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Sony HVR-Z1 / HDR-FX1
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Old January 21st, 2006, 06:35 PM   #1
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Reproducing Motion Artifact

I took my FX1 into my back yard to try to reproduce motion artifacts people seem to be indicating will occur at fast motion. So I started rolling tape, and turned 180 degrees very quickly panning from my pool with plants surrounding, to a fence with boards and plant in front to a bare fence. Then I chased the dog around the yard with a brick deck underneath, trying to create the artifact. Can't see it anywhere, but maybe I did reproduce it, but I just don't know what I am looking for. Can anyone tell me what I'm looking for or how to produce it ?

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Old January 21st, 2006, 07:17 PM   #2
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A tiny bump in the tape should create an artifact. (By dust is the best way.)
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Old January 21st, 2006, 07:30 PM   #3
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By Dust

Quote:
Originally Posted by Jack Zhang
A tiny bump in the tape should create an artifact. (By dust is the best way.)
Are you saying video tape a scene where dust is being raised ? Are there other situations ?

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Old January 22nd, 2006, 07:03 AM   #4
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I think you're talking about two different things. Chris is trying to recreate digital compression artifacts which occur at high motion in HDV because of the inter-frame compression. I think Jack is talking (if I'm correct) about getting a dropout on the tape, which might happen when dust comes in or something like that.

Chris: The (good) thing about motion artifacts is that the faster the camera moves, the more motion blur will be introduced, so that motion artifacts are less apparent. Also, the human brain notices less of these artifacts because it doesn't have time to look at one point for awhile. If you want to spot these artifacts, look at stills of your footage, and try to see things that aren't there in the real world. One of those things is macroblocking, which looks a bit like very large pixels which do have different colours in them, but with hard colour transitions to the next macroblock. It's a bit hard to explain, but it doesn't look right. You might also try to shoot with higher shutterspeed, so the motion blur will be less, so you can see the artifacts maybe more clearly.

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Old January 22nd, 2006, 07:20 AM   #5
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Also, here's a good thread about this subject: http://www.dvinfo.net/conf/showthrea...181#post417181

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Old January 22nd, 2006, 11:01 AM   #6
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Thanks for the Reply

Steven, Jack:

Thanks for the reply. I suppose I was looking for something more apparent and glaring. I will check out the thread Steve presented.

I just wanted to know what to avoid in future shoots.

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Old January 23rd, 2006, 04:19 AM   #7
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Very well.

And just for those interested, I found a more technical explanation for the macroblocking-syndrom. This quote is about the DVCPro-HD codec, but it applies to HDV as well. It is from www.cineform.com

"Finally, for those more technically minded, the DVCPRO HD codec compresses image data using a variation of a DCT (discrete cosine transform) algorithm. DCT compression divides images up into small (usually 8 pixel by 8 pixel) regions known as "blocks". Each block is transformed and compressed separately from all other blocks in the image. Under stressful (highly complex) images, these blocks begin to break down, yielding "block artifacts" familiar to many people."

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Old January 23rd, 2006, 05:41 AM   #8
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Chris,

You really don't need to worry. Obvious motion artifacts are not common. Don't believe the gloom and doom from people who have never used these cameras. I have seen many clips with incredible fast motion with no apparent "blockiness".

I have only seen one instance of compression motion artifacts from several hours of use on the FX1. It was from a scene of the sun setting over the ocean. The hundreds of tiny waves on the surface were varying between silhouette and brightly lit by the setting sun. The fast wave oscillation caused some areas to get blocky. It was only apparent on a large HD monitor with some scrutiny. Overall, it was a beautiful shot. I think the quick switching between black and nearly white in many areas of the scene caused the problem.

Fast objects moving through the scene don't seem to create artifacts as long as large portions of the scene aren't changing rapidly. For instance, a bird flying through the shoot looks like a bird and does not leave a bunch of blockiness behind in it's wake. People running through the scene look very clear.

Motion artifacts of the HDV codec are simply not a good reason to avoid the FX1/Z1. They are amazing cameras for the price and if you can display your work on an HD monitor they really look fantastic.

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Old January 23rd, 2006, 10:46 AM   #9
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i think the claims about motion artifacting are one of those tired, bogus sound bites that get repeated around the internet until it is wrongly mistaken for fact. i've done several motion studies with the FX-1. the one i posted online is available at this link:

http://www.dvinfo.net/conf/showthrea...t=fx-1+footage

i'm not saying it *can't* occur, just that, as someone else pointed out, it is not exactly a problem.

on the other hand, i've never seen anyone actually post online footage of a problem motion artifact. there are some claims of having produced motion artifacting with the FX-1/Z1 but nothing that i've seen in the way of actual footage shared for discussion. show us, please, instead of repeating internet chatter....

my challenge would be to the naysayers about the FX-1/Z1: put up something we can actually talk about and examine or stop blabbing on about this myth. and i won't believe in unicorns either until one walks through the door......
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Old January 23rd, 2006, 10:52 AM   #10
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Thanks

Marcus-

That is my experience, too, though I have limited time with this camera. I was out with grand kids this weekend, and I shot them playing ball on a windblown grassy field. The grass was long, and in constant motion. I haven't had a chance to look at it closely, but I am guessing that could create some blocking. I will take a close look. I did do one quick run through, and with the wind blowing kids' hair around, the video was still sharp and beautiful.... The best I have to view the HDV level on is my laptop screen, but even there it was pretty spectacular.

Chris Barcellos
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Old January 23rd, 2006, 06:16 PM   #11
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Chris,

I doubt waving grass will cause an apparent problem. The scene I described (briefly) was a long shot of the ocean at the bottom of the scene, the sun in the middle, and the clouds and pink/orange/yellow sky in the top. The ocean was shot from an acute angle, so about two miles of the surface was visible. From all this ocean, there were areas where the wave action (hundreds of little waves)caused one frame to the next to have almost all the pixels switch from black to white. This means that almost a third of the scene was doing a sort of reversal of a checkerboard pattern. Imagine a checker/chess board flipping back and forth between black and white 30 times a second. This is an EXTREME example of motion, where a large section of the scene is switching all the way from black to white (with the opposite color pixel right next to each other). Imagine the sparkling waves shimmering in the sun. There is a huge amount of movement.

Even with this, the image didn't look bad at all. Only with careful examination did the artifacting become apparent. A slight blur applied to the lower third in post would fix the problem. I would guess that the artifacts were so slight as to not be discernible on an SD monitor. Would this scene look better in DV? NO! HDV is a perfect replacement for DV. My friend that owns the camera now has a dual Athlon64 and editing is smooth and efficient. I used to edit DV on an 800Mhz Pentium III. His machine with Vegas Video is much faster editing HDV than I used to be with DV. It only took about 6 months for computers to catch up with HDV. It took 5 years to catch up with DV. SD isn't dead, but it's replacement is here.

Marcus
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Old January 24th, 2006, 11:29 AM   #12
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Waving Grass

I actually got a closer look at the "waving grass" and it looks pretty good. Mind you this is a field of short wavy grass, with individual blades moving with the wind. Not seeing motion artifacts.
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Old February 25th, 2006, 05:39 AM   #13
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Myth: That FX1's and Z1's cause motion artifacts in regular fast movement

BUSTED!

To give a better analogy/explanation of how motion artifacting work ont he FX1 and Z1 is that, its not fast motion, but the fast *processing* needed to record and *understand* the image. Like the checkerboard example, the pixels were not "moving" per say, the spectrum, and range of the processing was put to a challenge.

Its like a really bad case of indoor shooting then then quickly running out into the bright sunny day outside. It might... *might* cause artifacts because of the processing power of the cam to undersntad what the hell is going on.

In recent tests i've conducted myself, I've only come to the conclusion that movement only plays a small role in advacing the problem. So if you, for an extreem example, filmed in a pitch black room, and then turned on a million trobe lights with a bike race rushing by, it will create artifacts.... One from the drastic speed of light changes, and motion.

Motion IMOP is not the biggest and not the only cause for this problem.... And I dont see it a problem.... i've never seen this kind of artifacting happen in my exp.
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