Image Resolution Detection for OCR

Language:
EN
Product-Line:
FlexiCapture Engine, FineReader Engine
Version:
10, 11
Type:
Technology & Features
Category:
Imaging

The Problem:

  • When a document is scanned, then the scanning resolution is set in DPI, but…
  • When a document is digitized with a digital camera, there is no resolution, because it is a function/result of object distance, focal length of the lens, sensor size and sensor resolution.
  • The Question: When and how does our technology calculates the DPI of camera born images?

The Solution:

If the images come from a digital camera the images should come with meta data in the Exchangeable Image File Format (EXIF). This contains things like focal length, aperture, orientation (rotation).

  • So in theory the result can be calculated.
  • In reality older imaging software may not properly handle the information.
  • In case an image header has incorrect resolution data and user didn't set a value of resolution for overwriting the algorithm guessing reasonable resolution value:

Starting from ABBYY Technologies V9 the resolution detection is working this way:

In case an image header has incorrect resolution data and user didn't set a value of resolution for overwriting the algorithm guessing reasonable resolution value:

  • EXIF is present. Resolution is calculated by formula:
    max(96, RoundTo(max(width/8, height/12),10)).
  • EXIF is absent and an image is anisotropic:
    • One of dimensions is 50dpi or less – resolution is calculated by formula from p.1;
    • Degree of anisotropy is below 10% – resolution is equate with the biggest one;
    • Degree of anisotropy is above 10% – an image is resampled up to the biggest resolution.
  • EXIF is absent and an image is isotropic:
    • Resolution is below 50 dpi – resolution is calculated by formula from p.1;
    • Resolution is below 140 dpi and formula from p.1 gives bigger one – the image resolution is left intake but internally Engine works with resolution calculated by the formula from p.1;
    • Resolution is above 140 dpi – resolution is equate with it.
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