use Image::Density::TIFF; print "Density: %f\n", tiff_density("foo.tif"); # single-page print "Densities: ", join(", ", tiff_densities("bar.tif")), "\n"; # multi-page
A trivial density calculation would count the number of black pixels and divide by the total number of pixels. However, it would produce misleading results in the case where the image contains one or more target areas with scanned content and large blank areas in between (imagine a photocopy of a driver's license in the middle of a page).
The metric implemented here estimates the density of data where there is data, and has a reasonable correlation with goodness as judged by humans. That is, if you let a human look at a set of images and judge quality, the density values for those images as calculated here tend to correlate well with the human judgement (densities that are too high or too low represent "bad" images).
This algorithm is intended for use on bitonal TIFF images, such as those from scanning paper documents.
We omit the margins because there is likely to be noise there, such as black strips due to page skew. This does admit the possibility that we are skipping over something important, but the margin skipping here worked well on the test images.
Leading and trailing white on a row are omitted from counting, as are runs of white at least as long as the margin width. This helps out when we have images with large blank areas, but decent density within the areas filled in, which is what we really care about.
Gregor N. Purdy, Sr. firstname.lastname@example.org
Copyright (C) 2003-2012 Gregor N. Purdy, Sr. All rights reserved.
This program is free software. Its use is subject to the same license as Perl.