Sawtooth Motion Artifact

stevengotts wrote on 5/14/2009, 1:19 PM
Could a more experience camera person explain to me what my sawtooth edges artifact I get when their is accelerated motion or an accelerated pan is. and most importantly how to avoid it. I get it mostly in my broadcast cameras and my HD camera, not so often in my gl1. is it a shutter speed or interlace issue?
Thanks in advance for sharing your expertise
Thanks
Steven

Comments

jrazz wrote on 5/14/2009, 1:23 PM
This issue is usually caused by CMOS sensors as opposed to CCDs. I ran into this when I bought my first HD cams as they sported CMOS and capture the image differently than CCD. So, when you pan quickly you will experience this effect (that is, if we are indeed referring to the same issue). I have learned to work around it as there is no fix for it with the cams I have. I believe newer cams are compensating for this but I can't swear to it.

j razz
farss wrote on 5/14/2009, 1:53 PM
Agree, from the description it's impossible to know what we're talking about here. A frame grab would clear matters up immediately.

Assuming it is rolling shutter image skew two things can be done to mitigate the impact of it.

1) Limit panning rates.
2) Limit shutter speed. Fast(er) shutter speeds will make it more obvious as the natural motion blur of a 180 deg shutter tends to mask the appearance of the skew. This may not be all that easy to achieve. The only alternative way to control the amount of light for the camera is to close the iris. Closing the iris too far can soften the image once you reach the diffraction limit. You may need to add an external ND filter if the in-camera ND filters are not enough.

Bob.
Coursedesign wrote on 5/14/2009, 2:09 PM
If you're shooting interlaced and seeing the problem on an LCD or other progressive monitor, this is only a display problem.

If your deliverable is interlaced, preview your footage in Vegas on a CRT, using a BMD or AJA card for 4:2:2 broadcast SD/HD footage.

If your deliverable is progressive, but you shoot interlaced, you may need to put some effort into deinterlacing to get the best end result. Vegas does it well, but there are After Effects plug-ins that do better motion compensation, and of course hardware boxes that do it for a price.

CMOS cameras may have another problem often referred to as "jello cam" but I don't believe that is what you are seeing.
farss wrote on 5/14/2009, 2:18 PM
Another possibility.
If using Vegas to downscale from HD to SD you can get nasty "sawtooth" artifacts with interlaced footage on the edges of moving objects.
This is fixed by specifying a de-interlace method in the project settings.

If it's just plain old interlacing viewed on a LCD and you want to convert to progressive Mike Crash's Smart De-Interlacer does a reasonable job and it's free. Agreed the best plugins only run in After Effects and the best of the best are the hardware boxes however in HD even the Alchemist uses the same algorithm as the Smart De-Interlacer last I heard.

Bob.
stevengotts wrote on 5/14/2009, 3:29 PM
Wow, thanks for all the great leads. I will try to post a frame grab, im sure that will help identyfy it better for yo. Thank you all for your time.