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Hack 70. Composite a Green Screen Image

Hack 70. Composite a Green Screen Image

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Compositingyourfootagerequiresyoutouseakeyeffect,whichallowsyouto

removecertaincolorsorportionsofthevideosignal,suchasluminance,from

yourfinalimage.Whenusingakeyandmultiplelayersofvideo,thelayerbelow

theonethatiskeyedwillshowthrough.Figure6-36showsagreen-screenimage

withandwithoutakeyapplied.

Here'showtoapplyagreen-screenkey:



Avid

Tools



EffectsPalette



Key



ChromaKey



Figure6-36.Agreen-screenimagewithandwithoutakey

applied



FinalCut

Effects



VideoFilters



Key



BlueandGreenScreen



VideoEffects



Keying



Premiere

Effects



GreenScreenKey



Ifyoufindthatthespecifickeyeffectyouhaveuseddoesn'tworkaswellasyou

expected,youcantryotherkeyeffects,suchaslikeColorKey,ChromaKey,or



RGBKey.Figure6-37showshowaninitialcompositemightlook.

Figure6-37.Mergingthetwoimagesinaninitialcomposite



Unlessyou'rereallylucky(orreallygood),youwillhavetochangetheposition

ofyouractorswithintheframe.



6.11.2.ShiftingtheShot

Thenextstepincompositingyourshotinvolvesmovingittothelocationyou

desire.Bymovingthecenterpointofthefootage,youcanchangeyouractors'

spatialrelationshiptotherestofthescene.Thiseffectivelyenablesyoutoplace

youractorsinamorebelievableposition.



Avid

Tools



EffectsPalette



Image



Resize(Position)



FinalCut

Viewerwindow



Motiontab



BasicMotion



Center



Premiere

EffectControls



Motion



Position



Changingthepositionofyourvideoisreallyamatteroftrialanderror.Itmight

takeyouashorttimetogetthevideotowhereyouwantit.Onceyou'veplaced

yourvideo,eitherplayit(ifyoursystemiscapableofreal-timeplayback)or

spot-checkitbyclickinginvariousplacesonyourtimeline.Ifthereisalotof

movementinyourgreen-screenimage,youmightneedtorepositionyourvideo

untilitlooksright.Figure6-38showsthecompositeafterrepositioningthe

green-screenimage.

Figure6-38.Movingthecenteroftheimage



Again,unlessyou'rereallylucky,youwillneedtomakeafewmoreadjustments

inordertofoolyouraudienceintobelievingtheireyes.



6.11.3.AddingtheFinishingTouches

Dependingonhowyouhavelityourgreenscreen,thecolorsandshadowingof

yourbackground,andtheoverallmatchofyourfootage,youmightneedto

applyadditionaleffectstoachieveanylevelofrealism.Someeffectsyoumight

havetoworkwithincludesaturation/desaturation,brightnessandcontrast,and

generalcolorlevels.

Ifyouarecomfortableusingcolorcorrection,thatwillbeyourmostpowerful



tool.Additionally,ifthereareshadowsinyourbackground,youmightwantto

addshadows[Hack#71]tothegreen-screenimagetomatch.Figure6-39shows

theimageafterslightlydesaturatingitandadjustingandthebrightnessand

contrast.

Figure6-39.Alteringthesaturation,brightness,andcontrast



Gettingagreenscreentoachievealevelofbelievabilitycantakealongtime.

Whenworkingwiththefootage,youshouldtakeabreakandwalkawayatleast

everyhour.Otherwise,youmightbecometooengrossedintheeffecttothink

aboutitclearly.



Hack71.CreateaShadowforaGreenScreen

Image



Compositingsomeoneorsomethingintoasceneisanaccomplishment.But

tomakeitbelievable,youmightneedtoaddashadow.

Agreenscreenimageisonewhereanimageiscapturedwithabackground

consistingofonlyonecolor,green.Youcanalsoacquireabluescreenimage,

wherethebackgroundisblue,orjustaboutanyothercoloryoucanimagine.The

essentialingredientisthatyourimageisinfrontofasingle,solid-colored

background,andthatyoursubjectnothavethesamecolorasyourbackground

onit.Italsohelpsifyouareabletolightandsetthescenewithcompositingin

mind[Hack#22].

Whenyoucompositeasceneusingagreenscreen,youmightdiscoverthereis

noshadowbeingcastfromyoursubject.Ifyoudiscoverthesceneyouare

attemptingtocompositeneedsashadow,becauseitjustdoesn'tlookright,there

isaneasysolutiontoyourproblem.Evenbetterthanthat,thesolutioncanbe

foundwithintheproblemitself.



Thisprocessworkswellforashotofasinglemovingsubject,suchas

awomanwalkingacrossabeach.Itcanalsoworkjustaswellfor

morethanonesubject;however,themoremovementthereisbetween

thesubjects,suchasdancinginacircle,thelesssuccessfulthis

processwillbe,becauseyourshadowwon'tcastfromonesubjectonto

theother.



6.12.1.CreatingaCopyofYourImage



Thefirststepincreatingyourshadoweffectistoduplicateyourvideoimage.To

doso,simplycreateanothervideolayeronyourtimelineanddropacopyof

yourgreenscreenvideoontoit.Youwillwanttoapplythesameexacteffectyou

usedtoinitiallycompositeyourscene[Hack#70].Whenpreparedcorrectly,you

shouldhavetwolayersofvideo,withthesameeffectsapplied.



Thebestapproachtoduplicatingagreen-screenedimageistocopyand-pasteitintothetimeline.Afterapplyingthenecessaryeffectsto

keyoutthegreenscreenontheoriginalimage,copytheentireclip,

includingtheeffects,andpasteittoanother(lower)layeronyour

timeline.Thismightrequireshufflingyourfootageontonewlycreated

layersinyourtimeline.



6.12.2.LayeringVideotoPrepareforShadowCreation

Inorderfortheeffecttoappearcorrectly,youwillwantyouroriginalimageona

layerabovetheshadowimage'slayer.Ifyoudonotdostackthelayerscorrectly,

theshadowwillappearinfrontofyoursubject.Figure6-40showsthedifference

betweenhavingtheshadowlayerabovetheoriginallayerandbelowtheoriginal

layer.

Figure6-40.Thesubtlebutdistinctdifferenceinlayering



6.12.3.CreatetheShelloftheShadow



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Hack 70. Composite a Green Screen Image

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