SDL讲解,图形基本概念非常到位

SDL–2Video静宜大学信息工程系蔡奇伟副教授©2007纲要绘图系统设定显示模式绘图页BLITColorkeyAlpha值Clipping显示到荧幕SDL_image函式库图形显示framebuffer像素(Pixel,PictureElement)Pixel&Resolution荧幕上的一个光点称为一个像素（pictureelement,pixel）。分辨率（resolution）是荧幕像素的个数，通常用水平像素个数垂直像素个数的格式来表示，譬如：640480代表荧幕水平方向有640个像素以及垂直方向有480个像素。注：数位图片的最小组成单位也称为像素。图片的大小也常用上述的乘积方式来表示，譬如：一张640480的图片，水平方向有640个像素以及垂直方向有480个像素。CRTRetrace(回归讯号)DirectColor(直接色)FrameBufferRGB注：通常每个像素有16bitsor24bits。IndirectColor(间接色)FrameBuffer注：通常每个像素有8bits。ScreenMode设定显示模式SDL_Surface*SDL_SetVideoMode(intwidth,//宽度intheight,//高度intbpp,//bitsperpixel，像素位元数Uint32flags//旗标);SDL_SWSURFACE在系统存储器中建立视讯绘图页SDL_HWSURFACE在视讯存储器中建立视讯绘图页SDL_ASYNCBLITEnablestheuseofasynchronousupdatesofthedisplaysurface.ThiswillusuallyslowdownblittingonsingleCPUmachines,butmayprovideaspeedincreaseonSMPsystems.SDL_ANYFORMAT*使用荧幕目前的bpp。注：有星号的旗标只能用于荧幕设定。SDL_HWPALETTEGiveSDLexclusivepaletteaccess.WithoutthisflagyoumaynotalwaysgetthethecolorsyourequestwithSDL_SetColorsorSDL_SetPalette.SDL_DOUBLEBUF*Enablehardwaredoublebuffering;onlyvalidwithSDL_HWSURFACE.CallingSDL_Flipwillflipthebuffersandupdatethescreen.Alldrawingwilltakeplaceonthesurfacethatisnotdisplayedatthemoment.IfdoublebufferingcouldnotbeenabledthenSDL_FlipwilljustperformaSDL_UpdateRectontheentirescreen.SDL_FULLSCREEN*SDLwillattempttouseafullscreenmode.Ifahardwareresolutionchangeisnotpossible(forwhateverreason),thenexthigherresolutionwillbeusedandthedisplaywindowcenteredonablackbackground.SDL_OPENGL*CreateanOpenGLrenderingcontext.YoushouldhavepreviouslysetOpenGLvideoattributeswithSDL_GL_SetAttribute.SDL_OPENGLBLIT*CreateanOpenGLrenderingcontext,likeabove,butallownormalblittingoperations.Thescreen(2D)surfacemayhaveanalphachannel,andSDL_UpdateRectsmustbeusedforupdatingchangestothescreensurface.SDL_RESIZABLE*Createaresizablewindow.WhenthewindowisresizedbytheuseraSDL_VIDEORESIZEeventisgeneratedandSDL_SetVideoModecanbecalledagainwiththenewsize.SDL_NOFRAME*Ifpossible,SDL_NOFRAMEcausesSDLtocreateawindowwithnotitlebarorframedecoration.Fullscreenmodesautomaticallyhavethisflagset.范例SDL_Surface*screen;//640480,8bpp,存于系统存储器screen=SDL_SetVideoMode(640,480,8,SDL_SWSURFACE);//640480,32bpp,存于系统存储器screen=SDL_SetVideoMode(640,480,32,SDL_SWSURFACE);//1024768,与幕萤目前的bpp相同，存于系统存储器screen=SDL_SetVideoMode(1024,768,0,SDL_SWSURFACE|SDL_ANYFORMAT);//800600,32bpp，存于视讯存储器、用双缓冲区、全荧幕模式screen=SDL_SetVideoMode(800,600,32,SDL_HWSURFACE|SDL_DOUBLEBUF|SDL_FULLSCREN);Surface(绘图页)何谓绘图页SDL_Surface结构SDL_PixelFormat结构SDL_GetRGB()与SDL_MapRGB()Pitch直接存取绘图页的像素资料建立绘图页释放绘图页转换绘图页的像素格式何谓绘图页？绘图页(Surface)是一个抽象结构，用来描述一块储存图片的存储器区域，其中包含图片的宽度、高度、像素格式、储存位址、……、等等相关信息。SDL把framebuffer视为一种特殊的绘图页，称之为荧幕绘图页(DisplaySurface)。其他的绘图页称为幕后绘图页(Off-screenSurface)。某些绘图页的旗标仅能用于设定荧幕绘图页。SDL用SDL_Surface结构来定义绘图页。SDL_Surface结构typedefstructSDL_Surface{Uint32flags;/*旗标*/SDL_PixelFormat*format;/*像素格式*/intw,h;/*图片宽度与高度(pixels)*/Uint16pitch;/*绘图页实际宽度(bytes)*/void*pixels;/*像素资料的储存位址*//*clippinginformation*/SDL_Rectclip_rect;/*剪裁矩形*//*Referencecount--usedwhenfreeingsurface*/intrefcount;/*参照此绘图页的指标个数*//*Thisstructurealsocontainsprivatefieldsnotshownhere*/}SDL_Surface;SDL_PixelFormat结构typedefstruct{SDL_Palette*palette;Uint8BitsPerPixel;Uint8BytesPerPixel;Uint32Rmask,Gmask,Bmask,Amask;Uint8Rshift,Gshift,Bshift,Ashift;Uint8Rloss,Gloss,Bloss,Aloss;Uint32colorkey;Uint8alpha;}SDL_PixelFormat;范例32bpp:A8R8G8B8ARGB08162431成分maskshiftlossAlpha0xFF000000240Red0x00FF0000160Green0x0000FF0080Blue0x000000FF00范例16bpp:R5G6B5ARGB051115成分maskshiftlossRed0xF800=11111000000000002113Green0x07E0=0000011111100000252Blue0x001F=0000000000011111203范例//底下的函式用像素格式中的mask,shift,和loss的值，//取出对应的Red,Green,或Blue成份值。Uint32Get_PrimaryColor(Uint32,pixel,Uint32mask,Uint8shift,Uintloss){Uint32pc=((pixel&mask)shift))loss;returnpc;}假定变量screen是代表荧幕的绘图页，变量pv已经存入其中的某一像素值，则下列的函式呼叫可计算出此像素的三原色成分值：SDL_PixelFormat*fmt=screen-format;Uint32red,green,blue;red=Get_PrimaryColor(pv,fmt-Rmask,fmt-Rshift,fmt-Rloss);green=Get_PrimaryColor(pv,fmt-Gmask,fmt-Gshift,fmt-Gloss);blue=Get_PrimaryColor(pv,fmt-Bmask,fmt-Bshift,fmt-Bloss);SDL_GetRGB()voidSDL_GetRGB(Uint32pixel,SDL_PixelFormat*fmt,Uint8*r,Uint8*g,Uint8*b);依据像素格式fmt，取出像素pixel中三原色的值。SDL_MapRGB()Uint32SDL_MapRGB(SDL_PixelFormat*fmt,Uint8r,Uint8g,Uint8b);依据像素格式fmt，传回三原色合成的像素值。Pitch我们可以用pitch的值来求取下一列像素的起始位址。譬如：假定指标start指到第一个像素，则第k列像素的起始位址是start+k*pitchk=0,1,…,h-1。pitch可能不等于wBytesPerPixel。hw(pixels)pitch(bytes)gappixel/**Returnthepixelvalueat(x,y)*NOTE:Thesurfacemustbelockedbeforecallingthis!*/Uint32getpixel(SDL_Surface*surface,intx,inty){intbpp=surface-format-BytesPerPixel;/*Herepistheaddresstothepixelwewanttoretrieve*/Uint8*p=(Uint8*)surface-pixels+y*surface-pitch+x*bpp;switch(bpp){case1:return*p;case2:return*(Uint16*)p;case3:if(SDL_BYTEORDER==SDL_BIG_ENDIAN)returnp[0]16|p[1]8|p[2];elsereturnp[0]|p[1]8|p[2]16;case4:return*(Uint32*)p;default:return0;/*shouldn'thappen,butavoidswarnings*/}}/**Setthepixelat(x,y)tothegivenvalue*NOTE:Thesurfacemustbelockedbeforecallingthis!*/voidputpixel(SDL_Surface*surface,intx,inty,Uint32pixel){i