Originally Posted by mark haflich
RC is not a scaling alg. Sony's scaling algs (from whatever up to UHD or 4K all deliberately are designed to ring. Ringing is part of the process. scaling reduces sharpness because of the artificial creation of additional lines. Something must be done to bring back the sharpness lost. Ringing makes things look sharper to ones eyes. RC does not introduce more ringing. But RC is not scaling. It is applied to the results of the scaling. I prefer scaling which does not ring even though it will appear less sharp than a scaling which introduces (deliberately) ringing.
Please stop spreading misinformation. No, "scaling does not reduce sharpness because of the artificial creation of lines" - this shows a complete lack of understanding of sampling theory. You try to reconstruct the original image (as in before it was sampled in the first place) from a sampled representation on a higher resolution. So infact you are resampling the already sampled image, but always with the goal in mind of "faithfully" reconstruction the original (pre-sampled) image.
The following thought experiment should clear that up if you upscale to a resolution two times higher in both dimension (as in HD to UHD) you could just duplicate every pixel (called point sampling). This would NOT introduce any ringing, and the original sampled image is preserved completely (as the real output resolution of yourse is the same, despite upscaling). Neither is its sharpness reduced, nor is there any "artifical" creation of lines going on. This holds for linear interpolation and many other interpolation methods, if the interpolation kernel is made to be sharpness-neutral. I provided the link with images scaled to different resolutions by Bjorn Roy - these also do not exhibit any ringing, yet look more detailed (please for once look at the evidence). Linking sharpening to scaling is a design choice, but not because you have to get something back you lost, but to shift the trade-off between aliasing/bandwidth and "sharpness" (which is not a bad idea in itself ).
Also, Ringing (as I tried to explain earlier) is an visible artefakt of amplifying higher frequencies above the availabe bandwidth (equivalent to saying the resulting image lacks high enough frequencies due to being bandwidth limited - low passed) - but it remains an unwanted artefact. Ideally you want to boost detail (= higher frequencies) without ringing to become visible. It's not the ringing that looks sharper to ones eyes, but the larger amount of high frequencies does. Ringing is the overshoot/undershoot from excessive high frequency boosting in parts of the picture where our brain indentifies is as out of place. It just looks bad, and needs to be avoided. That's why for example limit-sharpen just defines a threshold that if exceeded reverts to a less sharpned version of the pixel.
Generally, the distinction between scaling and sharpening is not as you make it out to be, it can be folded into one operation (and very often is with for example lanczos scaling) or applied in sequence - this does not change any of the implications. Scaling does not need to "ring" and sharpening generally should try to avoid ringing. It is also possible to use an additional filter after scaling/sharpening to reduce ringing artefacts (e.g. JINC + AR in madVR)
If you would for one shed your "creationist" view of ringing and look at the links I provided, you would see that without RC the Sony reverts to point-sampling (or a very very mild linear interpolation, at best). If you do not trust the images in those links, you could just try for yourself with any 500es//600es and toggle RC on/off.. Also if you look at the upscaled images i provided, you will see no ringing with the RC. According to your theory, impossible. Yet, the ringing can clearly be seen with the Oppo upscaling and not on the sony RC picture. For your theory to be true, scaling would invariably introduce ringing, which would then be amplified by RC. But at least that does not show up in the images.
I stand by my opinion, that the Sony upscaling with RC does a pretty good job in regard to ringing. This does not mean that excessive use of RC comes without artefacts - grain and video noise is unaturrally amplified, and a generally the picture looks a tad artifical. I use general sharpness at 10 (as indentified by hdtvtest.uk as neutral - did not check myself yet) and RC-resolution at 20 with RC-noise-reduction at 10. Sony's defaults are much higher (resolution at 50, noise reduction at 25, sharpeness at 20?)