well on enik. I use this if the image is very brown.

If the image wraps around text, I simply use a paint brush to eliminate it. Ctrl-Click to pick up the background colour, then over-paint.

If the image is very speckled, a paint brush can help, or go the layer route as Jellby suggests (I've never tried that, sounds good, if complicated).

If you have an image, like a title headpiece, that spans two pages, you can easily put them together in Gimp, too. Just open each image, and crop just the left and right margins so the images are at the edge. Export the two images as new files. Then do File-New and make an image as wide as the two cropped images together. Then do File-->Open as Layers and select your two cropped, exported images. They will open on top of each other, so hit "M" (for "move"), and drag them so the edges match up. Then you can re-crop, fiddle the colours, and so on, and export the result.
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Old 08-19-2021, 07:39 PM   #6
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Quote:
Originally Posted by csr View Post
I am trying to produce some reasonably well-formatted, attractive ebooks from scanned public domain works from Internet Archive, [...] But the images, which were generally b & w sketches, have a very yellowed background. What is the simplest way to get rid of the yellowing?
Wrote a little about this last year:

2020: "How to handle images in books while doing OCR of books?"

But especially see the 2 older threads I linked to by GrannyGrump + SBT... they both explain how they use photo editors to get rid of the yellow background + do a better grayscale conversion.
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Old 08-27-2021, 10:24 AM   #7
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Quote:
Originally Posted by Tex2002ans View Post
Wrote a little about this last year:

2020: "How to handle images in books while doing OCR of books?"

But especially see the 2 older threads I linked to by GrannyGrump + SBT... they both explain how they use photo editors to get rid of the yellow background + do a better grayscale conversion.
That reminds me, did I miss a step? What happened to GG? Anybody know? Or have I just somehow managed to miss her, around here in different threads?

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Old 08-27-2021, 02:38 PM   #8
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That reminds me, did I miss a step? What happened to GG? Anybody know? Or have I just somehow managed to miss her, around here in different threads?

Hitch
Yes.

https://www.mobileread.com/forums/sh...d.php?t=341351
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Old 08-27-2021, 04:43 PM   #9
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OH, good. I just suddenly realized I hadn't seen her in ages. (And let's face it, we have a lot of...well, well-seasoned folks here...)

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Old 08-28-2021, 01:06 AM   #10
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(And let's face it, we have a lot of...well, well-seasoned folks here...)

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