Re: A "slanted edge" analysis program


"Lorenzo J. Lucchini" <ljlbox@xxxxxxxxxx> wrote in message
news:CGa0f.9459$133.3815@xxxxxxxxxxxxxxxxxxxxxxxx
SNIP

But my point is that sharpening algorithms should not necessarily
produce haloes. I don't have proof -- actually, proof is what I'm
hoping to obtain if I can make my program work! --, but just note
that my hypothesis is just that: halos need not necessarily occur.

That's correct, halo can be avoided while still boosting the high spatial frequencies' MTF. The boost however may be not too spectacular, it's just restoring some of the capture process losses. Some losses are inherent to the sampling process (e.g. area sampling versus point sampling will produce different MTFs from the same subject). Maybe it is more accurate to classify those as system characteristics rather than losses.

Sharpening on the other end, with further anticipated losses in mind (e.g. printing) should introduce small halos in order to trick human vision, but the halo should not be visible (IOW smaller than visual resolution). What *is* visible is the contrast boost (without halo) of the spatial frequencies we *can* resolve.

Capture loss restoration can be tackled by using a high-pass filter. Convolving the image with a smallish HP-filter kernel is rather fast and simple to implement. The best result can be achieved if the HP filter is modeled after the PSF. This is what it can look like on your (odd looking) "testedge.tif":
<http://www.xs4all.nl/~bvdwolf/main/foto/Imatest/testedge.zip>
The Luminance channel was HP-filtered in "Image Analyzer" with a "user defined filter" of 7x7 support.

The mean S/N ratio has decreased from 241.9:1 to 136.3:1, while the 10-90% edge rise went from 4.11 to 2.64 pixels. Unfortunately the scan suffers from some CA like aberration (especially the Red channel is of lower resolution), which may become more visible as well.

Bart 

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