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Canon XL and GL Series DV Camcorders
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Old August 21st, 2005, 11:32 PM   #1
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Ghost Images

According to the manual, setting the noise reduction to HIGH will create ghost images on the screen. Can someone comment on what these look like and whether they can be used as an effect?
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Old August 23rd, 2005, 04:57 PM   #2
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Basically the NR works like this: it compares each video frame to the last one. If there's no movement in the shot, the two images will be rather similar. Small color differences are probably just noise, so the NR changes the color values to match the previous frame.

So taking a static shot of a night city skyline results in virtually noise-free images.

But: if there's a moving object in the frame, there's a problem: The moving object reveals parts of the background, which weren't visible in the previous frame. In these areas the NR simply doesn't work and the noise remains visible.

So a moving object appears showing noise and even a slight noise trail following the motion of the object, while the rest of the background is noise-free.

With the maximum setting this ghosting is quite visible when you're watching closely. But I don't think normal viewers would notice the effect.

Shooting in 25P reduces this effect, because of the motion blur in the moving object.
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Old August 27th, 2005, 03:49 AM   #3
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Thank you Patrick. Sorry for the late reply.

Are you saying that if I have moving subjects in the frame, I should not bother to turn on the noise reduction?

What is the difference then between LOW, MEDIUM, and HIGH NR settings? Your explanation implies that NR either works or it doesn't. So what are the levels all about?

According to the XL2 manual, ghosting only occers with the HIGH NR setting. I've only used the MEDIUM and did not encounter any ghosting artifacts, even though there was considerable motion in all of my shots.
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Old August 27th, 2005, 01:21 PM   #4
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The type of NR can be gradual, depending on: how similar pixel color values of the last and current frame have to be, to notice them as noise. The low setting only takes very similar values, the high setting matches a wider range of color values.

The lower settings simply detect and remove less noise - and with less noise removed, you won't notice any ghosting.

It's true, if all of the image is in fast motion, the NR is basically useless. (but in my experience there's no need to turn it off.)

I always use the NR, even in the HIGH setting. In typical daylight situations, there's no difference, and no ghosting. (ghosting occurs due to remaining noise, so there's no ghosting if there never was any noise...)

In very noisy low-light conditions combined with some fast movements, the ghosting occurs. Most of the time it's not very obvious, especially in 25P. (ghosting only in moving subjects... which are motion-blurred anyway...)

But: I can remember one kind of shot where I had to turn down the NR from HIGH to a lower setting: Maximum gain, a dimly lit moving person in the foreground, in front of a nearly black night sky. The remaining noise following the persons movements simply stood out too much.
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