2007 September 30th, 10:35
Need help finding the couse of video artefact..
I have noticed a strange video artefact in my footage. Sometimes it has kind of soft and wide horisontal lines similar to what you can see when you film a tv set. I can't see that in camera's lcd, so it is really annoying when you notice it only when it is captured to my mac. :T HV20 is my first camera and i don't have enough experience to find the couse of my problem. Here is the sample footage with this problem: http://hv20.info/yopu/video_artefact.mov. It is just a 2.5Mb file.
It would be very helpful if more experienced users could point the reason of this artefact. I don't think that my hv20 is faulty, because i see this artefact just in some footage, not all. I guess something is wrong with camcorder settings. Thanks.
2007 September 30th, 11:04
Formerly known as Jo_the_big_O
Do you mean the Interlaced lines? This can be explained when you understand, that a couplete frame in Interlaced video consistst of a half frame with the even lines, followed by the other half frame with the uneven lines. When you move your camera abruptly, as you have done, the two concutive frame-halves differ so much in what is shot, that you wil see the horizontal lines.
2007 September 30th, 11:45
It looks like it's related to the auto focus. Maybe a combination of auto focus an image stabilization.
Do you really plan on whipping your camera around like that on a regular basis though?
2007 September 30th, 12:46
I don't think it is related to autofocus and image stabilisation because these are always the same (default).
2007 September 30th, 13:03
Then why i get the same artefact in footage that is filmed without lots of movement like this: http://hv20.info/yopu/video_artefact2.mov?
Originally Posted by Jo_the_big_O
2007 September 30th, 16:10
I was just about to post a similar thread. I have those too. It's not interlaceing. I don't see those lines in the material captured with HV20, but they appear in some videoas when i render them to another file, regardless of the compression type, even when i recompress them to uncompressed RGB. I know the logical thing would be to assume it's the encoder's fault, but i never get those lines with any other material other than from HV20.
2007 September 30th, 16:49
The second link makes it very clear that you are experiencing compression artifacting. I would guess that it is due largely in part to the light level you're shooting in, but I'm not an expert. I suppose it's possible that there's a problem with your capture. Does it look ok if you play it on a TV from the camera?
2007 September 30th, 17:25
How to avoid it if it is an compression artefact? I can also see these when i capture them to imovie of FCP before compressing.
I get these artefacts in room lighting quite often, which parameter (shutter speed, etc.) could impact it?
2007 September 30th, 17:33
I don't actually have an HV20 (yet) so I couldn't say... but you might consider adding more light to the room. At least to see if it alleviates the problem.
As a rule though, the slower the shutter speed, the brighter the image, be aware though that if the shutter speed is set too low there will be a lot of blurring to your footage.
2007 September 30th, 18:06
WHere are you located? Is it a PAL or NTSC cam that you have? Are you using flourescent lighting? This does not necessarily look like any type of compression artifact. It could have a lot to do with your lighting.
Do you get the same type of results when you film outdoors in good lighting etc.?
2007 October 1st, 02:53
These shots were taken in London, but the camcorder is NTSC.
I am not sure if the lighting was fluorescent. I have footage from the same room and same (or very similar) lighting conditions without the video artefact: http://hv20.info/yopu/without_artefact.mov. The only difference could be one small lamp (i don't remember if it is on in this footage).
Does the fluorescent lighting changes a lot?
2007 October 1st, 05:20
Fluorescent lighting can flicker, depending on the quality of the ballast in the fixture. I believe it does so at 60hz, at least on US power, not sure of the power frequency in the UK.. so a 60hz flicker in theory could affect a 60fps image, but would likely affect a 50fps image in more unique ways (think of the crawling line on a TV or CRT monitor that's not in sync with a camera)
It certainly looks like compression artifacting to me though, likely due to insufficient data from being seriously underexposed. It has the blocky compression artifact look to it, but it moves around a bit too.. Especially in the first clip, it was noticable mostly on the camera lens when the camera was pointing straight into the mirror.
Either way, I'd put my money on it being a lighting issue, since the footage all looks very poorly lit. If you look very closely at the dark areas of this third clip -- the bag or clothing hanging from the closet door, between the closet and the desk is where I noticed it.
MPEG compression, being a lossy compression method inherently has to loose some of the information. This tends to happen most in the dark areas, because if there isn't much data there to begin with, it goes away much quicker.
I think the reason you don't notice it in this last clip is because the majority of the image is lit. If you had the same level of light in the other two clips you probably wouldn't have had any problem with the footage.
Interesting. Now that I look at the first two clips again, on a better monitor than the crappy laptop I viewed them on before I see the horizontal banding you were talking about, which I did not see before. I saw compression artifacting and that was it. So, now that I know what we're looking at...
This looks like it's caused by the frequency of the camera and lighting being out of sync. It's got a weird strobing look to it. It is very strange indeed. As Ian asked, does this happen outdoors in bright sunlight?
2007 October 1st, 06:26
It mentions the fluorescent light issue in the HV20 manual.
2007 October 1st, 12:13
Originally Posted by tcindie
It doesn't happen outdoors. But it sometimes does indoors in a good lighting conditions as well, for example in a food store. I'll grab a manual tomorow, maybe i'll find what to do in such conditions..
2007 October 1st, 12:18
I got those lines on footage of the sky. On another occasion, while filming on a poorly lit stage i got them also, but only on the part of the stage that was very well lit. In fact, the light was hitting that part directly. The strange thing is it happens completely randomly. I've got like two hours of the footage of a band on a stage and the lines appear only on couple of minutes of the footage. The lighting was the same throughout the entire footage and all the later encoding was done straight to uncompressed video. Alas, when all is done and i output the project to xvid or Mpeg2, i can see those lines here and there.
2007 December 14th, 20:02
that happens to me with fluorescent light, I'm in a 220v country so I don't know if it has anything to do with it, I can't seem use halogen lamps either (which sucks) but with tungsten and natural light it works just fine, try also different shutter speeds, like at 1/30 and 1/24 it's REALLY nasty and then there's some other ones where you can't see it the flicker (or whatever that artifact is called) at all
2007 December 15th, 02:46
I'll second that!. Fluorescent lights can flicker in a weird way and play havoc with auto white balance. My digital stills camera can shoot MOV files, and under fluorescent lights it flickers just like that, but worse.
Try manual white balance.