[opendtv] Re: Math of oversampling - a simple comparison
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 27 Apr 2005 08:36:42 -0400
Oops. That url should have been:
<www.trbarry.com/Leno_1080i.jpg>
Sorry,
- Tom
Tom Barry wrote:
> Jeroen Stessen wrote:
> > So, Tom, if you would please send me a single frame ?
>
> Jeroen -
>
> I've place a jpeg compressed version of the full untouched frame at:
>
> <www.trbarry.com/Leno_1080i> so you or anyone else can compare and
> play with it better. Be interesting to see what you come up with.
>
> I think it was exactly the right frame but I foolishly did not
> before write down the frame number so I had to go hunt for it.
>
> - Tom
>
>
>
>>Hello,
>>
>>Tom Barry wrote:
>>
>>
>>>>Okay, here's one from Leno. I had to wait to record it last
>>>>night. As before the left half of the image is the original and
>>>>the right half is filtered to an effective 1/4 resolution of only
>>>>960x544, though no actual resizing was done.
>>>>(...)
>>>>See: http://www.trbarry.com/Leno_1080p_compare_qtr_rez.jpg
>>
>>
>>Kon Wilms wrote:
>>
>>
>>>Look at the flag on Leno's chest (yes, that's a flag). Look at the
>>>coffee cup on his table, vs the one on the left side of the photo. Look
>>>at the jaggies on the building set behind his head. Those are some bad
>>>artifacts.
>>
>>
>>
>>I've viewed the image with ACDSee, which can zoom in by using bicubic
>>interpolation instead of pixel repetition. The right half of the
>>picture is suspiciously blocky, which is not something that we would
>>expect from only 2D lowpass filtering...
>>
>>I have to say it again: that Tom's method of applying 8x8 DCT and
>>then setting the 48 higher coefficients to zero is NOT a good method
>>for simulating down-sampling and up-sampling. It is only a good
>>method for creating blocking artefacts. Sorry, Tom.
>>
>>We are trying to simulate the what-if case: first down-sample
>>1920x1080(p) to 960x540(p), then apply MPEG-2 compression and de-
>>compression (with 120x70 DCT blocks of 8x8 each, right ?) and then
>>up-sampling back to 1920x1080(p). I say (p), because this may have
>>been an image from a 1080i transmission, but with so little motion
>>the de-interlacing to 1080p is trivial.
>>
>>There is a big difference between letting high frequencies disappear
>>through proper down-sampling (with anti-aliasing filtering and all)
>>or by first cutting the image into blocks and then discarding the
>>higher DCT coefficients. That is because the block edges themselves
>>contain higher harmonics, which are not part of the original image.
>>Discarding these higher harmonics is what causes the blockiness...
>>Proper down-sampling works over the entire image, not over blocks,
>>it is therefore immune to blockiness.
>>
>>Therefore I ask Tom to provide me with the original image, lightly
>>re-compressed please (that is, a JPEG that is no worse than the
>>original MPEG), and then I can apply actual down- and up-sampling
>>to the entire picture and send it back to Tom.
>>
>>I find it more illustrative to toggle between two pictures (ACDSee
>>can do that, or we even have a window-wiper program for that), than
>>to compare two sides of a single image, with a different content.
>>
>>So, Tom, if you would please send me a single frame ?
>>Or else Hans, from the EBU, can give me some other HD frame.
>>
>>Greetings,
>>-- Jeroen
>>
>>+-------------------------------+------------------------------------------+
>>| From: Jeroen H. Stessen | E-mail: Jeroen.Stessen@xxxxxxxxxxx |
>>| Building: SFJ-5.22 Eindhoven | Deptmt.: Philips Applied Technologies |
>>| Phone: ++31.40.2732739 | Visiting & mail address: Glaslaan 2 |
>>| Mobile: ++31.6.44680021 | NL 5616 LW Eindhoven, the Netherlands |
>>| Pager: ++31.6.65133818 | Website: http://www.apptech.philips.com/
>>|
>>+-------------------------------+------------------------------------------+
>>
>>
>>
>>
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