计算机图形学computergraphics课件14.pptx

1、1ObjectivesIntroduce the OpenGL shading functionsDiscuss polygonal shading Flat Smooth Gouraud2Steps in OpenGL shading1.Enable shading and select model2.Specify normals3.Specify material properties4.Specify lights3Normals In OpenGL the normal vector is part of the state Set by glNormal*()glNormal3f(

2、x,y,z);glNormal3fv(p);Usually we want to set the normal to have unit length so cosine calculations are correct Length can be affected by transformations Note that scaling does not preserved lengthglEnable(GL_NORMALIZE)allows for autonormalization at a performance penalty4Normal for Trianglep0p1p2nPl

3、ane n (p-p0)=0n=(p2-p0)(p1-p0)normalize n n/|n|pNote that right-hand rule determines outward face5Enabling Shading Shading calculations are enabled byglEnable(GL_LIGHTING)Once lighting is enabled,glColor()ignored glEnable(GL_COLOR_MATERIAL);Must enable each light source individuallyglEnable(GL_LIGHT

4、i)i=0,1.Can choose light model parametersglLightModeli(parameter,GL_TRUE)GL_LIGHT_MODEL_LOCAL_VIEWER do not use simplifying distant viewer assumption in calculation(infinite viewer)GL_LIGHT_MODEL_TWO_SIDED shades both sides of polygons independently6Defining a Point Light Source For each light sourc

5、e,we can set an RGBA for the diffuse,specular,and ambient components,and for the positionGLfloat diffuse0=1.0,0.0,0.0,1.0;GLfloat ambient0=1.0,0.0,0.0,1.0;GLfloat specular0=1.0,0.0,0.0,1.0;Glfloat light0_pos=1.0,2.0,3,0,1.0;glEnable(GL_LIGHTING);glEnable(GL_LIGHT0);glLightv(GL_LIGHT0,GL_POSITION,lig

6、ht0_pos);glLightv(GL_LIGHT0,GL_AMBIENT,ambient0);glLightv(GL_LIGHT0,GL_DIFFUSE,diffuse0);glLightv(GL_LIGHT0,GL_SPECULAR,specular0);7Distance and Direction The source colors are specified in RGBA The position is given in homogeneous coordinates If w=1.0,we are specifying a finite location If w=0.0,we

7、 are specifying a parallel source with the given direction vector The coefficients in the distance terms are by default a=1.0(constant terms),b=c=0.0(linear and quadratic terms).Change bya=0.80;glLightf(GL_LIGHT0,GLCONSTANT_ATTENUATION,a);1/(a+bdL+cdL2)8SpotlightsUse glLightv to set Direction GL_SPO

8、T_DIRECTION Cutoff GL_SPOT_CUTOFF Attenuation GL_SPOT_EXPONENT Proportional to cosafq-qf9Global Ambient LightAmbient light depends on color of light sources A red light in a white room will cause a red ambient term that disappears when the light is turned offOpenGL also allows a global ambient term

9、that is often helpful for testingglLightModelfv(GL_LIGHT_MODEL_AMBIENT,global_ambient)10glLightfv(light,pname,param)11LIGHT_TYPE_SPOT(Demo Ogl-lighting)case LIGHT_TYPE_SPOT:GLfloat diffuse_light2=1.0f,1.0f,1.0f,1.0f;GLfloat position_light2=2.0f*x,2.0f*y,2.0f*z,1.0f;GLfloat spotDirection_light2=x,y,z

10、;GLfloat constantAttenuation_light2=1.0f;glLightfv(GL_LIGHT2,GL_DIFFUSE,diffuse_light2);glLightfv(GL_LIGHT2,GL_POSITION,position_light2);glLightfv(GL_LIGHT2,GL_SPOT_DIRECTION,spotDirection_light2);glLightfv(GL_LIGHT2,GL_CONSTANT_ATTENUATION,constantAttenuation_light2);glLightf(GL_LIGHT2,GL_SPOT_CUTO

11、FF,45.0f);glLightf(GL_LIGHT2,GL_SPOT_EXPONENT,25.0f);12Moving Light Sources(Demo)Light sources are geometric objects whose positions or directions are affected by the modelview matrixDepending on where we place the position(direction)setting function,we can Move the light source(s)with the object(s)

12、Fix the object(s)and move the light source(s)Fix the light source(s)and move the object(s)Move the light source(s)and object(s)independently13void display(GLint spin)GLfloat light_position=0.0,0.0,1.5,1.0;glPushMatrix();glTranslatef(0.0,0.0,5.0);glPushMatrix();glRotated(GLdouble)spin,1.0,0.0,0.0);gl

13、Lightfv(GL_LIGHT0,GL_POSITION,light_position);glPopMatrix();auxSolidTorus(0.275,0.85);glPopMatrix();glFlush();/旋转光源14Material Properties Material properties are also part of the OpenGL state and match the terms in the modified Phong model Set by glMaterialv()GLfloat ambient=0.2,0.2,0.2,1.0;GLfloat d

14、iffuse=1.0,0.8,0.0,1.0;GLfloat specular=1.0,1.0,1.0,1.0;GLfloat shine=100.0glMaterialf(GL_FRONT,GL_AMBIENT,ambient);glMaterialf(GL_FRONT,GL_DIFFUSE,diffuse);glMaterialf(GL_FRONT,GL_SPECULAR,specular);glMaterialf(GL_FRONT,GL_SHININESS,shine);15Front and Back Faces The default is shade only front face

15、s which works correctly for convex objects If we set two sided lighting,OpenGL will shade both sides of a surface Each side can have its own properties which are set by using GL_FRONT,GL_BACK,or GL_FRONT_AND_BACK in glMaterialfback faces not visibleback faces visible16Emissive TermWe can simulate a

16、light source in OpenGL by giving a material an emissive componentThis component is unaffected by any sources or transformationsGLfloat emission=0.0,0.3,0.3,1.0);glMaterialf(GL_FRONT,GL_EMISSION,emission);17TransparencyMaterial properties are specified as RGBA valuesThe A value can be used to make th

17、e surface translucentThe default is that all surfaces are opaque regardless of ALater we will enable blending and use this feature18Efficiency Because material properties are part of the state,if we change materials for many surfaces,we can affect performance We can make the code cleaner by defining

18、 a material structure and setting all materials during initialization We can then select a material by a pointertypedef struct materialStruct GLfloat ambient4;GLfloat diffuse4;GLfloat specular4;GLfloat shineness;MaterialStruct;19IMPLEMENTATION I20ObjectivesIntroduce basic implementation strategiesCl

19、ipping Scan conversion21OverviewAt end of the geometric pipeline,vertices have been assembled into primitivesMust clip out primitives that are outside the view frustum Algorithms based on representing primitives by lists of verticesMust find which pixels can be affected by each primitive Fragment ge

20、neration Rasterization or scan conversion22Required TasksClippingRasterization or scan conversion We can get the pixels based on verticesSome tasks deferred until fragement processing Hidden surface removal AntialiasingProjectionFragmentsClippingShadingSurface hiddenTextureTransparency23Rasterizatio

21、n Meta AlgorithmsConsider two approaches to rendering a scene with opaque objectsFor every pixel,determine which object that projects on the pixel is closest to the viewer and compute the shade of this pixel Ray tracing paradigmFor every object,determine which pixels it covers and shade these pixels

22、 Pipeline approach Must keep track of depths24Clipping 2D against clipping window 3D against clipping volume Easy for line segments polygons Hard for curves and text Convert to lines and polygons first25Clipping 2D Line SegmentsBrute force approach:compute intersections with all sides of clipping wi

23、ndow Inefficient:one division per intersection26Cohen-Sutherland AlgorithmIdea:eliminate as many cases as possible without computing intersectionsStart with four lines that determine the sides of the clipping windowx=xmaxx=xminy=ymaxy=ymin27The Cases Case 1:both endpoints of line segment inside all

24、four lines Draw(accept)line segment as is Case 2:both endpoints outside all lines and on same side of a line Discard(reject)the line segmentx=xmaxx=xminy=ymaxy=ymin28The CasesCase 3:One endpoint inside,one outside Must do at least one intersectionCase 4:Both outside May have part inside Must do at l

25、east one intersectionx=xmaxx=xminy=ymax29Defining OutcodesFor each endpoint,define an outcodeOutcodes divide space into 9 regionsComputation of outcode requires at most 4 subtractions(8 times for two endpoint)b0b1b2b3b0=1 if y ymax,0 otherwiseb1=1 if y xmax,0 otherwiseb3=1 if x xmin,0 otherwiseOutco

26、des applicationCase I:o1,o2AB:o1=o2=030Outcodes applicationCase II:o1,o2CD:o1 0,o2=031Outcodes applicationCase III:o1,o2EF:o1&o2 032Outcodes applicationCase IV:o1,o2GH and IJ:o1&o2=0Compute the intersection points;Recompute the code of intersection points.3334The Cohen-Sutherland Line-Clipping Algor

27、ithm(2)To perform trivial and reject tests,we extend the edges of the clip rectangle to divide the plane into nine regions,and encode each region a 4bit code.If both 4bit codes of the endpoint are zero,the line is Trivially Accepted If both endpoints lie in the outside halfplane of a particular edge

28、,the line is Trivially rejected,and the l o g i c a l A N D o f t h e endpoints is not zero.35The Cohen-Sutherland Line-Clipping Algorithm(3)Compute the codes of both endpoints and check for trivial acceptance and rejection.If neither test is successful,find an endpoint that lies outside,and then te

29、st the code to find the edge that is crossed and to determine the corresponding intersection point.Clip off the line segment from the outside endpoint to the intersection point by replacing the outside endpoint with the intersection point,and compute the code of this new endpoint to prepare for the

30、next iteration.36EfficiencyIn many applications,the clipping window is small relative to the size of the entire data base Most line segments are outside one or more side of the window and can be eliminated based on their outcodesInefficiency when code has to be reexecuted for line segments that must

31、 be shortened in more than one step37Cohen Sutherland in 3D Use 6bit outcodes When needed,clip line segment against planes38Parametric Lines and IntersectionsFor L1x=x0l1+t(x1l1 x0l1)y=y0l1+t(y1l1 y0l1)For L2x=x0l2+t(x1l2 x0l2)y=y0l2+t(y1l2 y0l2)The Intersection Point:x0l1+t1(x1l1 x0l1)=x0l2+t2(x1l2

32、 x0l2)y0l1+t1(y1l1 y0l1)=y0l2+t2(y1l2 y0l2)39Clipping Lines Clipping Lines by Solving Simultaneous Equations Two sets of simultaneous equations of this parametric form can be solved for parameters tedge for the edge and tline for the line segment.The value of tedge and tline can be checked to see wh

33、ether both lie in 0,1;if they do,the intersection point is a true cliprectangle intersection.Shortcoming:considerable calculation and testing40A Parametric Line-Clipping Algorithm(1)Liang-Barsky1.For each clip edge(to upright rectangle),we can easily calculate a value t corresponding to the intersec

34、tion point between the edge and the line segment.2.We get four values of in t total.3.Intersections can be classified as PE or PL(How?)PE:Point Entering the clipping rectanglePL:Point Leaving the clipping rectangle41A Parametric Line-Clipping Algorithm(2)Liang-Barsky3.on the basis of the angle betwe

35、en P0P1 and the normal of the edge.0)90,PE (Ni.D 0)4.Divide t into two groups(t0,t0”t1,t1”)according to its corresponding intersection belongs to PE or PL.5.We get t0=max(t0,t0”,0)t1=min(t1,t1”,1)If t0 tl)return nil;else return P(te)and P(tl)as true clip intersections;43AdvantagesCan accept/reject a

36、s easily as with CohenSutherlandUsing values of a,we do not have to use algorithm recursively as with CSExtends to 3D44Clipping and NormalizationGeneral clipping in 3D requires intersection of line segments against arbitrary planeExample:oblique view45Plane-Line Intersections)()(121ppnppnao-0n)(1210-PPaPP46Normalized Formbefore normalizationafter normalizationNormalization is part of viewing(pre clipping)but after normalization,we clip against sides ofright parallelepipedTypical intersection calculation now requires onlya floating point subtraction,e.g.is x xmaxtop view