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Old 07-27-2014, 03:17 PM   #1
u238110
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DPI to use when scanning images

Does anyone have a sense of what DPI to use for ebook images? It seems to me like many scans are artificially large in size. For example, 300 DPI is popular, and I don't know if that's always necessary. If the book is brand new, you can include large images in the ebook and then people will have the extra benefit of viewing the images in all their glory on a computer, but if you're just scanning an old book that has images that have been greatly downscaled already, then my only desire is to make the plainest digital copy of the book possible.

Also, if I were to turn a PDF into an EPUB, and the PDF had a table, I would have to screen shot the table to make it an image and add it to the EPUB as an image, right? Is there a more sophisticated way? And I'd make the screen shot at any zoom level I desired, right? Because PDF uses vector/raster images, which have no intrinsic size, right?

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Old 07-27-2014, 04:37 PM   #2
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Does anyone have a sense of what DPI to use for ebook images? It seems to me like many scans are artificially large in size. For example, 300 DPI is popular, and I don't know if that's always necessary. If the book is brand new, you can include large images in the ebook and then people will have the extra benefit of viewing the images in all their glory on a computer, but if you're just scanning an old book that has images that have been greatly downscaled already, then my only desire is to make the plainest digital copy of the book possible.
I guess it depends a lot on the kind of image. Anyway, a useful rule is to scan at the highest optical resolution you can get in your scanner, and downscale in postprocessing. Do not think in terms of DPI, but in terms of pixels in the final image (and resolution of the ebook readers).

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Also, if I were to turn a PDF into an EPUB, and the PDF had a table, I would have to screen shot the table to make it an image and add it to the EPUB as an image, right? Is there a more sophisticated way?
Yes, you could code the table in HTML (not likely to look nice in real devices) or reformat the information as a list or something similar.

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And I'd make the screen shot at any zoom level I desired, right? Because PDF uses vector/raster images, which have no intrinsic size, right?
I'm not sure what you mean. If the PDF is already a scan, those are already images which have an intrinsic size, and you'd better extract the image than take a screenshot. With vector data, you can indeed take a screenshot at whichever zoom level you like, that's basically equivalent to changing the DPI when scanning.
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Old 07-27-2014, 05:46 PM   #3
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Yes, you could code the table in HTML (not likely to look nice in real devices)
Can you upload a table that exemplifies why this is.


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I'm not sure what you mean. If the PDF is already a scan, those are already images which have an intrinsic size, and you'd better extract the image than take a screenshot.
I don't know, I thought the images used for PDFs were always converted to vector and/or raster data.
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Old 07-28-2014, 12:21 AM   #4
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Can you upload a table that exemplifies why this is.
Quite simply, any table.

Real devices don't do so well rendering tables.
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Old 07-28-2014, 01:19 AM   #5
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I usually scan in 400DPI, regardless if it is text and/or images. Images usually must be edited and resized anyway.
Within a PDF you can have encapsulated images. Actually a lot of scanners can output a pdf, which is just a collection of JPG images encapsulated in a PDF.
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Old 07-28-2014, 12:29 PM   #6
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Wouldn't really go any lower than 300 DPI scans (once you start getting lower, the accuracy of OCR begins to drop drastically).

As to HTML tables versus image tables, there was a large discussion here:

https://www.mobileread.com/forums/sho...d.php?t=223062

I posted examples + reasoning for the HTML Tables side. I believe the benefits of scalability + searchability, highly outweigh the "looks" + bloated filesize of an image.

But if you MUST use an image, please avoid JPG like the plague, and save it either as a GIF or PNG (preferably PNG).

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Old 07-28-2014, 12:33 PM   #7
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No, I'm going to be custom scanning all the images of the book. And I want the scan output to be the final size. I don't want to compromise the integrity of my images by interpolating them. So, what's your idea about an ideal plain/standard DPI setting for old-fashioned images? 189, 213, etc....

Totally agree with you about tables. E-readers need to improve. A simple side-scrolling feature would take care of the issue, would it not?

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Old 07-28-2014, 01:32 PM   #8
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No, I'm going to be custom scanning all the images of the book. And I want the scan output to be the final size. I don't want to compromise the integrity of my images by interpolating them. So, what's your idea about an ideal plain/standard DPI setting for old-fashioned images? 189, 213, etc....
Hmmm perhaps GrannyGrump would be able to give you more info on that one, I know that she works on cleaning up a lot of public domain images.

Although probably all of the advice is, scan images in as high of a DPI as your device can handle. THEN downscale to what is needed as the output.

When higher resolution devices come out in the future, this will allow you to create a higher resolution image from the source.

Always better to go high resolution and downscale, than trying to do the opposite.

300+ DPI for text. Probably something like 600+ DPI for images.

I mean, if you are going to be scanning, you want to get it done right THE FIRST TIME.

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Totally agree with you about tables. E-readers need to improve. A simple side-scrolling feature would take care of the issue, would it not?
Heh, never going to happen. (At least I don't see it happening in the current generation of ebook formats). :P

And you have to keep in mind the MILLIONS of ereaders out there that don't have that functionality.

And then you have to keep in mind people making the FONT SIZE HUGE, and breaking/overflowing even the most basic of HTML tables.

It is one of the reasons why when we convert to ebooks, we try to "verticalize" the tables. An informal rule that gets passed around is try not to go past "4 column" tables. There is much more vertical real estate than horizontal on ebook readers.

There are SVG Tables as well. MikeWV brought it up recently, and there was some discussion/examples:

https://www.mobileread.com/forums/sho...d.php?t=240980

Now, while that SVG is vector, and DOES look nice (and can scale to fit the entire screen). While theoretically, this would be WAY better than either HTML or bitmap images... sadly, the documentation/support in ereaders is crappy. (I am also not too sure how accessible it is, or how well Text-to-Speech software would handle it).

It would also require a massive amount of testing on a multitude of different devices. For example, as MikeWV figured it out, turns out that Kobo had some weird bug with rendering the fonts in his SVG.
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Old 07-28-2014, 01:38 PM   #9
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No, I'm going to be custom scanning all the images of the book. And I want the scan output to be the final size. I don't want to compromise the integrity of my images by interpolating them.
Let me then insist: I think this is the wrong choice. Scanning at higher resolution and then downscaling will very likely give you a better result than scanning at the reduced resolution directly. Besides, if you scan at a higher resolution, you can:

- Rotate, de-skew, crop the image.
- De-speckle, clean noise, adjust colors, remove spine shadows.
- Fix print errors, creases, holes, rips, etc.

all of these before downscaling, so that the traces of your actions will be much less noticeable after that.

Additionally, my advice is to save a copy of the initial unmodified scan and the high resolution modified version just before downscaling. If you later (or some else) want to try a different postprocessing, or if much higher device resolutions become common, you wouldn't have to start from scratch.

But if you want a quick and dirty job, you can probably assume the device resolution will be something around 200dpi, so if you want the display size to be the same, scan at 200dpi (if you want the ebook to be 50% smaller, scan at 100dpi, and so on).
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Old 07-28-2014, 07:21 PM   #10
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Originally Posted by u238110 View Post
Does anyone have a sense of what DPI to use for ebook images? It seems to me like many scans are artificially large in size. For example, 300 DPI is popular, and I don't know if that's always necessary. If the book is brand new, you can include large images in the ebook and then people will have the extra benefit of viewing the images in all their glory on a computer, but if you're just scanning an old book that has images that have been greatly downscaled already ...
There is a lot of confusion about DPI & PPI out there. Here's an outline:

DPI = Dots Per Inch. This refers only to the resolution of a "marking device" (read: that part of a printer that actually marks the paper). Electronic files dont't have "dots" and thus don't have DPI.

PPI = Pixels Per Inch. This is the proper term to use when referring to digital images despite Photoshop's incorrect usage of DPI. Electronic files don't have inches so this value doesn't actually specify the resolution of the file. Only the image's size in pixels does.

When scanning printed images you generally need to scan at around 300 DPI to retain all of the detail visible to the average eye. If the images are halftone images (printed using dots of varying sizes) you may encounter moire problems. These occur when the halftone frequency and the scanning frequency interfere with each other. When this happens simply rescan at a different DPI.

The PPI set in a digital image is merely a note to be read by the application that opens the image. If that application has the concept of a virtual inch (e.g. word processors, page layout programs, ..., anything that thinks in terms if a printed page size) then that application can use the PPI to decide how large, relative to its virtual page, to scale the image. True ebooks (read: ePub, MOBI, ..., but not PDF) don't have virtual page sizes so as a rule ebook reading software will totally ignore any PPI set in the file. They will usually display the image 1 pixel in the file to 1 pixel on the screen. Most will override this and scale the image down when the image exceeds the size of the screen.
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Old 08-03-2014, 11:07 AM   #11
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So, basically, I've done a bit of reading on this and determined that a desktop computer is actually like a miniature. Many people have gained a glimpse into this when scanning something and then seeing how large the resulting scan is on their computer. The reason it's so large is because the desktop environment was designed for cartoon-style computer icons. In order for photos to maintain their integrity in this environment, they need to consist of lots and lots and lots of pixels, and as a result, they're so huge. People downscale their images after seeing these huge scans, not realizing that they're actually working in a miniature environment. And downscaling that big scan is like pointing your finger at the book from which it was scanned and magically making the image in the book smaller (and images embedded in books are usually downscaled to begin with). Of course, the thumbnail-size images included in books are usually so compromised in quality already, but the point is that when you downscale images, you permanently compress their integral makeup. They might look huge on your computer monitor, but they're supposed to.

We could have desktop environments that had very high resolutions and then fonts would look just as fine and sharp as they do when printed. A good way to illustrate what I'm trying to convey is by referencing the iPhone's Retina display: http://gizmodo.com/5561351/the-iphon...ay-controversy

Even though it's just a hand held device, the graphics are made up of so many pixels that if you were to throw them onto a standard desktop computer, the text characters would be half the size of the screen. The reason that graphics and imagery appears so much bigger on a standard desktop is because the desktop's display is nowhere near as concentrated as the Retina display. The Retina display is more along the lines of reality; the desktop is more along the lines of a cartoon. And as a consequence, when a photograph is viewed in this cartoon environment, it's going to be huge as a result of the environment attempting to compensate for all the information.

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Old 09-06-2014, 12:51 PM   #12
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I usually scan in 400DPI, regardless if it is text and/or images. Images usually must be edited and resized anyway.
Within a PDF you can have encapsulated images. Actually a lot of scanners can output a pdf, which is just a collection of JPG images encapsulated in a PDF.
Which scanner / software do you use? I have a scansnap ix500 and I guess it's just 300 or 600, no way to go with 400 or else in between...

Thank you.

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Old 09-06-2014, 01:43 PM   #13
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The most I scan on an enormous Canon all-in-one copy machine at the office. Sheetfeeder and scanning to mail. Very easy.
At home I have a Brother DS-600. Scans in duplex in one go and I can do it in 400DPI. Speed is a slower, but good enough for me.
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Old 09-10-2014, 05:45 AM   #14
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Thank you. I would like to try them, but the scansnap works so smooth: it gives good results, load paper without issues, it's really a pleasure to work with and it's quick to... Thanks again.
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Old 03-06-2015, 05:18 AM   #15
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Totally agree with you about tables. E-readers need to improve. A simple side-scrolling feature would take care of the issue, would it not?
Heh, never going to happen. (At least I don't see it happening in the current generation of ebook formats). :P

And you have to keep in mind the MILLIONS of ereaders out there that don't have that functionality.

And then you have to keep in mind people making the FONT SIZE HUGE, and breaking/overflowing even the most basic of HTML tables.

It is one of the reasons why when we convert to ebooks, we try to "verticalize" the tables. An informal rule that gets passed around is try not to go past "4 column" tables. There is much more vertical real estate than horizontal on ebook readers.

There are SVG Tables as well. MikeWV brought it up recently, and there was some discussion/examples:

https://www.mobileread.com/forums/sho...d.php?t=240980

Now, while that SVG is vector, and DOES look nice (and can scale to fit the entire screen). While theoretically, this would be WAY better than either HTML or bitmap images... sadly, the documentation/support in ereaders is crappy. (I am also not too sure how accessible it is, or how well Text-to-Speech software would handle it).

It would also require a massive amount of testing on a multitude of different devices. For example, as MikeWV figured it out, turns out that Kobo had some weird bug with rendering the fonts in his SVG.

Kindle Paperwhite has this type of side-scrolling feature.
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