Thanks for a great reply!
The setup uses an imac with its built-in camera, and the green screen
is completely light green and very higly lit to prevent any shadow. I
guess the motion detection method doesn't really work with this type
of camera, so i'm looking at the method that simply masks out the
background somehow. Today i've tried making it work with javascript/
core image filter patches, but i'm still a bit new at that. You
wouldn't happen to have an old patch lying around that uses either of
these techniques? :-)
Jonathan
16 jan 2009 kl. 14.03 skrev Chris Wood:
Jonathan,
There's 2 ways I've used to do it. The first is to use a filter to
remove the background (you would need to have the camera pointing at
a plain coloured background though) and in the filter just compare
to see if the green component is much higher than the other
components, and filter on that. Noise is still an issue, but it can
at least be reduced a bit.
The other way is to do motion detection, and remove anything not
moving. To do that I just used a queue with a length of 2, so I
could feed the current and last frames into a filter. It's fairly
easy to mask out the background, but there are some quite nasty
catches.
Video noise is a huge problem, as it means the whole image is always
moving slightly. You really need a good camera to get rid of it,
otherwise you have to do a lot of noise reduction that can make the
image look blurry.
Another big problem is auto-white balance on the camera, or anything
like that.. if the camera suddenly adjusts anything, the whole image
changes so the background suddenly pops into view for a short time.
Again, a good camera with manual controls would help.
The last problem is that sometimes the thing you want to show stops
moving. Say you're filming somebody talking.. their mouth is moving,
but their eyes might stay still for a short time. They disappear..
and even if they are moving constantly, sometimes there will be an
area with similar colour that doesn't get detected, and you have a
hole..
I guess the way ichat works is to sample the background just once,
when there is nothing in front of it, and use that for comparison.
Then even if you're still, there is still a difference. You still
have the problem of white balance changes, video noise etc., though
so you'll need a fair bit of cleaning done on the image.
Chris
2009/1/16 Jonathan Selander <[email protected]>
Hi,
I've used a couple of patches from the QCTV example application to
create a green screen effect in my composition. However, i just
compared it to Photo Booth which does it much more accurately.
What's the best way to accomplish this? Is there any patch out there
i can use that works well?
The QCTV patch seems to be very susceptible to video noise, so lots
of pixels appear everywhere, so the backdrop isn't solid.
Thanks
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