1、Models and Architectures原著Ed AngelProfessor of Computer Science, Electrical and Computer Engineering, and Media ArtsUniversity of New Mexico編輯 武漢大學計算機學院圖形學課程組ObjectivesLearn the basic design of a graphics systemIntroduce pipeline流水線 architectureExamine software components for an interactive graphics

2、 systemImage Formation RevisitedCan we mimic模仿 the synthetic camera model to design graphics hardware software?Application Programmer Interface (API)Need only specify Objects對象Materials材質屬性Viewer觀察者Lights光源But how is the API implemented?Physical ApproachesRay tracing光線跟蹤: follow rays of light from c

3、enter of projection until they either are absorbed by objects or go off to infinity無窮遠Can handle global effectsMultiple reflectionsTranslucent半透明 objectsSlowMust have whole data baseavailable at all times Radiosity輻射度: Energy based approachVery slowPhysical Approaches ?(The answer is no)Lets see the

4、 Graphics Architectures Outline1.7 Graphics Architectures1.7.1 Display Processors 1.7.2 Pipeline Architecture1.7.3 The Graphics Pipeline1.7.4 Vertex Processing1.7.5 Clipping and Primitive Assembly1.7.6 Rasterization1.7.7 Fragment Processing. The Programmers Interface1.6.2 Three-Dimensional APIs1.7 G

5、raphics ArchitecturesEarly graphics systems used general-purpose computers.(通用計算機)CPUSystemMemoryVideoControllerMonitorSystem BusI/O DevicesDigital to analog(D/A) converter1.7.1 Display ProcessorsSpecial-purpose graphics systems, relieving the general-purpose computer from the task of refreshing the

6、 display continuously.CPUSystemMemoryVideoControllerSystem BusI/O DevicesDisplay Processormemory(Frame Buffer)VideoControllerMonitorDisplay listDisplay ProcessorRather than have the host computer try to refresh display use a special purpose computer called a display processor (DPU)Graphics stored in

7、 display list顯示列表 (display file) on display processorHost compiles display list and sends to DPU1.7.2 Pipeline ArchitecturePipeline流水線Concurrency并行Delay or latency延遲Throughput吞吐量1.7.3 The Graphics Pipeline (Practical Approach)Process objects one at a time in the order they are generated by the appli

8、cationCan consider only local lighting局部光照明Pipeline architectureAll steps can be implemented in hardware on the graphics cardapplication programdisplay1.7.4 Vertex Processing頂點處理Vertex頂點包含拓撲信息在內的幾何表示拓撲信息Topology，鄰接關系、次序幾何信息Geometry，點（一般意義的點point）坐標信息Vertex頂點 及其 頂點序列構成 線段， 多邊形，或者 多邊形集合曲線、曲面 的參數頂點圖形學中

9、最基本的幾何模型1.7.4 Vertex Processing頂點處理Much of the work in the pipeline is in converting object representations(幾何表示) from one coordinate system to anotherObject coordinates對象坐標Camera (eye) coordinates觀察坐標Screen coordinates屏幕坐標Every change of coordinates is equivalent to a matrix矩陣 transformation Vertex

10、 processor also computes vertex colors1.7.4 Vertex Processing (Projection投影)Projection投影 is the process that combines the 3D viewer with the 3D objects to produce the 2D imagePerspective透視 projections: all projectors投影線 meet at the center of projection投影中心Parallel平行 projection: projectors投影線 are par

11、allel, center of projection is replaced by a direction of projection投影方向1.7.4 Vertex Processing (Projection投影)Chapter 5 will discuss projection processComputer viewingPerspective projectionsParallel projectionChapter 6 will discuss lighting and shading光照與明暗繪制computes vertex colors1.7.5 Clipping and

12、Primitive Assembly 裁減 與 圖元組裝(Primitive Assembly)Vertices must be collected into geometric objects before clipping and rasterization can take placeLine segments線段Polygons多邊形Curves and surfaces 曲線 曲面 1.7.5 Clipping and Primitive Assembly (Clipping裁減)Just as a real camera cannot “see” the whole world,

13、the virtual camera can only see part of the world or object spaceObjects that are not within this volume are said to be clipped out of the scene1.7.5 Clipping and Primitive Assembly (Clipping)Chapter 7 (7.3, 7.4, 7.5, 7.6, 7.7) will discuss Clipping processClippingLine segments Clipping Polygon clip

14、pingClipping of other primitivesClipping in three dimensions1.7.6 Rasterization光柵化If an object is not clipped out, the appropriate pixels in the frame buffer must be assigned colorsRasterizer produces a set of fragments片元 for each objectFragments are “potential潛在 pixels”Have a location in frame buff

15、er位置信息Color attributes顏色屬性Depth information深度信息Vertex attributes are interpolated over objects by the rasterizer1.7.6 Rasterization光柵化Chapter 7 (7.8, 7.9, 7.10) will discuss rasterization process for Line segments Polygons基本圖元1.7.7 Fragment Processing片元處理Fragments are processed to determine the colo

16、r of the corresponding pixel in the frame buffer(真實感問題：兩類真實感問題)Colors can be determined by texture mapping紋理映射 or interpolation of vertex colors頂點顏色插值 一般意義上狹義上真實感Fragments may be blocked阻擋 by other fragments closer to the camera Hidden-surface removal隱藏面消除1.7.7 Fragment Processing片元處理Section 7.11 wi

17、ll discuss Hidden-surface removal隱藏面消除Chapter 6 will discuss lighting and shading光照與明暗繪制Section 8.7 will discuss texture mapping紋理映射 Chapter 9 will discuss programmable shaders可編程著色器. The Programmers InterfaceProgrammer sees the graphics system through a software interface: the Application Programme

18、rs Interface (API)Fig 1.21 Application Programmers model of graphics systemapplication program interface (API)application programming interface (API)1.6.2 Three-Dimensional APIs(API Contents)Functions that specify what we need to form an imageObjects對象Viewer觀察者Light Source(s)光源Materials材質屬性Other inf

19、ormationInput from devices such as mouse and keyboardCapabilities of system1.6.2 Three-Dimensional APIs （Object Specification對象指定）Most APIs support a limited set of primitives includingPoints (0D object)Line segments (1D objects)Polygons (2D objects)Some curves and surfacesQuadrics二次曲面Parametric polynomials 參數多項式曲面All are defined through locations in space 空間位置 or Vertices頂點1.6.2 Three-Dimensional APIs （Example）glBegin(GL_POLYGON)glVertex3f(0.0, 0