[opendtv] Re: Math of oversampling - a simple comparison

Dear Tom,=20

> I've place a jpeg compressed version of the full untouched frame at:
> <www.trbarry.com/Leno=5F1080i.jpg> so you or anyone else can compare and =

> play with it better.  Be interesting to see what you come up with.

I send you back two files:=20
<Leno=5F1080=5F544=5F1080p.jpg> and <Leno=5F1080=5F480=5F1080p.jpg>=20
(attachments deleted for OpenDTV).=20

The first  file was generated: 1920x1088 -> 960x544p -> 1920x1088,=20
the second file was generated: 1920x1088 -> 720x480p -> 1920x1088,=20

For the down-sampling I have used an 8-taps transposed polyphase=20
filter (equivalent to a 16-taps impulse response low-pass filter)=20
that has been optimized for low aliasing. It was applied in the=20
linear-light domain, to simulate what an SD (p) camera would do.=20

For the up-sampling I have used a 6-taps polyphase filter (6-taps=20
impulse response low-pass filter) that has been optimized more for=20
high sharpness than for low aliasing. It was applied in the gamma=20
domain, to simulate what a typical receiver would do.=20

There are some errors: the alignment of the output image is not=20
exactly correct with the input image, and there are black bars=20
around the image because I did not bother with filter run-in/out.=20
Those can be fairly easily corrected in the design, but I have=20
never bothered to. (That's why we have overscan, to not bother.)=20

I think that the sharpness loss is more significant than with=20
your example, but the artefacts are milder and this is a true=20
2-step process with real filters with finite bandwidth (slope).=20
So I think that this is a realistic example of what-if (480p).=20
It can be significantly improved by receiver-side sharpness=20
enhancement (=E0 la PixelPlus), but I did not apply that here.=20

I will post this same message, without attachments, on OpenDTV,=20
and will you please put the images on your web-site and post a=20
link to the messages ? Thanks.=20

Best regards,=20
-- Jeroen=20

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