Rotoscoping the only way?

Matias Baridon · November 16th, 2006, 08:39 PM

I shot a commercial where I had to cut-out the subject and place her in an all-white set with some graphics moving behind her. I chose to shoot in front of a bright white set rather than blue or green screen due to the fact that I was shooting on DV and I havent had good keying on DV footage, so by shooting in front of a white screen, the keying doesnt have to be perfect for it wont be noticed on the virtual white set.

I get good cut outs using After Effects 6 or 7 with only one Color Key. Whenever I get an undesired cut-out on the subjects bright spots, I just mask that hole out...

Charley Gallagher · December 3rd, 2006, 10:13 PM

I'm a little late jumping in here but if the camera was on a tripod, and stationary, wouldn't the "Difference Matte" do the job? I have not used it but my understanding is that its perfect for removing everything in a clip that remains the same, like on a stage.

If you start filming with only the stage and no actors, then when the actors come onto the stage they are the only new information in the clip and only the actors will show up with this filter. Frequently a still image is taken from the clip of the stage and this clip is on a layer by itself, the length of the entire clip. The overlying clip is compared to this image and only that which is different (ie; the actor) between the clip shows up. Everything but the actor would be transparenta.

Anyone ever use this filter?

Chris Hocking · December 3rd, 2006, 11:52 PM

I've tried using this kind of feature in the past with a variety of different software applications, but have never had much luck with it. Chroma keying has always worked out much better. But I'd be very interested to hear if others have had any luck with a difference matte approach. I do intend to have another go with it in the future though...

Oh, and by the way, in the end, despite the good results I got using FCP, I ended up just manually rotoscoping the whole footage, thus achieving the perfect matte.

Martin Chab · December 5th, 2006, 05:59 AM

To Charley:
The problem with the difference key is the noise. If you have a "really" good footage without any noise your key will be good, otherwise the noise will be interpreted by the keyer as a difference and the result will follow that.

To everyone: If your background is white (or black) try the extract keyer that is developed specially to make extractions with B or W backgrouds.

Martin Chab · December 5th, 2006, 07:06 AM

I forgot,
even if you have a "near perfect" situation (let´s say 4:2:2 sampling, an even blue or green with good saturation and low spill) is not very often that you can get a perfect key with only one key pass.

Cody Dulock · December 7th, 2006, 11:06 AM

heres a link to an ipod spoof me and some friends did.
http://www.youtube.com/watch?v=udSp7yhvYIk

we had a blue screen, then we used green tape on the bottle to key that out as well... it was pretty complicated getting all the compositing done, but it turned out okay in the end.

Glenn Chan · December 7th, 2006, 01:09 PM

I've had good results with a difference key.

Equipment used:
*Avid Media Composer recording HD-SDI to DNxHD compression. This is what I had to work with; an uncompressed FCP system should be better quality and cheaper.
*Panasonic HDC-900 camera with some HD zoom lens (it exhibits chromatic aberration)
*The subject was wearing colors not found in the background.

Good points about difference key:
*Perfect spill. You should light a difference key such that the floor, background, etc. give off the same light as your target environment. No spill suppression needed.
*May be slightly cheaper.

Bad points about difference key:
*Requires noise reduction... i.e. erode/blur in Combustion. Other effects packages should be able to do something like that.
*Requires some rotoscoping, because sometimes the colors with shadows overlap.
*Camera can't move (unless you have a motion control rig?).

?
*Motion blur was ok.
*On the HD zoom lens I used, there was chromatic aberration and that got picked up in the difference key.

I haven't tried:
*Seeing how difference key works on hair. I suspect it works poorly, since you need to erode + blur the alpha matte.
*Lighting a green screen really dark (to solve the spill problem), and using a combination of color keying + difference key techniques.

Aric Mannion · December 11th, 2006, 10:02 AM

I'll ad my experience in this if it's any help. I've done videos where I'd key out liquid (Luma) and people (green screen). In AE DV matte worked great, but NOTHING could key that liquid. So I did all my Luma keys in Final Cut and was surprised how perfect I got it. With a performer being keyed out the exposure would need to be perfect, and if it's DV someone told me 24p is best.
But green screen is better because if it's like the ipod ads they'd be black and white, and you wouldn't need to worry about the green reflecting.

November 16th, 2006, 08:39 PM	#16
Matias Baridon Regular Crew Join Date: Apr 2006 Location: Montevideo, Uruguay Posts: 52	I shot a commercial where I had to cut-out the subject and place her in an all-white set with some graphics moving behind her. I chose to shoot in front of a bright white set rather than blue or green screen due to the fact that I was shooting on DV and I havent had good keying on DV footage, so by shooting in front of a white screen, the keying doesnt have to be perfect for it wont be noticed on the virtual white set. I get good cut outs using After Effects 6 or 7 with only one Color Key. Whenever I get an undesired cut-out on the subjects bright spots, I just mask that hole out...

December 3rd, 2006, 10:13 PM	#17
Charley Gallagher Regular Crew Join Date: Jan 2004 Location: Collingswood, NJ Posts: 99	I'm a little late jumping in here but if the camera was on a tripod, and stationary, wouldn't the "Difference Matte" do the job? I have not used it but my understanding is that its perfect for removing everything in a clip that remains the same, like on a stage. If you start filming with only the stage and no actors, then when the actors come onto the stage they are the only new information in the clip and only the actors will show up with this filter. Frequently a still image is taken from the clip of the stage and this clip is on a layer by itself, the length of the entire clip. The overlying clip is compared to this image and only that which is different (ie; the actor) between the clip shows up. Everything but the actor would be transparenta. Anyone ever use this filter?

December 3rd, 2006, 11:52 PM	#18
Chris Hocking Major Player Join Date: Dec 2005 Location: Melbourne, Australia Posts: 595	I've tried using this kind of feature in the past with a variety of different software applications, but have never had much luck with it. Chroma keying has always worked out much better. But I'd be very interested to hear if others have had any luck with a difference matte approach. I do intend to have another go with it in the future though... Oh, and by the way, in the end, despite the good results I got using FCP, I ended up just manually rotoscoping the whole footage, thus achieving the perfect matte.

December 5th, 2006, 05:59 AM	#19
Martin Chab Regular Crew Join Date: Apr 2006 Location: Malmo Sweden Posts: 103	To Charley: The problem with the difference key is the noise. If you have a "really" good footage without any noise your key will be good, otherwise the noise will be interpreted by the keyer as a difference and the result will follow that. To everyone: If your background is white (or black) try the extract keyer that is developed specially to make extractions with B or W backgrouds.

December 5th, 2006, 07:06 AM	#20
Martin Chab Regular Crew Join Date: Apr 2006 Location: Malmo Sweden Posts: 103	I forgot, even if you have a "near perfect" situation (let´s say 4:2:2 sampling, an even blue or green with good saturation and low spill) is not very often that you can get a perfect key with only one key pass.

December 7th, 2006, 11:06 AM	#21
Cody Dulock Major Player Join Date: Sep 2004 Location: arlington, texas Posts: 420	heres a link to an ipod spoof me and some friends did. http://www.youtube.com/watch?v=udSp7yhvYIk we had a blue screen, then we used green tape on the bottle to key that out as well... it was pretty complicated getting all the compositing done, but it turned out okay in the end.

December 7th, 2006, 01:09 PM	#22
Glenn Chan Inner Circle Join Date: Jun 2003 Location: Toronto, Canada Posts: 4,750	I've had good results with a difference key. Equipment used: Avid Media Composer recording HD-SDI to DNxHD compression. This is what I had to work with; an uncompressed FCP system should be better quality and cheaper. Panasonic HDC-900 camera with some HD zoom lens (it exhibits chromatic aberration) The subject was wearing colors not found in the background. Good points about difference key: Perfect spill. You should light a difference key such that the floor, background, etc. give off the same light as your target environment. No spill suppression needed. May be slightly cheaper. Bad points about difference key: Requires noise reduction... i.e. erode/blur in Combustion. Other effects packages should be able to do something like that. Requires some rotoscoping, because sometimes the colors with shadows overlap. Camera can't move (unless you have a motion control rig?). ? Motion blur was ok. On the HD zoom lens I used, there was chromatic aberration and that got picked up in the difference key. I haven't tried: Seeing how difference key works on hair. I suspect it works poorly, since you need to erode + blur the alpha matte. Lighting a green screen really dark (to solve the spill problem), and using a combination of color keying + difference key techniques.

December 11th, 2006, 10:02 AM	#23
Aric Mannion Major Player Join Date: Apr 2006 Location: Boston, Massachusetts Posts: 616	I'll ad my experience in this if it's any help. I've done videos where I'd key out liquid (Luma) and people (green screen). In AE DV matte worked great, but NOTHING could key that liquid. So I did all my Luma keys in Final Cut and was surprised how perfect I got it. With a performer being keyed out the exposure would need to be perfect, and if it's DV someone told me 24p is best. But green screen is better because if it's like the ipod ads they'd be black and white, and you wouldn't need to worry about the green reflecting.