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MobiusStrip
12-09-2009, 01:43 AM
Hi all.

I read a few articles about this issue and looked at a bunch of footage. It seems to me that there are some suspect "explanations" floating around.

The HD-sized images from these cameras are being downsized from a vastly larger frame; I'd think any aliasing due to the chip and its OLPFs would occur at the native resolution of the chip. Resizing it down to HD resolution would reduce (if not totally obliterate) this extremely fine aliasing. Barry notes this effect too:

“...since web video is frequently scaled down in size (and scaling down helps to reduce aliasing artifacts) they released the 5D Mk II as-is...”

Scaling an image larger than 5000 x 3000 down to 1920 x 1080 should not result in more aliasing than a full-sized still. And 1280 x 720 should look smoother still. But in fact we don’t see that at all; most people think the 720 footage looks MORE aliased (although that’s probably because they’re blowing it up to the same size as the 1080 for viewing).

The aliasing in DSLR video almost certainly comes from one thing: a crappy resizing algorithm. What happens if you resize an image with no resampling? Try it in your favorite image-editing program. The result: wretched aliasing.

To achieve the necessary processing throughput to deliver “HD” video without cropping the sensor, these cameras are taking shortcuts. They’re not downsampling the images well, and the result is aliasing galore. They may even be doing some abbreviated deBayering to speed things up. I question the idea that somehow the aliasing occurs because the chip is "not optimized for video." The limitation here is computing power and data throughput. As we all know, these are likely to improve over time. My guess is that the images from the exact same sensor, run through a good-quality downsizing algorithm, would match anyone's expectations for HD. I also think that even with a more aggressive OLPF array on the sensor you'd still see aliasing, thanks to the bad downsizing.

Sumfun
12-09-2009, 04:19 AM
To achieve the necessary processing throughput to deliver “HD” video without cropping the sensor, these cameras are taking shortcuts. They’re not downsampling the images well, and the result is aliasing galore. They may even be doing some abbreviated deBayering to speed things up. I question the idea that somehow the aliasing occurs because the chip is "not optimized for video." The limitation here is computing power and data throughput. As we all know, these are likely to improve over time. My guess is that the images from the exact same sensor, run through a good-quality downsizing algorithm, would match anyone's expectations for HD. I also think that even with a more aggressive OLPF array on the sensor you'd still see aliasing, thanks to the bad downsizing.

If you took a full size image from the sensor and downsized it properly, then sure the image would look great. But the problem is that the sensors on current HDSLR's are not capable of doing 24 full scans per second. That's why they have to "cheat" by doing line skipping or binning, thus resulting in aliasing. So yes, they need more processing power to run a good scaling algorithm, but that processing power would be wasted if the sensors are not fast enough.

I believe the camera vendors are all aware of this, and are working towards a solution. So far, Red is the only one who has announced that they will scale HD video down from a full size sensor scan.

bwwd
12-09-2009, 04:43 AM
and over and overr and over again, we know that,we are aware of that.
Ive seen aliasing with my very own eyes,i was looking at the fence from angle when i was in the car, guess i dont have enough res or crappy resizing algorithm.
You can hide aliasing in many shots if you are careful.

trez
12-09-2009, 08:48 AM
While I agree with you, MobiusStrip, that aliasing is introduced by poor resizing algorithm there are some misconceptions in the discussion imo:

1. In general, downscaling does NOT reduce aliasing (already present in full raster image). Properly done (low-pass filtered) downscaling can hide some high frequency aliased components, but aliasing is usually spread over the entire (spatial) frequency range.
2. Canon claims that 5d2 and 7d do not use line skipping. They say they use pixel binning. As far as I understand this means that all sensor photocells are scanned and then groups of them are averaged (or subject to some primitive filtering). I suppose Canon uses simplified debayering scheme to reduce the computational complexity, which results in ugly chroma channel contamination, which is not present in other hdslrs (f.ex. in gh1 only luma channel suffers from aliasing). If they'd been skipping the lines, chroma channels would have stayed clean while shooting b&w resolution chart (nothing but the grayscale values would have showed up). Also, skipping the lines means much lower sensitivity and greater noise floor. Think this is the reason why Canon decided to use pixel binning instead.
3. Optical low-pass filter with cutoff frequency properly set for HD video would actually help the aliasing (even if simplest form of downscaling is applied - line skipping or pixel binning). However, in such case there would be no benefit of oversampled sensor (improved MTF, greater resolving power). Anyway, this would be better than ugly aliasing... Resolving potential is lost in primitive downscaling anyway.

Looks like we have to wait for next generation chips. Or 2mpix dslrs ;)

mbell75
12-09-2009, 12:57 PM
This is silly. First of all, Canon is never going to make a $1700 or even $2700 SLR camera with huge sensors take perfect video. They would kill their video business. Everyone should know by now that the still cameras are just that, STILL cameras that happen to shoot video. So please, all the video guys keep shooting traditional video cameras and be happy with that. The rest of us who dont need as much or care about certain things will be just happy with our crappy aliasing SLRs :happy:

Barry_Green
12-09-2009, 01:37 PM
This is silly. First of all, Canon is never going to make a $1700 or even $2700 SLR camera with huge sensors take perfect video. They would kill their video business.
I disagree. I really don't think the companies worry about that. Did Canon worry when they brought out the XHA1 that it would kill the XLH1? No, because every other manufacturer brought out equipment that killed the XLH1 anyway. At least Canon got to keep all the XHA1 sales they could manage.

If Canon doesn't bring out a huge-sensor video camera with great video, then they'll find that soon enough someone else will, and then their video business will be killed anyway. That's called progress, and it happens every day around here. Everyone's lineup gets killed off.

So the question is: will Canon keep the sales (by making their own) or let someone else have all those sales? If they've got any brains, they'll be the ones that make the big-sensor video camera, and if the rest of their lineup can't compete? Screw it. That's the law of the jungle.

Now, back to the question at hand -- the tech simply doesn't exist to do a sensor like what would be required. Scarlet's the closest, and they're only 2/3" and a complete kit is $5,000. The 7D's chip is about 5x bigger than the Scarlet's 2/3" chip.

You're not going to see a full-frame large-sensor camera that scans the entire chip and scales it down high-quality, realtime, at video frame rates, at DSLR price points, in the very near future. Yes they'll get there, but just like you're not seeing 10GHz 32-core CPUs out right now, so also you're not going to see 60fps full-frame video camera sensors.

mbell75
12-09-2009, 05:57 PM
I disagree. I really don't think the companies worry about that. Did Canon worry when they brought out the XHA1 that it would kill the XLH1? No, because every other manufacturer brought out equipment that killed the XLH1 anyway. At least Canon got to keep all the XHA1 sales they could manage.

If Canon doesn't bring out a huge-sensor video camera with great video, then they'll find that soon enough someone else will, and then their video business will be killed anyway. That's called progress, and it happens every day around here. Everyone's lineup gets killed off.

So the question is: will Canon keep the sales (by making their own) or let someone else have all those sales? If they've got any brains, they'll be the ones that make the big-sensor video camera, and if the rest of their lineup can't compete? Screw it. That's the law of the jungle.

Now, back to the question at hand -- the tech simply doesn't exist to do a sensor like what would be required. Scarlet's the closest, and they're only 2/3" and a complete kit is $5,000. The 7D's chip is about 5x bigger than the Scarlet's 2/3" chip.

You're not going to see a full-frame large-sensor camera that scans the entire chip and scales it down high-quality, realtime, at video frame rates, at DSLR price points, in the very near future. Yes they'll get there, but just like you're not seeing 10GHz 32-core CPUs out right now, so also you're not going to see 60fps full-frame video camera sensors.


You are right. What I was saying is if Canon is going to put out something killer like that, it will be in a true video camera. Not an SLR that shoots video. I think most are well aware of the faults that the SLR video has, but for some (including myself) those things wont matter. I love it for the awesome DOF I can get with my fast prime Canon lenses. I still have yet to come across a video camera in the $2k-$6k range that can give the same DOF I can get with a Rebel T1i and my 85 f/1.2 Not even close. I also love the ability to shoot in natural and lower light thanks to the bigger sensor. I am shooting models with it, not action or sports so for my needs, its great and the aliasing never comes into play with what I shoot.

MobiusStrip
12-10-2009, 12:42 AM
This is silly. First of all, Canon is never going to make a $1700 or even $2700 SLR camera with huge sensors take perfect video. They would kill their video business.

I don't think so. If anyone is in a position to do it, Canon is. They don't have any high-end video business to protect, like Sony. They've admittedly always shown a particular cluelessness about video with their foot-dragging on progressive scan and film-friendly frame rates, but if they pull their head out of their ass, they can own this space in the future.

Barry_Green
12-10-2009, 08:34 AM
Again, this idea of "protecting" your high-end video business is something I disagree with, and I don't think we see it happening; if a manufacturer artificially curtails advancements to "protect" their overpriced existing gear, they'll quickly find out that some other manufacturer will swoop in and steal their market share. Sony used to do this type of thing a lot, and look where it got them. I don't think they bother anymore.

Secondly, if anyone DID have to worry about protecting high-end video, it's Canon. Canon is a lens company, they make lenses for Panasonic and Sony and Ikegami and Hitachi and Thomson and every other high-end camera. Those companies are Canon's *customers*. In the 5D video of Tim Smith, he said Canon is a LENS company and they want to do whatever they can to sell more lenses. So if Canon goes and makes something that obsoletes other companies' high-end product lines, they will be slitting their own throat by killing off their high-end lens business.

So, it can't really go both ways. Either a company refuses to introduce new products because they're trying to artificially constrain the marketplace and "protect" their higher end products, or they don't.

If they do constrain, then Canon is ill-positioned to make the introductions.

If they don't constrain, then the technological restrictions are what prevent new product introductions.