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1080i has rectangular pixels

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Lorin Thwaits1080i has rectangular pixels
by on Apr 6, 2004 at 1:07:28 am

Wow, I just got back from the Bahamas never realizing I was being pummelled all the while!

[Guy Barwood] "isn't full 1080i 1920x1080 (square pixles)? "

[Borjis] "Yeah, where did you get that 1080 uses rectangular pixels?
Those days are over. Both Formats use square pixels."


[Rick Spurway] "Absolutely. See this page on my website..."

What I was referring to with a claim of rectangular pixels is two-fold: both on the CCD itself and in the HDV recording format. The CCDs being used in Sony's new HDV camera have an effective resolution of 1440x1080. At 16:9 that ends up being rectangular pixels, as each pixel is stretched 33% more horizontally. (Really!) No big loss since rarely do 1080i displays employ a full 1920 pixels. High-end units can address 1440 pixels, and low-end ones only 1000 or so. The HDV format itself only records 1440 pixels across, but then so does Sony's professional HDCAM format, the same used in Star Wars I. Rectangular pixels, guys...

Panasonic's Varicam actually throws out even more with 1080i, ending up with really wide 2:1 rectangles for pixels. (They lay down 960x1080 on the tape.) With the 1080i format, currently you can get square pixels only if you capture using SDI directly to a hard drive or the newest tape formats. So for the rest of us it's gonna be rectangular. Of course in time this will change, but it will be a few years until we're capturing at a full 1920x1080 resolution with prosumer gear. It will require at least 50Mbps bandwidth to do it any justice, and 100Mbps to get excellent 4:2:2 color.

Anyway, back to the here and now. Here's more detail about 1080i OTA signals according to ABC's FAQ on HD: "The 1080i format cannot be compressed to fit in a 6MHz channel without creating objectionable artifacts and it has been recommended that the 1920 pixels be sub-sampled to 1440 to reduce compression artifacts. Therefore, encoder manufacturers have elected to discard approximately 25 percent of the picture for over-the-air transmission. This compromise is not required for 720p. More of the original picture information remains through the transmission chain."

(Note that 6 MHz broadcast spectrum does NOT translate into 6 megabits per second. It means 19.4 Mbps according to the ATSC standard.)

Now I've probably opened "Pandora's Box" as people become flabbergasted with it all. Just promise you won't respond until you've done your homework.

-Lorin


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Steve MullenRe: 1080i has rectangular pixels
by on Apr 6, 2004 at 2:11:21 am

Amazing --I've been thinking about pixels too.

It turns out that since most CRTs cannot display much more than 500-600 lines vertically that some HDTVs simply lay down the second 540-line field right over the first 540-line field.

Now I know you believe the Sony CCDs for the consumer HDV are 1440x1080 -- but I've been wondering if they might only be about 576 which is the PAL CCD spec. There is a curious note in the Sony release about "using all 576-lines."

In short, I'm wondering if Sony will take advantage of the fact that there can be far fewer vertical lines of rez than are recorded. In fact, even the JVC maps the CCD to 720-line spec.

Why am I thinking along these lines? The fewer the CCD elements, the more light sensitivity and the greater the light latitude.



Steve Mullen
Digital Video Consulting
http://www.mindspring.com/~d-v-c
========
* GR-HD1 and JY-HD10 HDV Shooting Guide
* HDVcinema Pro -- realtime Edit HDV with FCP
* DVX100 User's Handbook


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Guy BarwoodRe: 1080i has rectangular pixels
by on Apr 6, 2004 at 4:00:24 am

I won't argue with you as I am trying to learn not teach. What I would like to come across is a clear explanation of how this all effects the image we see in different display technologies and from different CCDs etc.

ie, say with a Plasma, or LCD, we have discrete pixels. Are they all square or rectangular (if so what A/R? and is it consistant) etc Then take all these various HD formats and explain how each is displayed. ie if a pixel is origonally square, then resampled to recangular (1920 to 1440), and then shown on a device with square pixels, isn't there going to be 2 generations of image degridation due to all this resampling etc

Perhaps at the moment its not as important but in the future it may be as displays get higher and higher res (until the point where everything we have is 1920x1080p capable.....). Should we be looking for a display with a native res of 1440x1080 (16:9 rect), 1440x810 (16:9 square), 1280x720 (16:9 rect) etc

What about CCDs? Excluding again native 1920x1080, for the HDV specs, would the primo CCD be 1440x1080 16:9 rect? How will that effect 1280x720p capturing? Will there be a sharpness loss due to converting a rect sampling area to a square sample even though we are starting with a higher res image?

I guess I would like to have more experience in it myself. For a while there I thought this confusion of rect vs sqr pixels was over.

Regards

Guy


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Steve MullenRe: 1080i has rectangular pixels
by on Apr 6, 2004 at 5:10:16 am

I've always been confused by the difference, just followed the rules.

But now it seems the confusion is back -- this time about HD.

And, your concern about remapping is valid.

A lot of Plasma and LCD problems seem to come from resampling to their native rez from NTSC, PAL, VGA, XGA, and HD. This is necessary because Plasmas/LCDs are not always built to be either 1280x720 or 1920x1080. Some very odd sizes are being built.

And the JVC remaps 6XX lines to 720. So, yes, there could be multiple mappings. One at th CDD and one at the Plasma/LCD.

Any clarification will be welcome. Particularly as it relates to HDV.


Steve Mullen
Digital Video Consulting
http://www.mindspring.com/~d-v-c
========
* GR-HD1 and JY-HD10 HDV Shooting Guide
* HDVcinema Pro -- realtime Edit HDV with FCP
* DVX100 User's Handbook


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Lorin ThwaitsRe: 1080i has rectangular pixels
by on Apr 6, 2004 at 4:14:40 pm

[Guy Barwood] "With a Plasma, or LCD, we have discrete pixels. Are they all square or rectangular (if so what A/R? and is it consistent?)"

Not consistent unfortunately, but there are some standard groupings of displays. A few 1024x576, some 1280x768, many 1366x768, some 1440x1080, and a very few truly square pixeled 1920x1080. Aspect ratio of the full panels of all these is 16:9, from which you can further calculate individual pixel aspect ratio.

[Guy Barwood] "If a pixel is origonally square, then resampled to recangular (1920 to 1440), and then shown on a device with square pixels, isn't there going to be 2 generations of image degridation due to all this resampling etc "

Yeah, upon playback of footage we record there are then two separate chances the image has had to degrade. Here's a rundown:

First chance for loss: During Capture
Hopefully we're recording on tape a 1:1 mapping to what's on the CCD. That's the case with 720p in the JVC GR-HD1 cams and possibly for the upcoming XL2. Also the case with 1080i in the Sony cam. No loss from resizing that way. But if you record 720p with the Sony then there is some loss. I don't know the mechanism they've decided upon for this, but I would expect the downsampling has been done in one of two possible ways:

  • (More unlikely) By taking a middle 960x720 16:9 rectangle of pixels out of the full 1440x1080, preserving the same resolution in the Y-axis, and then stretch the image up to the full 1280x720 size

  • (More likely) Downsample both vertically and horizontally to 1280x720
I honestly don't know which of the two would actually look better, but I assume that the second option would probably retain more resolution. Pretty tricky stuff since I believe the CCD is made to be scanned in an interlaced way.

All speculation about Sony's CCD aside, it's best to record at whatever the native resolution of the CCD actually is.

Second chance for loss: During Playback
When displaying the image, if it's being upsampled horizontally as with going from 1440 up to 1920, there's some loss, but fairly minimal. More troublesome with 1080i is upsampling vertically, which does have some loss. Getting around the interlacing is tricky when stretching like that. For those with lower-res displays, downsampling is always lossy.

[Guy Barwood] "Perhaps at the moment its not as important but in the future it may be as displays get higher and higher res (until the point where everything we have is 1920x1080p capable.....). Should we be looking for a display with a native res of 1440x1080 (16:9 rect), 1440x810 (16:9 square), 1280x720 (16:9 rect) etc"

The best option is of course the full 1920x1080 resolution. Short of that, something with the same vertical resolution would be better when displaying 1080i. Because 1080i is interlaced, more artifacts emerge when downsampled.

[Guy Barwood] "What about CCDs? For the HDV specs, would the primo CCD be 1440x1080 16:9 rect? "

For capturing at 1080i, yes it would. But then for capturing 720p, a 1280x720 would be best. 1:1 mapping is preferred. If it's done right, downsampling from 1440x1080 to 1280x720 shouldn't be too bad. It will be fun to take a peek frame by frame in the Sony cam with that type of downsampled video to see how many artifacts emerge.

[Guy Barwood] "For a while there I thought this confusion of rect vs sqr pixels was over."

How I wish! Oh well, we're still getting a tremendous boost in resolution through all this.

-Lorin


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BorjisRe: 1080i has rectangular pixels
by on Apr 6, 2004 at 7:48:13 pm




Well, All i know for a fact is, when rendering CGI or F/X work
with After Effects and 3D Studio Max, you render square pixels
when doing 720 or 1080 HD rez. Thats the way the cookie crumbles.


what happens from there to display I won't argue with and
leave to those experts (Lorin) :)



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Steve MullenRe: 1080i has rectangular pixels
by on Apr 6, 2004 at 11:49:56 pm

And the JVC definitely has square pixels.

Moreovr, the JVC CCD generates less than 720-lines (656 I think) that JVC has told me are mapped to 720-lines.

Steve Mullen
Digital Video Consulting
http://www.mindspring.com/~d-v-c
========
* GR-HD1 and JY-HD10 HDV Shooting Guide
* HDVcinema Pro -- realtime Edit HDV with FCP
* DVX100 User's Handbook


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Lorin ThwaitsRe: 1080i is rectangular / JVC doesn't use a 1280x960 CCD?!?
by on Apr 7, 2004 at 3:43:18 am

[Steve Mullen] "the JVC CCD generates less than 720-lines (656 I think) that JVC has told me are mapped to 720-lines"

Wow! I didn't know that. I thought the effective resolution of their imager was a full 1280x960, and recording in 720p was done by just trimming off a 120-pixel high swath on both the top and bottom, leaving a 1280x720 window to draw from. You're sure there's not a full 720 pixels vertically when recording in HD?

Taking this thought further, if this is true and the CCD indeed has square pixels, that means that there's only 1166 pixels on the X-axis. Messy business upconverting that to 1280x720. If that's the case then I would prefer a letterboxed true 1166x656 image rather than the upscaled 1280x720 that blurs some pixels in the process.

I could have sworn I had seen specs on the JVC CCD as having 1280x960 raw pixels native, but if it's less then I'll stand corrected. Can anyone substantiate this notion?

-Lorin


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Lorin ThwaitsRe: JVC cams -- I stand corrected!
by on Apr 7, 2004 at 4:21:49 am

[Steve Mullen] "the JVC CCD generates less than 720-lines (656 I think)"

Okay, I think I've found the source you mention for 659 line video. There's both info about the European version, and info about the North American / Japanese version to support this claim. (Scroll down to the Specifications in the second document for all the real native resolutions being used in different modes).

The JVC cams do in fact have the limited vertical resolution you describe. The pixels are apparently not perfectly square, but rather stretched a bit vertically because the imager does still render a full 1280 pixels in the X-axis. Fairly strange conversion then going up from 659 to 720 in the Y-axis. A real shame it's not really 1280x720 to start with. And now it makes me even more curious to see what Sony's offering may be doing in terms of upsampling.

-Lorin


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Steve MullenRe: 1080i is rectangular / JVC doesn't use a 1280x960 CCD?!?
by on Apr 7, 2004 at 4:58:27 am

You are right about the CCD -- JVC grabs a 16:9 window from the 4:3 chip.

Then the DSP computes the luma and chroma: "the JVC chip in 720p mode generates 842,861 luma samples from 659 CCD rows." AND "All single-CCD cameras generate luma samples by means of a sliding filter that moves across pairs of CCD columns. This filter decreases horizontal resolution by about 25%. Therefore, 1,280 elements (HD mode) yield a video row with 960 pixels."

JVC confirmed the camera's DSP scales the image up to 720 lines. But I never asked about scaling 960 to 1280.

However, I guess these 960 samples probably are mapped up.







Steve Mullen
Digital Video Consulting
http://www.mindspring.com/~d-v-c
========
* GR-HD1 and JY-HD10 HDV Shooting Guide
* HDVcinema Pro -- realtime Edit HDV with FCP
* DVX100 User's Handbook


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