small image and OCR

[alex kelly]

Hello,

I'm trying to OCR a small grey scale image, its for a energy meter but it says the image is to small  - the full response is below: 

pi@OCRReader:~ $ tesseract test_images/cropped_image.png out

Tesseract Open Source OCR Engine v3.04.01 with Leptonica

Error in pixGenerateHalftoneMask: pix too small: w = 230, h = 50

Empty page!!

Error in pixGenerateHalftoneMask: pix too small: w = 230, h = 50

Empty page!!

How can I make tesseract-ocr read the value? 

I can send this to google computer vission API but i would rather do this on the device (rather than sending it to the cloud) and I was recommended tesseract.  If there is a better solution, please let me know. 

Thanks 

[Lorenzo Bolzani]

Hi Alex,

you need to pre process the image a little.

First negate it, tesseract expect dark on white background text.

Then use --psm 6 to tell tesseract that this is a single block or text and not a complex page to split in paragraphs. Also try psm 7, single line.

tesseract --psm 6 cropped_image.jpg -

1.4 95500>0

Now by stretching the contrast (or and OTSU/adaptive threshold) and straightening the image I get (almost) correct results, see the attached image.

1 4 9 55 0 5

Ideally you want an image as simple as possible, black text on white background. You may also try to crop the black border out, if possible.

Have a look here on how to isolate blocks of text:

https://www.pyimagesearch.com/2017/07/17/credit-card-ocr-with-opencv-and-python/

https://www.pyimagesearch.com/2017/02/20/text-skew-correction-opencv-python/

Bye

Lorenzo

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[alex kelly]

Thanks for getting back to me.  When i run it i get an error, any ideas why and how to resolve it?  

pi@ShopFloorOCRReader:~ $ tesseract --psm 6 "test_images/cropped_image.jpg"

Tesseract Open Source OCR Engine v3.04.01 with Leptonica

read_params_file: parameter not found: ����

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[Zdenko Podobny]

What about:

tesseract --help

;-)

Zdenko

ut 23. 4. 2019 o 16:59 alex kelly <alex.k...@gmail.com> napísal(a):

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[Lorenzo Bolzani]

Hi,

I suspect you did a cut and paste or some edits and now you have some non-printable characters in your command line (the question mark boxes). Write it again from scratch.

And you are missing one parameter in the command line, the output file, you can use "-" for standard output.

$ tesseract --psm 6 image.png -

You are also using version 3.x, you should probably upgrade to 4.x.

Lorenzo

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