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Oh wow, this is my father's site! Will try to get him here to answer questions, if anyone is interested.

by nkoren 8y ago

dahart 8y ago
I’m curious what he’d say about anti-aliasing for digital imagery, if anything. In CG, there seems to be a difficult balance between sharpness and antialiasing. If you try to get too close to the Nyquist limit, you suffer more aliasing, and to get rid of all signs of aliasing, you have to compromise on sharpness. Personally, I’ve started to prefer using a Gaussian filter, but many people in digital imaging feel it’s way too blurry. My theory is that we are pretty good at seeing sub-pixel detail even in frequencies that are well (e.g. 2x) below the Nyquist limit, that having something less sharp than ideal only compromises the aesthetic, but not our ability to see the details of what’s there. I did a video project a long time ago in old 480i NTSC and found that filtering vertically twice as much as should be called for improved my ability to see fine details (that is, filtering as if the vertical image resolution is 240). I realize that motion and interlaced video is super different than still photography, but my lesson there has been influencing all my filtering decisions ever since...
- deleted 8y ago
  [deleted]
- theoh 8y ago
  So, I'm assuming you know this, but the theoretically optimal pixel reconstruction filter is the sinc function. https://en.wikipedia.org/wiki/Sinc_filter https://en.wikipedia.org/wiki/Sinc_filter For very high-quality work, I guess we have enough processing power to approximate sinc quite well these days. Is that what you mean by trying to get close to the Nyquist limit? The sinc filter does tend to introduce ringing artifacts. Very high resolutions are now practical, too, which changes things a lot. Probably in favour of using filters which never introduce ringing artifacts, like the Gaussian. (And things like the Lanczos or Mitchell filters exist as compromises between sinc and Gaussian.)
  - dahart 8y ago
    Yeah, I know about sinc and the ideal theory, but nobody uses sinc for production work. Your filter has to be too wide, and the ringing artifacts are too undesirable. The max sharpness in a digital image, the Nyquist limit, is 1 cycle across 2 pixels. It’s not possible to get any sharper than that. But actually rendering images with frequencies exactly at that limit generally cause aliasing artifacts of some kind. That’s what I mean about getting close to Nyquist. > Very high resolutions are now practical, too, which changes things a lot. Probably in favor of using filters which never introduce ringing artifacts, like the Gaussian. Yeah, this is mostly true, and it is one way I justify using a Gaussian. But it’s important to know and remember that without the right filter, you can get visible ringing artifacts no matter how high your resolution is, and no matter how much you super-sample your pixels. This doesn’t come up all that much in CG film or photography, but I have a digital art project where it matters a lot and comes up all the time. (And I order expensive prints too, where artifacts not very visible on my monitor can become plain & obvious on paper). > And things like the Lanzos or Mitchell filters exist as compromises between since and Gaussian. Yes, and I suspect those are the most common ones used in VFX and CG film & print production. But those are balancing sharpness and aliasing. For situations where I really need no aliasing, they aren’t good enough. I can’t usually see the difference between Gaussian, Lanczos and Mitchell without zooming and studying high frequency regions, but I’ve watched VFX supervisors comment on the softness they feel after watching like 2 seconds of film. Some people have an incredible sensitivity to sharpness.
    - brudgers 8y ago
      + My understanding of the way the Nyquist limit plays out in digital photography is that it guarantees no moire. If moire isn't an issue in a scene then it is possible, but not guaranteed to get higher resolution (that of the individual pixel). Of course resolution and sharpness are not exactly the same thing...that's why a 16 mega pixels is a resolution not a sharpness and why a lot of studio photography uses cameras without anti-aliasing filters (e.g. medium format digital). In the studio, conditions can be set up to avoid moire in ways that aren't practical for sports or wedding photography. Similarly, moire isn't going to be a problem with astro-photography and some subjects in landscape photography. + For what it's worth, ringing artifacts are often caused by diffraction or lens aberrations rather than aliasing from sampling frequency (though they can be). + The shape of the modulation transfer function (MTF) is different between film and digital. The line pair comparison in the article illustrates it without being explicit. The Nyquist limit means that sharpness falls off a cliff in digital images and the example line pairs go from resolved to gray abruptly. The example line pairs for film fade to gray, what is and isn't sharp is determined by observation rather than an equation. Digital images often appear sharper because they are only ever so sharp and film appears softer because it is kinda sorta sharp beyond the point where it is clearly sharp. It's why film has a different look...or is a different artistic medium than digital when it comes to some forms of expression.
manaskarekar 8y ago
Thanks for sharing his inspirational story in this old thread: https://news.ycombinator.com/item?id=7540832 https://news.ycombinator.com/item?id=7540832

Oh wow, this is my father's site! Will try to get him here to answer questions, if anyone is interested.

by nkoren 8y ago

Oh wow, this is my father's site! Will try to get him here to answer questions, if anyone is interested.

I’m curious what he’d say about anti-aliasing for digital imagery, if anything. In CG, there seems to be a difficult balance between sharpness and antialiasing. If you try to get too close to the Nyquist limit, you suffer more aliasing, and to get rid of all signs of aliasing, you have to compromise on sharpness. Personally, I’ve started to prefer using a Gaussian filter, but many people in digital imaging feel it’s way too blurry. My theory is that we are pretty good at seeing sub-pixel detail even in frequencies that are well (e.g. 2x) below the Nyquist limit, that having something less sharp than ideal only compromises the aesthetic, but not our ability to see the details of what’s there. I did a video project a long time ago in old 480i NTSC and found that filtering vertically twice as much as should be called for improved my ability to see fine details (that is, filtering as if the vertical image resolution is 240). I realize that motion and interlaced video is super different than still photography, but my lesson there has been influencing all my filtering decisions ever since...

deleted 8y ago
[deleted]
theoh 8y ago
So, I'm assuming you know this, but the theoretically optimal pixel reconstruction filter is the sinc function. https://en.wikipedia.org/wiki/Sinc_filter https://en.wikipedia.org/wiki/Sinc_filter For very high-quality work, I guess we have enough processing power to approximate sinc quite well these days. Is that what you mean by trying to get close to the Nyquist limit? The sinc filter does tend to introduce ringing artifacts. Very high resolutions are now practical, too, which changes things a lot. Probably in favour of using filters which never introduce ringing artifacts, like the Gaussian. (And things like the Lanczos or Mitchell filters exist as compromises between sinc and Gaussian.)
- dahart 8y ago
  Yeah, I know about sinc and the ideal theory, but nobody uses sinc for production work. Your filter has to be too wide, and the ringing artifacts are too undesirable. The max sharpness in a digital image, the Nyquist limit, is 1 cycle across 2 pixels. It’s not possible to get any sharper than that. But actually rendering images with frequencies exactly at that limit generally cause aliasing artifacts of some kind. That’s what I mean about getting close to Nyquist. > Very high resolutions are now practical, too, which changes things a lot. Probably in favor of using filters which never introduce ringing artifacts, like the Gaussian. Yeah, this is mostly true, and it is one way I justify using a Gaussian. But it’s important to know and remember that without the right filter, you can get visible ringing artifacts no matter how high your resolution is, and no matter how much you super-sample your pixels. This doesn’t come up all that much in CG film or photography, but I have a digital art project where it matters a lot and comes up all the time. (And I order expensive prints too, where artifacts not very visible on my monitor can become plain & obvious on paper). > And things like the Lanzos or Mitchell filters exist as compromises between since and Gaussian. Yes, and I suspect those are the most common ones used in VFX and CG film & print production. But those are balancing sharpness and aliasing. For situations where I really need no aliasing, they aren’t good enough. I can’t usually see the difference between Gaussian, Lanczos and Mitchell without zooming and studying high frequency regions, but I’ve watched VFX supervisors comment on the softness they feel after watching like 2 seconds of film. Some people have an incredible sensitivity to sharpness.
  - brudgers 8y ago
    + My understanding of the way the Nyquist limit plays out in digital photography is that it guarantees no moire. If moire isn't an issue in a scene then it is possible, but not guaranteed to get higher resolution (that of the individual pixel). Of course resolution and sharpness are not exactly the same thing...that's why a 16 mega pixels is a resolution not a sharpness and why a lot of studio photography uses cameras without anti-aliasing filters (e.g. medium format digital). In the studio, conditions can be set up to avoid moire in ways that aren't practical for sports or wedding photography. Similarly, moire isn't going to be a problem with astro-photography and some subjects in landscape photography. + For what it's worth, ringing artifacts are often caused by diffraction or lens aberrations rather than aliasing from sampling frequency (though they can be). + The shape of the modulation transfer function (MTF) is different between film and digital. The line pair comparison in the article illustrates it without being explicit. The Nyquist limit means that sharpness falls off a cliff in digital images and the example line pairs go from resolved to gray abruptly. The example line pairs for film fade to gray, what is and isn't sharp is determined by observation rather than an equation. Digital images often appear sharper because they are only ever so sharp and film appears softer because it is kinda sorta sharp beyond the point where it is clearly sharp. It's why film has a different look...or is a different artistic medium than digital when it comes to some forms of expression.

manaskarekar 8y ago

Thanks for sharing his inspirational story in this old thread: https://news.ycombinator.com/item?id=7540832 https://news.ycombinator.com/item?id=7540832