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Canon XL and GL Series DV Camcorders
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Old March 6th, 2005, 01:52 AM   #1
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Edge ghosting with XL2

Hi !

When I have contrasty edges in some of my XL2 footage I get edge gostings on the left side of the edge.

Here is an example:
http://home.comcast.net/~chalbers/ghosting.jpg

1. Is that supposed to happen in video ?
2. If it is , how can I clean it up ?

Thanks !

Frank
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Old March 6th, 2005, 02:46 AM   #2
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Frank, I suggest you sit further from the screen. :)

Seriously, I think this is quite common with DV images, I see it a lot with many different cameras. It's probably a "feature" of the compression process. You could try playing around with the Sharpness setting on your XL2, but I don't know how much difference that would make.

Richard
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Old March 6th, 2005, 06:31 AM   #3
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Frank, this is a somewhat unavoidable feature of the DV compression which occur at the boundary of dark and light objects. For example, when shooting the sun setting behind a mountain or whatever dark object, a bright edge will appear on the footage between the sky and the object. Still, in my experience the XL2 seems to manage much better than XL1 in thise sense. What comes to your example, there is nothing wrong withit: That's what the DV-compression generates.
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Old March 6th, 2005, 12:43 PM   #4
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Thanks for the feedback !

I'll just have to learn to live with it I suppose ...

I tried changing several settings, including sharpness. But that didn't work.

I suppose adding more details in my framing is what to do to get rid of attracting attention to the edges. :-)

Frank
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Old March 6th, 2005, 07:19 PM   #5
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That may be compression artifacts as suggested but it does seem odd to me.

I must admit I never noticed it on my XL1 DV footage even when projected. So I don't think I totally agree that it may be normal. But now I will have to examine my footage closer to see. Maybe I just wasn't looking closely enough.

Is it noticeable when screened directly off the camera playback onto a monitor?
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Old March 6th, 2005, 07:58 PM   #6
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Frank,

I won't swear to it, but looking at that image makes me think you might have been working with a wide open iris. An indoor shot with what I guess is available room lighting. Since the reflection on the face of the vertical column seems fairly sharp, your DOF can actually be shallow enough in this condition to have the back edge of that column slightly out of focus. If you were zooming in, the DOF will get shallow indeed. Just a thought because I have done this same thing in my house trying to create a shallow DOF. Also, the position of the light reflection suggests that the light source might have been a little to the right which would create a bit of a shadow on the back left side of the column.

Or, maybe I missed your point and none of what I just typed is relevant. :-)

regards,

-gb-
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Old March 6th, 2005, 10:59 PM   #7
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Hi Greg ! It's all relevant indeed !

But it wasn't a controlled lighting setup. Just a little test I was doing. And that's when I noticed the ghosting. I'm surewith carefull lighting conditions you can take care of a lot of bad artifacting. I'm still in my chidren shoes on real lcontrolled lighting setups though. That's next on my "to learn" list.

Thanks for the info though !

Frank
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Old March 7th, 2005, 01:27 AM   #8
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Jim, Greg, Have you ever sorted out what the DV compression does?

The point is that, the compression is about loosing a significant amount of information (the ratio is 5:1), and obviously, there is a price to pay for that. According to the saying, there are no free lunches. This problem Frank demonstrates is a well known consequence of the compression (technically a so called "discrete cosine transform"). In some conditions it becomes more visible, and thus, the other way around, the artifact depends bit on the settings of the camera. Still, there is no way to avoid the problem completely.

An easy way to convince oneself of the problem is to take a footage with a DV camera and a still photo of the same target with a digital SLR camera yielding raw images. Then, if one compares the images, the edge ghosting effect Frank talks about is apparent. Tu sum up Jim, just look again carefully of your XL1 footages, and you will definitely discover the effect.
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Old March 7th, 2005, 07:17 AM   #9
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I suspect that this is not compression (DCT) related but rather luminance leakage. DV compression artifacts usually manifest themselves as jaggy edges because the DCT works on "tiles" i.e square groups of pixels as exemplified in the current thread "XL2 vertical streaking". Luminance leakage isn't easy to explain but is a consequence of matrixing after gamma correction and band limiting (sub sampling) of the color difference signals. See Poynton for details. If I'm right then this is not a fault of the camera but of the NTSC system. Given that the camera uses SD NTSC encoding (and PAL will show it too) it's going to exhibit this phenomenon under certain conditions.
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Old March 7th, 2005, 08:52 AM   #10
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A.J.....you are WAY TOO smart! So with the limits of dv25mbs being said. Remember the "Clive" thread? If you have moire ie ..like shooting a building or house with shingles, is there anyway to still shoot the building and minimize the moire damage and have acceptable footage. Or is this just something a 1/3 3CCD miniDV camera just can't do?

Thanks
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Old March 7th, 2005, 08:38 PM   #11
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It is something no digital camera can do unless that camera is equipped with a blurring filter which, for example, the Nikon D still cameras are but the Canon XL video cameras are not. Given that, you must obtain a blurring filter from another source and use it (at the cost of a less sharp overall image) or try to avoid scenes that are known moire producers i.e. anything with regular patterns with a pitch comparable to the pixel spacing on the CCD. You can learn a lot about what causes it to the point it is noticeable by looking for it in broadcast SD and HD and I guarantee you will find it! Moving farther away from the pattern to the point where it looks uniform or closer to it to the point where the individual shingles (or whatever) are clearly resolved is an approach that you may be able to use. Some people have reported success in blurring the images in post.
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Old March 7th, 2005, 09:04 PM   #12
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In film I know some diffusion filters can help with this problem and a good film-to-tape transfer on a Spirit will resolve it as well.

My understanding is that it has to do with the display medium more than the acquisition medium. AJ, does HD display have the same moire problems?
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Old March 7th, 2005, 09:18 PM   #13
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As A.J. says, moving the cam will affect the moire pattern. If you can't do that, try changing the zoom setting instead. In fact, if you zoom in/out and watch the viewfinder, you will see the moire patterns come and go. With any luck you can find a setting that works for your framing and also does not show the moire. (Of course if your shot involves a zoom you can't really avoid the moire!)

Richard
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Old March 8th, 2005, 03:33 AM   #14
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<<<-- Originally posted by A. J. deLange : I suspect that this is not compression (DCT) related but rather luminance leakage. -->>>

A.J., I remember seeing examples of applying just DCT and then getting egde ghosting in the output. Intuitively, as the very idea is to map an image from the spatial space to frequency domain, and then neglect higher frequencies, it is likely that the loss of information should be visible at the interfaces between bright and dark objects. ("Rapid" spatial changes correspond with high frequencies) Still --unless one has good frieds who are experts in signal processing-- the best way to find out is to write a Matlab code and test the whole thing oneself.
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Old March 8th, 2005, 09:28 PM   #15
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Jim,
Yes, any rastered or digitized image collected with a system that presents higher spatial frequencies to the sensor than the sensor's pixel spacing (i.e. no antialiasing filter) will show moire. Go to a store that sells HDTV gear and watch the screens for a few minutes. You'll see it. The monitor does have a great effect on when it occurs. A properly antialiased image presented on a screen with pixel spacing too coarse to support the resolution of the image will show it. So with respect to Richard's comment - the relatively low resolution of the stock color viewfinder may deceive you in trying to eliminate moire by zoom/angle/composition. The higher resolution B&W viewfinder would be better in this regard and a studio monitor better still.

Lauri - yes, the DCT will upset edges and I'm not saying that the ghost in Frank's frame definitely isn't caused by the DCT but rather that I suspect it isn't because DCT artifacts are usually blocky resulting in a jagged appearance like the artifacts in this frame grab: http://www.wetnewf.org/DCTEdges.jpg (note that I have used USM to emphasize the effect in this frame - it's not that visible in the original). As you note the DCT throws out high frequency information so that the tiles tend to be more uniform than they would be without compression and it is this that makes the tiles stand out from one another. This happens more in a picture with lots of detail (like the one I've posted here) because it has more high frequency content. Frank's frame doesn't have nearly as much detail. In fact it has so little that I doubt the DCT algorithm would have to throw out any high frequency info.
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