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BuffersSoftwareCollege,ShandongUniversityInstructor:ZhouYuanfengE-mail:yuanfeng.zhou@gmail.comReview2•Rasterization:Polygonscanconversionalgor

ithm;•Hiddensurfaceremoval•Aliasing3Objectives•IntroduceadditionalOpenGLbuffers•Learntoreadandwritebuffers•Learn

touseblending4BufferDefineabufferbyitsspatialresolution(nxm)anditsdepth(orprecision)k,thenumberofbits/pixelpixel5OpenGLFrameBuffer6OpenGLBuffers•Co

lorbufferscanbedisplayed-Front-Back-Auxiliary-Overlay•Depth•Accumulation-Highresolutionbuffer•Stencil-Holdsmasks7WritinginBuffers

•Conceptually,wecanconsiderallofmemoryasalargetwo-dimensionalarrayofpixels•Wereadandwriterectangularblockofpixels-B

itblocktransfer(bitblt)operations•Theframebufferispartofthismemory(Doublebuffer)•LeftandrightbuffersforStereroi

magesframebuffer(destination)writingintoframebuffersourcememoryBit-blockwriting•Ascan-linewritingwhilepolygonfilling•Cha

racterdisplay•Clearallpixelbuffer8BitBltcopy•BOOLBitBlt(intx,inty,intnWidth,intnHeight,CDC*pSrcDC,intxSrc,intySrc,DWORDdwRop);//SRCCOPY•Copie

sabitmapfromthesourcedevicecontexttothiscurrentdevicecontext.(FromaBitmapinmemorytocurrentCDC)910WritingModelReaddestinationpixel

beforewritingsourceSource:sDestination:d’d’=f(s,d)11BitWritingModes(16modes)•Sourceanddestinationbitsarecombinedbitwise•

16possiblefunctions(onepercolumnintable)replaceORXORMode3&6Mode6Mode31213XORmode•RecallfromChapter3thatwecanuseXORbyenablinglogicopera

tionsandselectingtheXORwritemodeS(SM)M•XORisespeciallyusefulforswappingblocksofmemorysuchasmenusthatarestoredoffscreenIfSrepresentsscreenandMrep

resentsamenuthesequenceSSMMSMSSMswapstheSandM14ThePixelPipeline•OpenGLhasaseparatepipelineforpixels-Writingpixelsinvolves•Movingpixelsf

romprocessormemorytotheframebuffer•Formatconversions•Mapping,Lookups,Tests-Readingpixels•Formatconversio

n15RasterPosition•OpenGLmaintainsarasterpositionaspartofthestate•SetbyglRasterPos*()(worldcoord)-glRasterPos3

f(x,y,z);•Therasterpositionisageometricentity-Passesthroughgeometricpipeline-Eventuallyyieldsa2Dpositioninscreenc

oordinates-Thispositionintheframebufferiswherethenextrasterprimitiveisdrawn16BufferSelection•OpenGLcandrawintoorread

fromanyofthecolorbuffers(front,back,auxiliary)•Defaulttothebackbuffer•ChangewithglDrawBufferandglReadBuffer•Notethatformatofthepix

elsintheframebufferisdifferentfromthatofprocessormemoryandthesetwotypesofmemoryresideindifferentplaces-Needpacki

ngandunpacking-DrawingandreadingcanbeslowOpenGLcodeLib3dsMesh*mesh=f3ds->meshes[i];//由3ds文件中读取出需要数据glGenBuffers(1,&m_Vertex[i]);glGenBuffers(1,&m_I

ndex[i]);glBindBuffer(GL_ARRAY_BUFFER,m_Vertex[i]);glBufferData(GL_ARRAY_BUFFER,(sizeof(float)*9*mesh->nfaces)+(siz

eof(float)*3*mesh->nvertices),TotleV,GL_STATIC_DRAW);glVertexPointer(3,GL_FLOAT,0,BUFFER_OFFSET(0));glNormalPointer(

GL_FLOAT,0,BUFFER_OFFSET(V_size));glBindBuffer(GL_ARRAY_BUFFER,m_Vertex[0]);glBindBuffer(GL_ELEMENT_ARRA

Y_BUFFER,m_Index[0]);glDrawElements(GL_TRIANGLES,DrawCount[0],GL_UNSIGNED_SHORT,NULL);1718Bitmaps•OpenGLtreats1-bitpixels(bitmaps)differe

ntlyfrommulti-bitpixels(pixelmaps)•Bitmapsaremasksthatdetermineifthecorrespondingpixelintheframebuffer

isdrawnwiththepresentrastercolor-0colorunchanged-1colorchangedbasedonwritingmode•Bitmapsareusefulf

orrastertextcursor(XOR)-GLUTfont:GLUT_BIT_MAP_8_BY_1319RasterColor•SameasdrawingcolorsetbyglColor*()•Currentrastercol

orisfixedbylastcalltoglRasterPos*()•Geometrydrawninblue•OnesinbitmapuseadrawingcolorofredglColor3f(1.0,0.0,0.0);glRasterPos3f(x,

y,z);glColor3f(0.0,0.0,1.0);glBitmap(…….glBegin(GL_LINES);glVertex3f(…..)20DrawingBitmapsglBitmap(width,height

,x0,y0,xi,yi,bitmap)firstrasterpositionsecondrasterpositionoffsetfromrasterpositionincrementsinrasterpositionafterbitmap

drawnTextdefinitioninOpenGL•Rastertextisfasterthanvectortext;•Times,Courier,ComputerModern;•Size:10pt,24pt(1inch=72pt)21TextinOpenGL•8

x13textisdefinedinGLUT_BITMAP_8_BY_13;•Thesizeoftextisrelatedwithresolutionofscreen;•Define8x13text:•GLu

bytemy_font[128][13];•base=glGenLists(128);•for(i=0;i<128;i++){•glNewList(base+i,GL_COMPILE);•glBitMap(8,13,0.0,0.0,10.0,0.0,my_font[i]);•glE

ndList();•}2223Example:CheckerBoardGLubytewb[2]={0x00,0xFF};GLubytecheck[512];inti,j;for(i=0;i<64;i++)for(j=0;j<8,j+

+)check[i*8+j]=wb[(i/8+j)%2];glBitmap(64,64,0.0,0.0,0.0,0.0,check);24ImageFormats•Weoftenworkwithimagesinastandardformat(JPEG,TIFF,GIF,

PS,EPS)•Howdoweread/writesuchimageswithOpenGL?•NosupportinOpenGL-OpenGLknowsnothingofimageformats-Somecodeavaila

bleonWeb-Canwritereaders/writersforsomesimpleformatsinOpenGLPS&EPS•PostScript(PS)imageforcontrollingprinterRGBimageconvertsto7bi

tsASCII;soPSimagecanbeidentifiedbyprinter,butitislarge.•EncapsulatedPostScript(EPS)issimilartoPSimage.25GIFimage

•CompuServeIncorporated(1987)GraphicsInterchangeFormatColorIndexImagelow-resolutionsmallanimations(imageslices)26TIFF•Tagge

d(标签)ImageFileFormat•Highquality•Firstforscannedcopy•Losslesscompression•Cansavevectorboundaryofimage27JPEG•lossycompression•Hi

ghcompressionratio•Discretecosinetransform28Sizecomparison•Image1200x1200•TIFF:1440198bytes(about1.37m)•JPE

G:80109and38962bytes(about78kb)•EPS:abouttwiceofTIFF•CompressedTIFF:abouthalfsize•ZIPcompression:TIFFandEPSaresimilar29TIFFJPEG-18JPEG

-3730DisplayingaPPMImage(Linux)•PPMisaverysimpleformat(24bits)•Eachimagefileconsistsofaheaderfollowedbyallthepixeldata•HeaderP3#comment1#comment

2.#commentnrowscolumnsmaxvaluepixels31ReadingtheHeaderFILE*fd;intk,nm;charc;inti;charb[100];floats;intred,green,blue;printf("enterfil

ename\n");scanf("%s",b);fd=fopen(b,"r");fscanf(fd,"%[^\n]",b);if(b[0]!='P'||b[1]!='3'){printf("%sisnotaPPMfile!\n",b);exit(0

);}printf("%sisaPPMfile\n",b);checkfor“P3”infirstline32ReadingtheHeader(cont)fscanf(fd,"%c",&c);while(c=='#'){fscanf(fd,"%[^\n]",b);printf("%s\n",b

);fscanf(fd,"%c",&c);}ungetc(c,fd);skipovercommentsbylookingfor#infirstcolumn33ReadingtheDatafscanf(fd,"%d%d%d",&n,&m,&k);printf("%drows%dco

lumnsmaxvalue=%d\n",n,m,k);nm=n*m;image=malloc(3*sizeof(GLuint)*nm);s=255./k;for(i=0;i<nm;i++){fscanf(fd,"%d%d%d",&red,&green,&blue);image[3

*nm-3*i-3]=red;image[3*nm-3*i-2]=green;image[3*nm-3*i-1]=blue;}scalefactor34PixelMaps•OpenGLworkswithrectangularar

raysofpixelscalledpixelmapsorimages•Pixelsareinonebyte(8bit)chunks-Luminance(grayscale)images1byte/pixel-RGB3bytes/pixel•Threefunctio

ns-Drawpixels:processormemorytoframebuffer-Readpixels:framebuffertoprocessormemory-Copypixels:framebuffertoframebufferPixelformat•Imageinmemory:RG

B,RGBA,Index•Format:intorfloat•glPixelMap3536OpenGLPixelFunctionsglReadPixels(x,y,width,height,format,type,myimage)startpixelinframebuffersizetypeo

fimagetypeofpixelspointertoprocessormemoryGLubytemyimage[512][512][3];glReadPixels(0,0,512,512,GL_RGB,GL_UNSIGNED_BYTE,myimage);glDrawPixels(wid

th,height,format,type,myimage)startsatrasterposition37ScalingtheImageDataWecanscaletheimageinthepipelineglPixelTransferf(GL_RED_SCALE,

s);glPixelTransferf(GL_GREEN_SCALE,s);glPixelTransferf(GL_BLUE_SCALE,s);Wemayhavetoswapbyteswhenwegofromprocessor

memorytotheframebufferdependingontheprocessor.Ifso,wecanuseglPixelStorei(GL_UNPACK_SWAP_BYTES,GL_TRUE);38Thedisplay

callbackvoiddisplay(){glClear(GL_COLOR_BUFFER_BIT);glRasterPos2i(0,0);glDrawPixels(n,m,GL_RGB,GL_UNSIGNED_INT,image);glFlush();}