prefiltering and resize

[hanfrunz]

Hello everyone,

do the internal resize filters of avisynth use any kind of pre/post filtering to avoid aliasing? I read this (http://www.xs4all.nl/~bvdwolf/main/foto/down_sample/down_sample.htm) very interesting article about downsampling with photoshop and some other tools and asked myself how does avisynth handle it? Can someone give me more information about how the resizers work? Should i normaly use prefiltering (lowpass FIR filter) to get better quality?

regards,
hanfrunz

[Gavino]

When downsizing, Avisynth uses a wider filter kernel (width proportional to scale factor) which eliminates aliasing as far as possible in a single operation.

For more details, see here (http://avisynth.org/mediawiki/Resampling).

[hanfrunz]

Would it be possible to add a "destination is progressive but viewed on a crt monitor as interlaced so avoid flicker" option? directly to the resizer? I often have to downsize hd or computer generated video in high resolution to PAL, and have to prefilter myself.

[IanB]

@hanfrunz,

That's not really an 'option', that's really a choice of vertical resizer. Most of the resizers are selected for sharpness. You want something soft like BiLinear, BiCubic with soft settings for B and C or Gauss with moderate P.

[wonkey_monkey]

I'm not sure any resizer, even bilinear, would be "soft" enough to mitigate interlace flicker that much - though if it does, you should use a sharper resizer to resize horizontally first to keep as much horizontal resolution as possible.

Much easier just to use a single nice resize, and apply a slight vertical blur (or do a 0.25-0.5 opacity overlay of a 1px vertically shifted clip). Okay, it's an extra "resample", but the rounding errors are going to be pretty insignificant.

David