John Jackman's Green Screen Tips


Here's a post to the VLM/DraCo mailing list made by John Jackman. December 2, 1996 (Slight editing done)

Several folks expressed interest in info on Blue/GreenScreen work, so I'll be posting several tips. 

Paint:

Gordon wrote:
Have you used Rosco Ultimatte, Chroma Key or similar paints?


We used Rosco Chroma Key Green, $45 a gallon (!) on a 24' x 18' wall.  TheUltimatte is just a shade different, specified by Ultimatte and moreexpensive.You don't need that unless you are using the Ultimatte plug-in forAfterEffectsor the Ultimatte hardware for a keyer.  You don't really need to pop$45/galloneither, you can get pretty darn close at the local paint store.  The green andblue are supposed to be "pure," matching the green or blue in the SMPTEcolor bars as closely as possible -- but in the ballpark works just fine.  Itmustbe flat latex, not semi gloss.

Straight out painting a wall is best.  I've done this with a blue clothbackdrop,but it is not easy to get right, to get all the wrinkles out, and the clothabsorbsmore light than paint.  However, If the wall is out of the question, you canrigup a large cloth as a cyc.  If you only need a small area, paint a piece ofpanelling and then you can move it away when not needed.

Is blue or green better?  I've heard several answers, but it's kind of a cointoss.  You're supposed to lean toward green with brunette actors andblue with blondes.  All the talent on our set was brunette, and they certainlylooked fine.  We did a test with a blonde, however, and it was equally clean.I think the differences may show up more if the lighting is "iffy."

Ash wrote:I'm interested :) How well does it work with anims? and what arethe RGB settings for the Background?

It works great with anims.  For most of this work, we will be keying 3Drendered "virtual sets" -- some still and some animated -- behind the talentand some real props in the foreground.  But in using IFX CineMatte, youdon't set an RGB range as you would with the Mask operator, for instance.  Theprogram automatically analyses the green or blue bandpass; you havesettings to "Darken BG" and "Lighten FG."

Other than a big green (or blue) wall, the most important part of this is thelighting.  The techniques are fairly straightforward if you obey the rules.

Lighting:

Lighting is VERY IMPORTANT.  If it's not right, it won't work.  While lightingour set, we had the advantage of using a GVG SEG with componentChromaKey option, so we could get immediate feedback on our lighting.Failing that, if you're running MovieShop 3.x or lower, you could try thecrudechromakey effect while setting lights, but it's really pretty crummy.  (Ithinkthat's why they dropped it, Steve!)

You don't really have to be able to monitor the effects live.  I've done thisacouple of times without that possibility.  Just follow the rules on thelightingreally carefully.

LIGHTING THE SCREEN:
Actually, the screen doesn't have to be as perfectly illuminated as most folksthink.  We used three scoops, which of course gave three intersecting softcircles.  No problem.  However, your talent needs to be a distance from thescreen -- 6-8 feet is best.  This allows you to light the talent withoutcastinga shadow on the screen, which MAY goof you up.  IFX cannot handle castshadows, the only thing that I know will do that is the new UltiMatte stuff.

Light the screen as evenly as possible, but DO NOT OVER ILLUMINATE!This is actually the mistake I've seen a couple of times.  Your enemy isgreen or blue spill on your talent's back.  The more light you pour on thescreen, the more light reflects off.  Hey, 3d folks, this is radiosity!  I'veseena couple of studios (this was a few years ago) which used back lit blueplexiglass; they had a nice even blue, but there was always spill and theresulting shimmering shoulders.  It's better to have even, mediumillumination.  While the illumination can vary, don't let there be anyreally dark areas.

LIGHTING THE FOREGROUND:
Talent should be 6-8 feet in front of screen, if possible.  WATCH OUT forany reflective surfaces, they will vanish when they pick up the BG.  Watchout also for thin stringy stuff, which won't tend to compose well.  Use thestandard three-point lighting approach of key, fill, back -- except use astronger back light than you might usually use.  Looking at the scene ona monitor, watch for any green or blue spill or reflections.

Here's  where a live ChromaKeyer or a quick test render will pay off.  Thisstuff tends to have problems on one side only -- one side will key fine, theother will flicker or catch spill.  Seeing the effect live or doing a testrenderwill show you any problems.  Put the back light strongly on the side withproblems.  In some ticklish situations (most folks won't want to get thisfancy) you can treat problem areas with very subtle gels on the back light.Rosco makes a color wheel for their gels, find one that is opposite tothe pure green or pure blue (I don't remember the numbers).  These willhelp to cancel out the spill.  Like most light mixing, this iscounter intuitiveand seems like it won't work.  The "counter" gel for green is a sort of lightpinkish-yellow which doesn't seem to affect the light color much -- butit works!

Oh, yeah -- tell the talent not to wear any green or blue!  Seems obvious,but our talent forget when she was out buying costumes.

Compositing

Once you have shot your green or blue screen video andeither shot or rendered background material, digitizethem in your DraCo or VLM.  (BTW, this works almost aswell on VLM as on DraCo, as long as you are using areasonable datarate ((70% +)) and use a clean tapeformat like Hi-8 or DVC.  VHS will not work well).

Next, create a special directory on a regular partitionfor your files.  Export your green screen foregroundframes -- using direct JPEG fields.  It's easier if youturn off SETTINGS/SAVE ICONS so you don't have to dealwith them.

 WARNING: 

      DO NOT export full frames thru multipic.lib, and esp.    do not use the deinterlace function!!!!  This creates    jaggy edges on your talent hat CineMatte cannot deal    with.  Always deal with fields.


Next, export an equal number of frames for the background.You MUST have exactly the same number of frames. If usinga static single frame, you can use a single fieldrepeatedly.

Now, run ImageFX. CineMatte is found under the "HOOKS"button.

There seem to be two bugs in Cinematte.    1. When you click on the dynamic range box and then    unclick it, subsequent changes have no effect.  You    have to cancel out of the hook and start again.    2. When you run Cinematte from either IMP or AutoFX,    the program uses the current settings and does not    import the settings from the Arexx script.  This is    true every way I have tried, using the supplied    Cinematte.ifx script, recording my own script, or    typing the command string into IMP.  The workaround    is to load main & swap buffers and actually run    Cinematte by hand to obtain the proper settings.    (You have to do this anyway to find the settings!)    Then hook out to IMP, and run a series calling    Cinematte.  The settings that will be used are the    ones you entered in Cinematte earlier, no matter    what settings are specified in the script.

Load a representative foreground frame into the mainbuffer.  Load a background frame into the swap buffer.Run CineMatte.

The CineMatte requester has a bunch of settings.  Setthe screen color to either blue or green, as appropriate.Set output to COMPOSITE ONLY.  Set the preview windowto MATTE and run a preview.  The BG will be gray and FGa lighter gray.  You want the matte to be BLACK in thegreen or blue areas, and WHITE for your FG subject.Usually I have to click on the DYNAMIC RANGE gadget, whichexpands the contrast between FG & BG.  Then move theDARKEN BG gadget up and test until you get a nice solidblack BG.  The FG will be compromised.  Now move theLIGHTEN FG gadget up to clear the FG to white.  Don'tgo higher than you have to on either one!  The previewscreen is really too small to fine-tune, so click on OKAYto run the settings.  Hey Kermit, could we get biggerpreview windows?

The output composite will be half-height, of course,since its a single field.  If it helps, double the heightand the render to eyeball your results.  If you can't getrid of the blue aura by twiddling with with the BG/FGgadgets, try the REMOVE HALO.

I haven't figured out the PROTECTION and RESTORE RGBitems yet.  REMOVE BG should be set to 0, and BLENDset to 255. Write down your settings!

When you get a satisfactory test render, you're ready.But MovieShop JPEGs have something just a little"wrong" which will trigger a warning requester everyfew frames.  Unless you want to stay up all nightclicking OK, hit the ESC button on your keyboard topull up the IFX cli.  type the command "requesters off."Hit return, then return again to close the field. Thiswill prevent the warnings.

Now run IMP or AutoFX.  I use IMP.  DO NOT quit IFX,and do this in another session, you'll losethe CineMatte settings and the script settings won'twork. Load the series of FG frames into the MAIN andthe BG frames into SWAP.  You can put a single fieldin SWAP and IMP will use it repeatedly.  Set the LOADformat to JPEG, no options.  Set the SAVE format toJPEG, and set the OPTIONS to [QUALITY %] BEST.  I use"85 BEST".  If you don't specify BEST, you'll getblocky garbage.  Make sure all the other settings inIMP are right, and type in the processing call in thebottom field.  Here's the string I used last (shouldbe a single line):

HOOK CINEMATTE GREENSCREEN OUTPUTCOMPOSITE 1 101 56 5 0 0 0 0 0 255

The second # is the DARKEN BG, third is LIGHTEN FG,fourth is REMOVE HALO, etc.  Actually, since CineMattedoesn't seem to read these in right, you couldprobably have these numbers wrong and it would stillwork as long as you set CM properly earlier.  I likea belt and suspenders, though.  Now click BEGIN.

Go to bed.

When you get up, viola!

Import the fields back into your MS project, usingmultipic.lib.  MS will not directly import JPEGs fromIFX.  View your work.  Clap yourself on the back andraise your prices.

GOOD LUCK!

John Jackman

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