From-ShaderX-2-–-Shader-Programming-Tips-and-Trick.docx

AdvancedImageProcessingwithDirectX9PixelShadersJasonL.Mitchell,MarwanY.AnsariandEvanHart3DApplicationResearchGroupATIResearchIntroductionWiththeintroductionoftheps_2_0pixelshadermodelinDirectX9.0,weareabletosignificantlyexpandourabilitytouseconsumergraphicshardwaretoperformimageprocessingoperations.Thisisduetothelongerprogramlength,theabilitytosamplemoretimesfromtheinputimage(s)andtheadditionoffloatingpointinternaldatarepresentation.InthefirstShaderXbook,weusedtheps_l_4pixelshadermodelinDirectX8.1toperformbasicimageprocessingtechniquessuchassimpleblurs,edgedetection,transferfunctionsandmorphologicaloperatorsMitchell02.Inthischapter,wewillextendourimageprocessingtoolboxtoincludecolorspaceconversion,abetteredgedetectionfiltercalledtheCannyfilter,separableGaussianandmedianfilters,andareal-timeimplementationoftheFastFourierTransform.ReviewAsshowninouroriginalimageprocessingchapterinthefirstShaderXbook,post-processingof3Dframesisfundamentaltoproducingavarietyofinterestingeffectsingamescenes.ImageprocessingisperformedonaGPUbyusingthesourceimageasatextureanddrawingascreen-alignedquadrilateralintothebackbufferoranothertexture.Apixelshaderisusedtoprocesstheinputimagetoproducethedesiredresultintherendertarget.Figure 1 - Using a pixel shader for image processing by rendering from one image to anotherOutputImageImageprocessingisespeciallypowerfulwhenthecolorofthedestinationpixelistheresultofcomputationsdoneonmultiplepixelsfromthesourceimage.Inthiscase,wesamplethesourceimagemultipletimesandusethepixelshadertocombinethedatafromthemultiplesamples(ortaps)toproduceasingleoutput.ColorSpaceConversionBeforewegetintointerestingmulti-tapfilters,we,llpresentapairofshaderswhichcanbeusedtoconvertbetweenHSVandRGBcolorspaces.Theseshadersperformsomerelativelycomplexoperationstoconvertbetweencolorspaceseventhoughtheyareonlysingle-tapfilters.ForthosewhomaynotbefamiliarwithHSVspace,itisacolorspacewhichisdesignedtobeintuitivetoartistswhothinkofacolor'stint,shadeandtoneSmith78.InterpolationinthiscolorspacecanbemoreaestheticallypleasingthaninterpolationinRGBspace.Additionally,whencomparingcolors,itmaybedesirabletodosoinHSVspace.Forexample,inRGBspace,thecolor100,0,0)isverydifferentfromthecolor0,0,100.However,theirVcomponentsinHSVspaceareequal.Colors,representedbyhue,saturation,valuetriplesaredefinedtoliewithinahexagonalpyramidasshowninFigure2below.Thehueofacolorisrepresentedbyananglebetween0°and360oaroundthecentralaxisofthehexagonalcone.Acolor,ssaturationisthedistancefromthecentral(achromatic)axisanditsvalueisthedistancealongtheaxis.Bothsaturationandvaluearedefinedtobebetween0and1.WehavetranslatedthepseudocodeRGB-to-HSVtransformationfromFoley90totheDirectX9HighLevelShadingLanguage(HLSL)andcompileditfortheps_2_0target.IfyouareunfamiliarwithHLSL,youcanreferbacktotheintroductorychapter"'IntroductiontotheDirectX®9HighLevelShadingLanguage."AsdescribedinSmith79,youcanseethattheRGB_to_HSV()functioninthisshaderfirstdeterminestheminimumandmaximumchannelsoftheinputRGBcolor.ThemaxchanneldeterminesthevalueoftheHSVcolor,orhowfaralongtheachromaticcentralaxisofthehexagonalconetheHSVcolorwillbe.ThesaturationisthencomputedasthedifferencebetweenthemaxandminRGBchannelsdividedbythemax.Hue(theanglearoundthecentralachromaticaxis)isthenafunctionofwhichchannelhadthemaxmagnitudeandthusdeterminedthevalue.float4RGB_to_HSV(float4color)(floatrzg,b,delta;floatColorMaxzcolorMin;floath=0,s=0,v=0;float4hsv=0;r=color0;g=color1;b=color2;ColorMax=max(rzg);ColorMax=max(colorMax,b);colorMin=min(rzg);colorMin=min(colorMin,b);v=colorMax;/thisisvalueif(colorMax!=O)(s=(colorMax-colorMin)/colorMax;if(s!=O)/ifnotachromatic(delta=colorMax-colorMin;if(r=colorMax)h=(g-b)/delta;elseif(g=colorMax)Ih=2.0+(b-r)/delta;else/bismaxIh=4.0+(r-g)/delta;h*=60;if(h<0)(h+=360;hsv0=h/360.0;/movinghtobebetween0and1.hsv1=s;hsv2=v;returnhsv;TheHSV-to-RGBtransformation,alsotranslatedfromFoley901,isshownbelowinHLSL.float4HSV_to_RGB(float4hsv)(float4color=0;floatfzpzq,t;floath,s,v;floatr=0,g=0,b=0;floati;if(hsvl=0)(if(hsv2!=0)(color=hsv2;else(h=hsv.x*360.0;s=hsv.y;v=hsv.z;if(h=360.0)(h=0;)h/=60;i=floor(h);f=h-i;p=v*(1.0-s);q=V*t=V*(1.0-(s(1.0-(s*f)；*(1.0-f);if(i=0)(r=v;g=t;b=)P；elseif(i=1)r=q；g=v;b=)P；elseif(i=2)r=P；g=v;b=)t;elseif(i=3)r=P；g=q；b=V；elseifI(i=4)r=t;g=P；b=V；elseif(i=5)r=v;g=P；b=)q；color.r=r;color.g=g；color.b)=b;returncoloOtherColorSpacesItisworthnotingthatRGBandHSVarenottheonlycolorspacesofinterestincomputergraphics.Forexample,theoriginalpaperSmith78whichintroducedHSValsointroducedacolorspac