Daniel Reetz, the founder of the DIY Book Scanner community, has recently started making videos of prototyping and shop tips. If you are tinkering with a book scanner (or any other project) in your home shop, these tips will come in handy. https://www.youtube.com/channel/UCn0gq8 ... g_8K1nfInQ

How 2 brighten / lighten 2 colours (1 BitsPerPixel) image?

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b0bcat
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How 2 brighten / lighten 2 colours (1 BitsPerPixel) image?

Post by b0bcat » 14 Nov 2016, 22:52

In the foothills of learning GIMP ably assisted by the local library's now slightly dated 'Beginning GIMP: From Novice to Professional' by Akkana PECK, I have been unable however to find any means of brightening / lightening a (part) page of a scanned image where (say) the upper half gives the appearance of normal exposure and the lower half of over-exposure (reflecting the uneven ink saturation of original printed page), in the following case:

- image was scanned using Plustek OpticBook 3600 and its bundled "Action Express" scan interface in black and white mode;
- e.g. IrfanView reports image properties as "2 (1 BitsPerPixel)".

This I take to be 'true' monochrome mode, because in the later model Plustek OpticBook 4800 scanning using its (more confusing) bundled interface "Book Pavilion" seems to produce a greyscale image whether or not one selects black and white or greyscale; the properties of an image scanned this way being reported by IrfanView as "256 (8 BitsPerPixel)".

GIMP obliges by allowing one to adjust brightness of the greyscale image (or part selected) but not of the (true) monochrome image.

This is one reason I now scan text in greyscale rather than in b&w, but I need to process some old b&w scanned pages and find their too-dark areas can't be adjusted.

I suspect the reason for this is given in Peck's book and/or elsewhere but that I haven't found it for want of the correct terminology to search against. That said, does anyone have any means of securing the objective of brightening the text on a b&w page or part thereof?

cday
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Re: How 2 brighten / lighten 2 colours (1 BitsPerPixel) image?

Post by cday » 15 Nov 2016, 08:05

An image saved as black and white (1-bit colour depth) can only contain pixels that are pure black or pure white: if part of an image is darker than required, those black pixels could only be replaced by pixels that are pure white, which is not what you want...

When scanning pages that are nominally black and white (black text or line drawings on white paper) there are two possible options:

o Scan in grayscale (8-bit colour depth) and then enhance the image as required before, optionally, converting the images to black and white. You could alternatively scan in colour, but enhancement of black and white images in normally better done in grayscale.

o Scan directly in black and white after adjusting the scanner settings to obtain optimal images: scanner interfaces usually have a number of settings that can be adjusted that are often not explored. In addition to adjusting the settings to obtain a clean white background, it may also be possible to make fine threshold adjustments to control the 'weight' or thickness of the text to obtain a more pleasing result.

Be aware that images may render better on screen in grayscale, as the different shades available can smooth the edges of characters, unless the image is scanned at a higher DPI setting. Ultimately acceptable quality is a subjective judgement, and grayscale images will always have a substantially larger file size than suitably compressed 1-bit images.

b0bcat
Posts: 38
Joined: 30 Nov 2012, 21:37
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Country: UK

Re: How 2 brighten / lighten 2 colours (1 BitsPerPixel) image?

Post by b0bcat » 15 Nov 2016, 20:15

Thanks for that.

In my experience it's true that fiddling with the scanner controls can often overcome this problem, however in the present case each affected page is a half and half of normal type (faint, under-inked) and emboldened italic letters (which being over-exposed 'merge ' into each other). Disregarding the existing batch of b&w images, 'going forward' this issue won't bother me as over the last year I've taken to scanning b&w text in greyscale having learnt my lesson :)

Curiously, ScanTailor's own darker/brighter (or thicker/ thinner) setting potentially resolves the problem to some degree, but applies to the whole page - no selecting of the desired selection possible so of course it's not a fix here. That said, I wonder if that functionality is available in a utility (or some feature of GIMP so far undiscovered by me) where it can be applied to a selected part of the image.

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