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

onalgorithm;•Hiddensurfaceremoval•Aliasing3Objectives•IntroduceadditionalOpenGLbuffers•Learntoreadandwritebuffers•Learntouseblending4Bu

fferDefineabufferbyitsspatialresolution(nxm)anditsdepth(orprecision)k,thenumberofbits/pixelpixel5OpenGLFrameBuffer6OpenGLBu

ffers•Colorbufferscanbedisplayed-Front-Back-Auxiliary-Overlay•Depth•Accumulation-Highresolutionbuffer•Stencil-Holdsmasks7WritinginBuff

ers•Conceptually,wecanconsiderallofmemoryasalargetwo-dimensionalarrayofpixels•Wereadandwriterectangularblockofpixels-Bitblocktransfer(bitblt)operati

ons•Theframebufferispartofthismemory(Doublebuffer)•LeftandrightbuffersforStereroimagesframebuffer(destination)writingintoframebuffersou

rcememoryBit-blockwriting•Ascan-linewritingwhilepolygonfilling•Characterdisplay•Clearallpixelbuffer8BitBltcopy•BOOLBitB

lt(intx,inty,intnWidth,intnHeight,CDC*pSrcDC,intxSrc,intySrc,DWORDdwRop);//SRCCOPY•Copiesabitmapfromthesourcedevicecontexttothiscurre

ntdevicecontext.(FromaBitmapinmemorytocurrentCDC)910WritingModelReaddestinationpixelbeforewritingsourceSource:sDestination:d’

d’=f(s,d)11BitWritingModes(16modes)•Sourceanddestinationbitsarecombinedbitwise•16possiblefunctions(onepercolumnintable)replaceORXORMode3&6Mode6Mode3

1213XORmode•RecallfromChapter3thatwecanuseXORbyenablinglogicoperationsandselectingtheXORwritemodeS(SM)M•XORisespecia

llyusefulforswappingblocksofmemorysuchasmenusthatarestoredoffscreenIfSrepresentsscreenandMrepresentsamenuthesequenceSSMMSMSSMswapstheSand

M14ThePixelPipeline•OpenGLhasaseparatepipelineforpixels-Writingpixelsinvolves•Movingpixelsfromprocessormemoryt

otheframebuffer•Formatconversions•Mapping,Lookups,Tests-Readingpixels•Formatconversion15RasterPosition•OpenGLmaintainsarasterpositionaspartofthestat

e•SetbyglRasterPos*()(worldcoord)-glRasterPos3f(x,y,z);•Therasterpositionisageometricentity-Passesthroughgeometricpipeline-Eventuall

yyieldsa2Dpositioninscreencoordinates-Thispositionintheframebufferiswherethenextrasterprimitiveisdrawn16Buffe

rSelection•OpenGLcandrawintoorreadfromanyofthecolorbuffers(front,back,auxiliary)•Defaulttothebackbuffer•ChangewithglDrawBuf

ferandglReadBuffer•Notethatformatofthepixelsintheframebufferisdifferentfromthatofprocessormemoryandthesetwotypesofmemoryresideindif

ferentplaces-Needpackingandunpacking-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_Index[i]);glBindBuffer(GL_ARRAY_BUFFER,m_Vertex[i]);glBufferData(GL_ARRAY_BUFFER,(sizeof(

float)*9*mesh->nfaces)+(sizeof(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_ARRAY_BUFFE

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

ntlyfrommulti-bitpixels(pixelmaps)•Bitmapsaremasksthatdetermineifthecorrespondingpixelintheframebufferisdrawnwit

hthepresentrastercolor-0colorunchanged-1colorchangedbasedonwritingmode•Bitmapsareusefulforrastertextcursor(XOR)-GLUTfont:GLUT_BIT_MAP_8_BY_1319Rast

erColor•SameasdrawingcolorsetbyglColor*()•CurrentrastercolorisfixedbylastcalltoglRasterPos*()•Geometrydrawnin

blue•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)fi

rstrasterpositionsecondrasterpositionoffsetfromrasterpositionincrementsinrasterpositionafterbitmapdrawnTextdefinitioninO

penGL•Rastertextisfasterthanvectortext;•Times,Courier,ComputerModern;•Size:10pt,24pt(1inch=72pt)21TextinOpenGL•8x13textisdefinedin

GLUT_BITMAP_8_BY_13;•Thesizeoftextisrelatedwithresolutionofscreen;•Define8x13text:•GLubytemy_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]);•glEndList();•}2223Exa

mple: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];glBitma

p(64,64,0.0,0.0,0.0,0.0,check);24ImageFormats•Weoftenworkwithimagesinastandardformat(JPEG,TIFF,GIF,PS,EPS)•Howdoweread/writesuchima

geswithOpenGL?•NosupportinOpenGL-OpenGLknowsnothingofimageformats-SomecodeavailableonWeb-Canwritereaders/writersforsomesimpleformatsinO

penGLPS&EPS•PostScript(PS)imageforcontrollingprinterRGBimageconvertsto7bitsASCII;soPSimagecanbeidentifi

edbyprinter,butitislarge.•EncapsulatedPostScript(EPS)issimilartoPSimage.25GIFimage•CompuServeIncorporated(1987)GraphicsInterch

angeFormatColorIndexImagelow-resolutionsmallanimations(imageslices)26TIFF•Tagged(标签)ImageFileFormat•Highquality•Firstforscannedcopy•Loss

lesscompression•Cansavevectorboundaryofimage27JPEG•lossycompression•Highcompressionratio•Discretecosinetra

nsform28Sizecomparison•Image1200x1200•TIFF:1440198bytes(about1.37m)•JPEG:80109and38962bytes(about78kb)•EPS:abouttwiceofTIFF•Comp

ressedTIFF:abouthalfsize•ZIPcompression:TIFFandEPSaresimilar29TIFFJPEG-18JPEG-3730DisplayingaPPMImage(Linux)•PPMisaverysim

pleformat(24bits)•Eachimagefileconsistsofaheaderfollowedbyallthepixeldata•HeaderP3#comment1#comment2.#commentnrowscolumnsmaxvaluepixels3

1ReadingtheHeaderFILE*fd;intk,nm;charc;inti;charb[100];floats;intred,green,blue;printf("enterfilename

\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("%sisaPPMf

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

f("%s\n",b);fscanf(fd,"%c",&c);}ungetc(c,fd);skipovercommentsbylookingfor#infirstcolumn33ReadingtheDat

afscanf(fd,"%d%d%d",&n,&m,&k);printf("%drows%dcolumnsmaxvalue=%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•OpenGLworkswithrectangulararraysofpixelscalledpixelmapsorimages•Pixelsareino

nebyte(8bit)chunks-Luminance(grayscale)images1byte/pixel-RGB3bytes/pixel•Threefunctions-Drawpixels:processormemorytoframeb

uffer-Readpixels:framebuffertoprocessormemory-Copypixels:framebuffertoframebufferPixelformat•Imageinmemory:RGB,RGB

A,Index•Format:intorfloat•glPixelMap3536OpenGLPixelFunctionsglReadPixels(x,y,width,height,format,type,myimage)startpixelinframebuffersizetypeofimage

typeofpixelspointertoprocessormemoryGLubytemyimage[512][512][3];glReadPixels(0,0,512,512,GL_RGB,GL_UNSIGNED_BYTE,myimage);glDrawPixels(w

idth,height,format,type,myimage)startsatrasterposition37ScalingtheImageDataWecanscaletheimageinthepipelineglPixelTransferf(GL_R

ED_SCALE,s);glPixelTransferf(GL_GREEN_SCALE,s);glPixelTransferf(GL_BLUE_SCALE,s);Wemayhavetoswapbyteswhenwegofromproce

ssormemorytotheframebufferdependingontheprocessor.Ifso,wecanuseglPixelStorei(GL_UNPACK_SWAP_BYTES,GL_TRUE);38Thedisplaycallbackvoid

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