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0 pointseCa12y ago0 comments

It produces symbols, not characters.

Say that the scanner internally splits the scan into regions of 10x10 pixels that it saves in memory. If another region differs on less than (say) 10% of the pixels it is assumed that the two zones are identical and the first one is used in the second place too. The regions have no semantic meaning.

OCR translates the scan into a character set.

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Dylan1680712y ago

The only thing that's missing is a mapping from 'symbol #28' into 'ascii #63'. Internally it's storing instances of symbols plus font data for those symbols.

Also, something to think about: an EBCDIC document accidentally printed as ASCII/8859-1 would have equally zero semantic meaning when fed into an OCR program. But I don't think anyone would argue it wasn't OCR.

Groxx12y ago

That "only thing that's missing" is a very very big thing, and difficult to get correct. And where does it say it's storing font data for the symbols?

Dylan1680712y ago

A font doesn't need to be anything more than a series of bitmaps. And then each character location on the image, ignoring errors, references one of these bitmaps. That's how documents with embedded bitmap fonts generally work.

That mapping isn't a very big thing. Sometimes text-based PDFs don't even have it, and you don't notice unless you try to copy out and get the wrong letters.

j / k navigate · click thread line to collapse

0 comments

Dylan1680712y ago

The only thing that's missing is a mapping from 'symbol #28' into 'ascii #63'. Internally it's storing instances of symbols plus font data for those symbols.

Groxx12y ago

That "only thing that's missing" is a very very big thing, and difficult to get correct. And where does it say it's storing font data for the symbols?

Dylan1680712y ago

That mapping isn't a very big thing. Sometimes text-based PDFs don't even have it, and you don't notice unless you try to copy out and get the wrong letters.

j / k navigate · click thread line to collapse