From: "Jeroen Stessen" <jeroen.stessen@xxxxxxxxxxx> > Maybe. You're comparing optical low-pass filtering with > electronic (digital) low-pass filtering. The former is done > in the linear-light domain by a gaussian filter, the latter > is (probably) done in the gamma domain by a sinc-like filter. > Both have opportunities for making sampling artefacts, so it > is impossible to tell a priori which would be best. Yup - my point exactly. > The electronic sinc-like low-pass filter can have a sharper > transition band than the optical gaussian filter, and this > implies that it will create ringing. This ringing contributes > to the sharpness impression, but it is unnatural too... Overshoot is unnatural, but if one only uses a single negative lobe (on a sinc or sinc-like filter) and the over- and preshoot doesn't get too deep it contributes to the perception of edge sharpness. Certainly a completely unsharpened image looks soft to most people. > That is not a good filter for down-sampling ! You would need > at least a 12-taps polyphase FIR filter for decent performance. > You could use a transposed polyphase filter with fewer taps > (between 4 and 8), see our patent US5892695. The bi-cubic is > not good enough for transposing, unless you apply some further > tricks. Better to use a polyphase filter with lookup tables. Well, I beg to differ on the first point. If you like, you could use a two-lobe windowed sinc, but for all intents and purposes, Catmull-Rom is equivalent to two-lobe Lanczos-windowed sinc. The two curves overlay almost perfectly. Yes, if you use a bicubic for downsampling you are venturing outside the original interpolation math because Catmull-Rom was not intended to be a low-pass filter, but it does work fine. I just use it because it's equivalent in performance to a windowed sinc but is faster to calculate. Given that the calculation of the filter kernels is a tiny portion of the whole scaling operation, it's a false economy, but I just don't see any reason to switch to windowed sinc. And yes, I agree completely that you need quite a few taps and a polyphase filter to get acceptable results. The current scaler I'm working on uses as many taps and phases as are mathematically necessary, as it's not intended to be real-time. > Probably, but it is always a trade-off between sharpness and > (aliasing) artefacts. Sometimes less sharpness gives better > pictures. I find that one of the potential attractions of > HDTV: to give a bit more sharpness for a lot less artefacts. Agreed. Best regards, Don ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.