DIY Book Scanner

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?

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.

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!

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.

Misty is quite right. The number of pixels that corresponds to 1 inch is your DPI.

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.

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.

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.

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).

Yeah, I got the question wrong. Seems like a good solution then.