this is a simple chroma key function. doing a little java function in QC could do a basic key calculation for you. passing in your rgb values to the function.
f(r,g,b)->a is applied to every pixel in the image. a <= 0 means the pixel is the green screen, a >= 1 means the pixel is in the foreground object. Values between 0 and 1 indicate a pixel that is partially covered by the foreground object. A usable green screen example, which matches how chroma key was done on an optical printer is f(r,g,b)->K0*b-K1*g+K2 (K0..2 are user-adjustable constants, 1 is a good initial guess for all of them). i just noticed that FX in photo booth. kind of cool. then i noticed this facial recognition thing eye photo thought that was neat to. there are a whole bunch of ways to do the background key. first off if you are trying to replicate the photo booth fx. that is not a green screen key or chroma key etc..or what normal motion graphics programs do. like key light luma key primatte etc... seeing there are so many of those algorithms and if you have a program that can key a green screen it can make an alpha matte for you as well that you can import into QC and use as a matte on your green screen footage. in all honesty i have not attempted to do this in QC but i will today. this is the sort of old way to do motion tracking you subtract the back ground then track the pixel change. well photo booth seems like it is doing this back ground subtraction, i don't work at apple so i could be all wrong and this is only a guess is to how it works. so if you run your image through a few filters, thresholding maybe blurring a bit, histograming there are tons of options you want to get the pixel information of what will be the back ground you subtract later, but you want to filter to make the calculation easier. so in photo booth it takes a pic of the back ground without you in it then when you enter the frame it knows what are new pixels. so probably the best way to do this is to do a before and after difference of frames of video feed the difference of the two frames should then make some sort of luminance for you thus enabling you to add that calculation as a mask. this is done a lot in motion tracking and stuff like i said before. i like to split the screen up into a 6x6 grid really big but i do this to make music so im not trying to trigger the whole piano every time i move. you will want to do this grid a lot larger and compare the pixels. pixels are squares usually. i suggest just using a program that like i said does a green screen key all ready, as chris stated fg noise, back ground noise etc become a problem when you are making an alpha. you really want to get a good black and white alpha channel or mask can be a very difficult task with poor green screen footage. a proper green screen is simple, all the softwares out there can do the key pretty much with one button click, a bad green screen with shadows maybe wires and or wrinkles can be much more difficult because of the noise. seeing that this is a apple list the easiest of there software in my testing is motion to pull a screen. you have hardly any options but if its good footage you don't need controls. i guess its the best in all fairness of being the quickest. FCP is rrr i never use it to make a key. Shake is the best apple consumer product for keys because you have lotos of control. i found the other day trying to work with a bad key i could feed my key back into its self as a garbage matte and it cleans up a lot of shadows. a little trick i came up with. with x code, core image, and quartrz composer etc.. you should be able to make your own key program if it interests you. pretty much all the softwares use the same key algorithms so look them on google figure out what they do and re create them. some would be primate, chroma, luminance, color, or key light is a popular one. On Fri, Jan 16, 2009 at 8:03 AM, Chris Wood <[email protected]> wrote: > 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 >> _______________________________________________ >> Do not post admin requests to the list. They will be ignored. >> Quartzcomposer-dev mailing list ([email protected]) >> Help/Unsubscribe/Update your Subscription: >> >> http://lists.apple.com/mailman/options/quartzcomposer-dev/psonice%40gmail.com >> >> This email sent to [email protected] >> > > > _______________________________________________ > Do not post admin requests to the list. They will be ignored. > Quartzcomposer-dev mailing list ([email protected]) > Help/Unsubscribe/Update your Subscription: > > http://lists.apple.com/mailman/options/quartzcomposer-dev/dustin.oconnor%40gmail.com > > This email sent to [email protected] > -- Dustin O'Connor http://numedia-sci.info
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