Re: Apple II Books

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

Paul R. Santa-Maria wrote:
> The biggest scanning mistake I see folks making is using
> greyscale instead of monochrome.  This increases the file
> size and makes the scan useless for OCR.

Michael J. Mahon wrote:
> Scanning in grayscale, even if only to 2- or 3-bit depth, 
> should preserve some sub-pixel resolution which can
> be recovered by thresholding at higher resolution if
> desired.

That's true, except that people typically store greyscale images using
JPEG lossy compression, which was designed for continuous tone images
rather than text and line art.  JPEG discards high frequency
information.  Guess what the frequency spectrum of text and line art
looks like?  JPEG makes the edges blurry and indistinct, and introduces
other artifacts within the 8x8 pixel blocks.  This makes it MUCH worse
for OCR or even just for viewing.

Lossless greyscale coding (such as is available with JPEG 2000 wavelet
compression) would be worthwhile, but typically doesn't get very good
compression ratios, and software supporting it is not widely available.