I did some experiments trying something similar with image conversion from colour --> greyscale --> black & white images. (see 6 Symphonies, Op.9 (Stamitz, Carl Philipp) ) I found a result using the following....
- convert colour to greyscale (possibly adjust colours first if paper colour is off-white)
- increase pixels by 300% - this gives a nice number for curved interpolation - namely B-spline.
- program I used is freeware irfanview
- command in irfanview is Image --> Resize/Resample --> Set new size: pixels and Size Method: Resample and select B-Spline filter
- then use appropriate conversion for change of grey --> black and white as you have described.
(posted by Homerdundas)
- Thanks for the suggestion. Unfortunately, irfanview seems to be available only for Windows (I'm on Linux). I'm using GraphicsMagick, which can be easily called from a script.
- I'm not sure what's the rationale for the intermediate greyscale step. In my tests, this didn't seem to make much of a difference (except for the slower processing...). Did you get different results? As for the resize factor, I tried 200%, 300% and 400% (all with Lanczos resampling), and found that 200% is a good trade-off between file size and quality (tested by printing on a 600dpi laser). I'm not sure if GraphicsMagick does B-splines; I didn't find it in the manual, but maybe it goes by some other name. If I find it, I can give it a try.
- I've uploaded the first files to the FTP server (folder "Munich Digitization Centre"; ask Feldmahler if you don't have access yet). If you like, you can take a look and tell me if you can get better results with different settings.
- --Leonard Vertighel 02:39, 20 January 2009 (EST)
Hi Leonard. The source scans were in colour - with the full gamut of colour levels and at a lower resolution that one would want for a printable copy. My rational for the grayscale step was to reduce processing time from the colour while preserving information contained in the grey levels. I used 300% because the originals were 200 pixels per inch - and 600 is nice. I judged the quality of the result by some very slender white lines between 16th note beams. The spline seems to have made the lighter grey pixels between the beams lighter - enough that when converted to monochrome, the slender white lines were visible. Which method is best? I guess it depends on original scanning conditions etc. --Homerdundas 09:49, 26 January 2009 (EST)