Re: [Gimp-developer] Some questions about tiff

[Liam R E Quin <liam holoweb net>]

On Sun, 2022-09-18 at 23:53 +0200, Adalbert Hanßen wrote:
> Attached is an example scan from XSane.

Hello! I'm sorry to have taken a while to reply - it's been a difficult
time here.

First, i opened your sample image, and i do see a histogram under
Curves. However, it's not very obviopus. If you switch from linear to
logarithmic histogram using the rightmost icon near the top by Reset
Channel, you'll see it more clearly.

 
> 1. reduce the size of the file by reducing the 600 dpi resolution
> which was chosen during scanning for better OCR results, keep the OCR
> result,

Image->Scale Image, and use a multiple of two, For example, your sample
image is 4976x3190 pixels; in Scale Image, put a /2 after the width, in
the text box, and press the tab keyu, and it'll divide by two. Or use
/4 to divide by 4.


> 2. reduce the bits per pixel for the scan image plane, e.g. by
> posterizing or even binaizing,
Probably i'd keep 8 bits per pixel, but you can use Curves to reduce
teh amount of detail stored: drag the bottom left of the diagonal line
(the curve) right by four boxes, and the top right corner of the
diagonal line left by one box or half a box, making sure you can still
read the text.

> 3. improve the contrast of the displayed pdf file by some contrast
> enhancing function, e.g. as it is done after applying a contrast
> curve
> in Gimp.
>
> I want to do all this maintaining the OCR-plane from input files.
> Manipulating sandwich PDF-files (like scans made searchable by OCR)
> is
> probably out of the scope of Gimp. But the functions used for the
> image
> plane are in it.

> gs (ghostscript) can reduce the dpi e.g. from 600 dpi (good for OCR)
> down to 150 dpi (insufficient for OCR but  sufficient to display most
> documents. I wish, they would also provide 200 dpi requiring a bit
> more
> storage space. Unfortunately gs only handles 72 dpi (/screen), 150dpi
> (/ebook) and 300 dpi for output. It can do this keeping the OCR pane.

I don't know about planes, are we flying somewhere??
You could use imagemagick or (on Linux at least) netpbm and a shell
script, to automate this. GhostScript isn't the tool i'd use normally.


> To my knowledge, gs can't apply any color or grey level
> transformations,
> even none which could be made by a look up table.

gs is a PostScript renderer, not an image processing tool.


Hope this helps

ankh / liam / demib0y


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