72 dpi imports for OCR from my iPhone?

DEVONthink Tips
1

Hi everyone! I finally broke down yesterday and, after years and years of DTPro, decided to upgrade to DTPro Office. Things are nuts at both the colleges I teach and I thought I’d archive all the emails relevant to all the issues we’re working on (that’s another story, I’m using Airmail 3 and it doesn’t work with DT, sigh).

So I decide to scan some paper documents using my iPhone 7+ camera. When I go to OCR them it tells me the OCR won’t be accurate, because the capture is only 72dpi. What’s up with that and how can I fix it? I looked in prefs and it is set for 150dpi.

Thanks in advance for any help from anyone!

Bob

2

The DPI is determined at the point of capture. In other words, it is determined by the camera and the application used to scan the document in the first place. Obviously the camera in the iPhone is capable of much greater than 72DPI so it is likely controlled by the scanning application.
I typically use Scanner Pro by Readdle, and while I cannot find the DPI that Scanner Pro spits out, I’ve never had an issue using DTPO or PDF Pen Pro to OCR scans from it.

There is nothing you can do to increase the DPI of the scans you have after the fact, the setting you are referring to that is set to 150 is the resulting resolution AFTER OCR, which allows users to downsample the resolution of their OCRd documents to reduce the file size (Since as you will see, file size can get a bit out of hand when OCRing in DTPO and many other applications). For example, if you have a 300dpi scan, you can set DTPO to downsample it to 150 after OCRing.

You’ll either need to try your luck with OCR at 72DPI or re-scan them with an application that doesn’t down-sample so aggressively.

3

Hi, thanks for your reply. OK, I’ve got Scanner Pro and use it routinely to scan documents into my iCloud account. I thought from the DTProO instructions that I had to use the camera function; that is what was delivering 72dpi photos for some reason. I just tried Scanner Pro with the iPhone connected and I don’t see that DTProO is seeing it or it’s output directly. Is there some toggle that I need to turn on either in DTProO or Scanner Pro to make the direct connection?

4

Ah, now I understand your workflow.

Honestly I never used the
File>import>scanner or camera
feature.
There is no way to make a direct connection between Scanner Pro and the Import feature on DEVONthink Pro Office.

My typical workflow is to scan it using Scanner Pro, then either air-drop it to my Mac or more commonly just send it to DEVONthink To Go straight from Scanner Pro.

For most small documents (receipts, invoices), I just use the OCR in Scanner Pro which keeps file size under control and the accuracy is fine for most shorter documents. Some other things I’ll do with DTPO’s OCR.

5

Actually there’s some misunderstanding here.

An image capture device like a phone or digital camera, etc. captures a fixed amount of data - 8MP on an iPhone 6s. This means is captures 8,000,000 (rounded value) pixels - no more, no less.

Here is an image I just shot and Synced to DTPO…
iPhone6Image.png
2448 * 3264 = 7990272 (~8,000,000MP)

But what about the 72dpi? That’s a relative value. Relative to what? Relative to the requirements of the output or process. Follow this…
2448 / 72 = 34
3264 / 72 = 45.3
ImgSize_72.png
Ok, so what? Those values are inches! Remember, dpi = dots per INCH. So you could output this image at 72dpi at 34x45 inches. That’s the amount of data in it.
Now, outputting at 72dpi wouldn’t be a good quality product, but unless you have a large format printer, you’re not doing such a thing. (On a potentially interesting side note: 72 dpi at large sizes are used for billboards as: (1) higher resolutions would make for terabyte-sized images, and (2) the viewing distance is so great you don’t notice any quality issues!)

Now, let’s look at the math again, but using a value appropriate to commercial printing and OCR: 300dpi.
2448/300 = 8.16
3264/300 = 10.88
ImgSize_300.png
So there’s enough data in the image to print or OCR a(n approximately) US Letter size document.

By contrast, here’s a marqueed selection from a non-Retina Mac…
MacScreencap.png
331/72 = 4.6
296/72 = 4.1
So you could use this screencapture at 72dpi at 4.6x4.1 inches. Good for onscreen, passable for a quick print at home, bad for commercial printing.
331/72 = 1.1
296/300 = .98
So you’d get a little over a 1x1 inch image at 300dpi.

Now if you scan a US Letter page at 72dpi, you’d have pixel values of 612x792. Less than half a megapixel of data. This has insufficient data for anything but onscreen viewing.
Scan it at 200dpi = 1700*2200 = 3.4MP.
And at 300dpi, 2550x3300, 8.4MP. This is sufficient for OCR as well as commercial printing.

So, blah blah blah, what does this mean… it means there is actually sufficient data for the process. Granted, the higher the megapixels of the capture device and the quality of the lens will factor into things, but you should be able to process the image.

Here is a PDF at 150dpi and 75% Quality (264.7KB)…

PDF_150_75.jpg
PDF_150_75.jpg1185×810 223 KB

And one at 300dpi and 100% Quality (5MB - consider that)…
PDF_300_100.jpg
PDF_300_100.jpg1185×810 277 KB

Not an appreciable difference and NO, you shouldn’t be looking at them this closely to determine quality. You don’t read a PDF zoomed in this closely.

(And to head off the obvious question: No, scanning at 600 or 1200, etc. isn’t better. If you have a document with a lot of fine or thin detail, 600 could help but more data in other cases is increasing file size with no real benefit.)

On a side note:
Upsampling increases the megapixel value.
Downsampling decreases the megapixel value.

6

Jim this is a great post. Thank you! :smiley:

7

That is great info, but I’m still a little stumped. The 72 dpi came straight out of my iPhone 7Plus camera when I had it connected to my MBPro with DTPO open and the window File --> Import --> From Camera or Scanner open. So I had the camera take the photo, it showed up in the DTPO window, I told it to save it (or whatever, don’t remember the exact term in the window; just got the O version of DTP today) and DTPO told me that the 72DPI might not be good enough. Is there somewhere in DTPO or anywhere to change the incoming resolution?

8

@scott: Thanks. I guess 30+ years in Graphic Arts and Printing is useful here too. :smiley:

@ChemBob:

No. I think you missed the point of the post. There’s no need to change the resolution as it’s not a fixed point.

Just import an image from your phone into DTPO. Don’t OCR it. Do a Tools > Show Info and look at the pixel dimensions. What are they?

9

I must be doing something wrong; it’s not working. I connect the iPhone, select File --> Import --> From Camera or Scanner, see the camera, take the photo, it sees the photo, select the photo and use the “Download” button in the window, it puts it in “Scans and Images.” If I select it and tell it to “Save” it throws up a window telling me it probably won’t OCR well, so I don’t select OCR, I select Import. Then it once again tells me it might not be able to create an OCR from the document, but I can try it later and tells me how; I tell it OK, it throws a “Processing” banner across the image and it never, ever goes away. Nothing is saved into DTPO anywhere, it just stays like that until I either close the window or hit the delete button. It isn’t in any Inbox, it isn’t anywhere other than on my iPhone. The opportunity to look at it’s properties inside DTPO is never possible. If I select the Properties button on the scan import page it shows nothing about the photo.

What am I doing wrong? I was really hoping this would work without me having to resort to exporting PDFs from Scanner Pro into DTPO. I could do that before paying for the upgrade.

Thank to anyone who has tried and who can help me with this.

Bob

10

Select the image in Scans and Images and uncheck OCR. When you hit Save it will save to the Global Inbox by default.

11

Hi Jim,

OK, I finally found the checkbox to turn off OCR and saved it. Indeed the photo is huge with 72dpi resolution. Is there any way in DTPO to edit and reduce the dimensions that also increases the DPI, or do I need to do that in Affinity Photo, Acorn, or something like that?

I’m still trying to find the best way to manage this stuff in DTPro, a workflow, and I appreciate your help.

Bob

12

No, and again… you do not need to. The dpi doesn’t matter in this case (as described above). Note the Image Size screen caps I posted (from Affinity Photo). I changed the dpi to 300 which displayed the relative inch dimensions but… this did NOT change the image or pixel dimensions at all.

Just process one of the files with OCR (as I showed in my screencaps from Preview) and you’ll see it OCR’s as expected.