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

Custom DPI

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ibr4him
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Custom DPI

Post by ibr4him » 19 Oct 2010, 11:11

The tutorial shows 6 lines = 1nch (usually) which turns out to be 180 in that case, but my book has just 3 lines in 1 inch, and if I select 3 lines in an image editor, it shows 240px (height), should I set that as my DPI?

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Misty
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Re: Custom DPI

Post by Misty » 19 Oct 2010, 11:14

It's possible that Arabic text uses different standard line sizes from Roman and Cyrillic characters. I'd recommend going with what you've actually measured on the page instead of the tutorial guidelines for your books.
The opinions expressed in this post are my own and do not necessarily represent those of the Canadian Museum for Human Rights.

ibr4him
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Re: Custom DPI

Post by ibr4him » 19 Oct 2010, 11:16

Right, so I measured (physically) 3 lines to be approx 1inch. And using the marquee tool in an image editor, 3 lines are 240px in height.

I'm not sure what should I set the DPI to.

Thanks!

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Misty
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Re: Custom DPI

Post by Misty » 19 Oct 2010, 11:25

You should enter the DPI as 240 in that case. DPI is the number of pixels divided by the number of inches. Since you've measured 240 pixels to be 1 inch, you can go with 240/1 which gives you 240DPI.

For more detail you can check my blog post on DPI, but you should be okay with the measurements you've done here.
The opinions expressed in this post are my own and do not necessarily represent those of the Canadian Museum for Human Rights.

Tulon
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Re: Custom DPI

Post by Tulon » 19 Oct 2010, 11:50

Misty is quite right. The number of pixels that corresponds to 1 inch is your DPI.
Scan Tailor experimental doesn't output 96 DPI images. It's just what your software shows when DPI information is missing. Usually what you get is input DPI times the resolution enhancement factor.

StevePoling
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Re: Custom DPI

Post by StevePoling » 20 Oct 2010, 01:19

Does it make any sense to image a ruler then count the pixels between the 1" and the 2" marks? Then use that as your DPI estimate.

Tulon
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Re: Custom DPI

Post by Tulon » 20 Oct 2010, 02:48

StevePoling wrote:Does it make any sense to image a ruler then count the pixels between the 1" and the 2" marks? Then use that as your DPI estimate.
It's a chicken and egg problem. You don't know how far apart to draw you 1'' and 2'' marks until you know the DPI.
Scan Tailor experimental doesn't output 96 DPI images. It's just what your software shows when DPI information is missing. Usually what you get is input DPI times the resolution enhancement factor.

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daniel_reetz
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Re: Custom DPI

Post by daniel_reetz » 20 Oct 2010, 03:00

If we had people to hack on the code, we could make a "DPI page" with some markers on it to determine DPI automatically. You'd just print a copy of this page and shoot it once. As it is, this is in no way a feature request, just more blue-sky thinking.

I wish more people would specify their Scan Tailor wishes as blue-sky thinking. We can imagine anything... now we just need to recruit a talent pool that can implement it alongside Tulon.

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strider1551
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Re: Custom DPI

Post by strider1551 » 20 Oct 2010, 06:03

Tulon wrote:
StevePoling wrote:Does it make any sense to image a ruler then count the pixels between the 1" and the 2" marks? Then use that as your DPI estimate.
It's a chicken and egg problem. You don't know how far apart to draw you 1'' and 2'' marks until you know the DPI.
I read the original question differently. I always do a reference image where I lay a flat plastic ruler on the page. When I load my images I use gimp to count the pixels within an inch on the ruler, and then ImageMagick to set the dpi in the files (mogrify -density).

Tulon
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Re: Custom DPI

Post by Tulon » 20 Oct 2010, 06:19

Yeah, I got the question wrong. Seems like a good solution then.
Scan Tailor experimental doesn't output 96 DPI images. It's just what your software shows when DPI information is missing. Usually what you get is input DPI times the resolution enhancement factor.

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