Must scaling always take place in the progressive domain? - AVS Forum
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post #1 of 6 Old 05-19-2012, 09:21 AM - Thread Starter
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I recall reading ages ago that scaling must always be done with progressive frames. So for example, if you're sending 480i to a processor and then outputting 1080i to your display, the processor will deinterlace to 480p, scale to 1080p and then re-interlace to 1080i.

Is this true? I'm wondering how significant the consequences of poor deinterlacing are when considering the above process. Even if the processor does a fantastic job of scaling, if it doesn't have good cadence detection, then it will stay in 'video' mode, thus reducing the overall resolution of the scaled image.

Further to that, if its video mode is also poor, e.g. it introduces lots of jaggies, then presumably these jaggies will remain as artefacts in the image even when the output is 1080i, because those artefacts will become an integral part of the image as far as the scaling engine is concerned.

As an example, I send 1080i from my satellite box to my Edge, because otherwise I would have to wait about 10 seconds for the Edge to re-sync every time I switched between an HD channel and an SD channel. But since the sat box is scaling from SD to 1080i and probably doesn't contain a decent deinterlacing/scaling solution, then I'm wondering how much image quality I'm losing through this process?

Thoughts anyone?

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post #2 of 6 Old 05-27-2012, 05:25 AM
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Your assumptions are correct. If there is poor deinterlacing occurring, those artifacts will be scaled along with the rest of the image.

What I'd recommend doing is to DVR some SD content (if possible some film and video) ... and then do some testing to see how the image quality compares between using the satellite and Edge for scaling.





On a side note, you're exaggerating the sync delay on Edge. I have one, and my FiOS DVR is set to native output. Granted I don't watch much SD content, but it switches to 720p when I hit Disney Television Group channels ... and also switches to SD for the on-demand menu (they've upgraded the UI/guide for everything but that). Yes the sync delay is annoying, but you would have shot your TV by now if it was really 10 seconds
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post #3 of 6 Old 05-30-2012, 09:19 AM
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Yes, scaling must be done in progressive form. Interlacing is always line-by-line, so scaling an image with interlacing (before deinterlacing) will result in >1 pixel lines (and most likely not even an integer multiple, which makes it even worse) which will be essentially impossible to feed through a deinterlacer since deinterlacers expect 1-pixel vertical weaves, not >1pixel. And even if you did scale them, you better hope it's a bilinear scaler (which is not the best quality scaler, though it's a lot better than nearest-pixel) and not a bicubic or better, because bicubic would take information from the lines adjacent in the field, which is incorrect, since adjacent lines in a field are not adjacent lines in a frame. So bicubic scaling, while usually better than bilinear/NP, will make things even worse.

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post #4 of 6 Old 06-13-2012, 12:14 AM
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Interesting topic. Nothing in practice does this, for good reason, but theoretically you can. In addition to the many practical issues, you also impose a lower upper-limit in quality if you reverse the steps.

Consider a simple (mathematically convenient) case: 640x480i30 -> 1280x960p60

And the two block diagrams which switch the order of operations:
1) Input -> Deinterlace -> Resize(2x,2x) -> Output
2) Input -> Resize(2x,2x) -> Deinterlace -> Output
Where H & V in Resize(H,V) are the scale factors in the horizontal and vertical directions respectively.

Modern deinterlacers have three main modes of operation:
Video: Intra-Field (Interpolation)
Video: Inter-Field (Merging)
Film (Inverse Telecine)


When operating purely in intra-field mode, you can think of the input as 640x240p60. Then the deinterlacing operation is effectively a x2 resize in only the vertical direction: Resize(1x,2x). You can simplify the block diagrams by combining the resizing blocks and now both diagrams become the same thing:
Input -> Resize(2x,4x) -> Output
So in this case, there is no difference.

Motion adaptive deinterlacing is now the norm for video content. This is where intra-field and inter-field modes are selected on a pixel-by-pixel basis depending on if the pixel is determined as moving or stationary. When a pixel is deemed as stationary, inter-field mode is engaged and the fields are merged to reconstruct that point at the full resolution (detail). A moving pixel triggers intra-field mode. In film mode, entire fields are merged to undo the Telecine process.

Blah, blah, blah the takeaway is: by scaling the image prior to deinterlacing, you ruin the opportunity to perfectly reconstruct stationary portions of video content and also entire frames of film content. The lowpass filtering involved in resizing operations destroys this advantage, so that's why you don't reverse the steps.
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Originally Posted by DonoMan View Post

Yes, scaling must be done in progressive form. Interlacing is always line-by-line, so scaling an image with interlacing (before deinterlacing) will result in >1 pixel lines (and most likely not even an integer multiple, which makes it even worse) which will be essentially impossible to feed through a deinterlacer since deinterlacers expect 1-pixel vertical weaves, not >1pixel.

Great example of one of the practical issues you run into when reversing the operations.

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Originally Posted by DonoMan View Post

And even if you did scale them, you better hope it's a bilinear scaler (which is not the best quality scaler, though it's a lot better than nearest-pixel) and not a bicubic or better, because bicubic would take information from the lines adjacent in the field, which is incorrect, since adjacent lines in a field are not adjacent lines in a frame. So bicubic scaling, while usually better than bilinear/NP, will make things even worse.

I don't understand. Why would a bicubic filter do that while bilinear and nearest-pixel don't? They are the same type of filter; they just have different coefficients. I can easily make a bilinear filter that uses lines adjacent in the field. Are you talking about specific implementations?
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post #5 of 6 Old 06-14-2012, 08:09 AM
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Quote:
Originally Posted by LordSte View Post

I don't understand. Why would a bicubic filter do that while bilinear and nearest-pixel don't? They are the same type of filter; they just have different coefficients. I can easily make a bilinear filter that uses lines adjacent in the field. Are you talking about specific implementations?

Bilinear by definition uses only the current line. If you change bilinear to use the adjacent vertical lines, then you have turned it into a bicubic filter.

Many types of scaling have coefficients that can be changed, but each type of scaling is different. The algorithms between NN, bilinear, bicubic, spline, Lanczos, etc., are all different. The type of scaling used has a major effect on the picture quality.

"Vintage" is good for wine, not for A/V equipment.

-Dan D.
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post #6 of 6 Old 06-16-2012, 09:18 AM - Thread Starter
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Thanks for the responses. Some interesting reading.

And yes Raistlin, I was exaggerating the sync delay. It's still a pain though! smile.gif

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