高级计算机图形学ACG05

高级计算机图形学Advanced Computer Graphics Advanced Computer Graphics Course No. 04814560主讲教师：裴玉茹Instructor: Yuru PeiEmail: peiyuru@ Email:peiyuru@cis pku edu cn Department of Machine IntelligencePeking UniversityPeking UniversityNon-PhotorealisticRendering/~hxu/publications/index.htmlUsing a term like ‘nonlinear science’ is likereferring to the bulk of zoology as ‘the studyof nonelephant animals.of nonelephant animals’-Stanislaw UlamNPR’s applications NPR s applicationsNPR s applications NPR’s applicationsOutline •Toon Shading(painting)•Silhouette Edgeg(g)Rendering(Inking)•Other Styles•LinesT Sh diToon Shading •Toon Shading(celshading)–The most popularform of NPR–Amplification“Amplificationthroughsimplification”-UnderstandingComics, McCloudg•How to do toon shading?–Painting& inking(shading&silhouette rendering)silhouette renderingMost common methods for paintingSolid color Two-tone approach (hard shading)Gouraud-shaded duck silhouettes rendered,two tone shading specular/diffusewith solid shading two-tone shading specular/diffuse three-toneN li hti d l(t b d)New lighting model (tone-based)•Traditional shadingTraditional shading•Tone-based shadingNew lighting model (tone-based) N li hti d l(t b d)New lighting model (tone-based)Black shaded regions hide details & some edge linesHighlight and fine detail are lost in white shaded regionsN li hti d l(t b d)New lighting model (tone-based) Colored Phong-shaded spheres with edge lines and highlightsC l d Ph h d d h ith d li d hi hli htColored spheres shaded with hue and luminance shift, including edge lines and highlightslines and highlightsg pShading method-1D texture map •Lake et al. and Lander’s shading methoduse the idea of computing the diffuse shading dot product –use the idea of computing the diffuse shading dot product n•L –for each vertex and using n•L as a texture coordinate to access a 1D texture map pg pShading method-1D texture map •Cartoon shadingSilh tt d i(I ki)Silhouette rendering (Inking)•Different types of edgesDifferent types of edges–boundary or border edge•Edges not shared by two polygonsEdges not shared by two polygons–crease or hard edge•The dihedral angle is greater than somethreshold•Further subcategorized into ridge & valley –material edget i l d•The two triangles sharing it differ in material –silhouette edgesilhouette edge•One triangle is front-face, and the other isback-faceSilh tt d i(I ki)Silhouette rendering (Inking)•Methods used can be roughly categorized as based on Methods used can be roughly categorized as based on Surface angle–Surface angle–Procedural geometry–Image processing–HybridSurface angleg•Similar to the idea of Lake for shading–Use dot product between the direction to the viewpoint and the Use dot product between the direction to the viewpoint and thesurface normal•Environment Map p•1D texture (in practice)Surface angle •Advantage–Extremely fast•Disadvantage–Only work for some models•For a model such as cube, it fails.–The silhouette lines are drawn with variable widthProcedural geometryProcedural geometry•Basic idea:–Render the front faces normally, then render the back faces in a Render the front faces normally then render the back faces in a way as to silhouette edges visible.•Thick lines–Draw only the edges of theback facesb k fNehe productions : Lesson 37p/Procedural geometryProcedural geometry•Z-bias–Move the backfaces forward in screen Z by biasing themf f•Fixed amount•k z where z is the average distance from polygon tok*z where z is the average distance from polygon tocamera and k is constant•Use API such as glPolygonOffset()g yg()–Render the backfaces themselves in blackProcedural geometryProcedural geometry•Fattened triangle–Fatten the backfacesf•Pushing outward each edge by a distance proportional toz/(v Nz/(v•N)B–Render the backfaces themselves in blackProcedural geometryProcedural geometry•Fattened triangle–make the resulting edge the same thickness in screen spaceResults WireframeTranslation Fattening40 fps 50 fps 11.5 fpsThick lines Z bias Fattened trianglesProcedural geometryP d l t•shell or halo methodshell or halo method–Shifting normal by an amount proportional to their z-distance from the eyethe eyeg g p •Moving vertices outwards along their normals is a perfect task for a vertex shader–Without any help from the CPUProcedural geometryProcedural geometry•shell or halo methodshell or halo method–Simple to implement, efficient, robust, and gives steady performancep–Used by the game Cel DamageP d l tProcedural geometry•Curved surface generated by the acceleratorCurved surface generated by the accelerator•Fattening method–can’t•Shell method–can work as long as the representation can be displacedoutwards along the surface normaloutwards along the surface normal•Z-bias method–works with all curved surfaces•Limitations–Limited control over the edge appearanceLimited control over the edge appearance–Semi-transparent surface–Need antialiasing gProcedural geometryProcedural geometry•Ridge edges [Raskar01]Rid d–To generate an additional polygon along each edge ofthe triangle being renderedthe triangle being rendered–These edge polygons are bent away from thetriangle’s plane by the user-defined critical dihedralangle.–Valley edges, this tech. can be performed by using the stencil buffer and up to 3 passesstencil buffer and up to3passesImage processing•Image-based method–Operates entirely on data stored in buffers and does not Operates entirely on data stored in buffers and does notmodify (or even know about) the geometry in the scene•Z-buffer•Normal bufferImage processingImage processingNormal maps depth mapsImage processingI i•Mitchell, Image processing with pixel shaders in Direct3D Mitchell“Image processing with pixel shaders in Direct3D”–Using version 1.4ppixel shaders–Sample texture 6 timesin a single passS–Implement a Sobel edgedetection filterImage processingI i•Advantages–Handle all primitives, even curved surfacesMeshes do not have to be connected or even consistent –Meshes do not have to be connected or even consistent •Disadvantages–Needing powerful pixel shader–Some silhouette edge can be missed•E. g. a sheet of paper on a desk•Solution : object ID–Still can’t deal with paper folded onto itselfSilhouetteEdge DetectionSilhouette Edge DetectionB t f•Brute force–Loop through all the edgesSpeed-up:–Speed-up :•Cull out edges inside planar polygons •Edge bufferg–Store two additional bits per edge (F and B)•Loop through all faces, determine whether theyf t b k f th XOR th ti F B 10are front or back face, then XOR the respective bits of the adjacent edges.•the FB bits are11->silhouette 01the FB bits are 11 silhouette–Could use only one bit (if no boundary edges)•XOR in 0 for frontfacing and 1 for backfacing •If the edge is silhouette, the bit is 1Silh tt Ed D t tiSilhouette Edge Detection •Stochastic methodStochastic methodSilhouette Edge DetectionSilh tt Ed D t ti •Gauss mapsGauss maps–Precomputation methodface normal a point on the Gaussian sphere•face normal->a point on the Gaussian sphere•mesh edge -> an arc on the Gaussian sphereEdgeNormal 2 Normal 1Silh tt Ed D t tiSilhouette Edge Detection•Gauss mapsGauss maps–For orthogonal projectiona view of the scene a plane through the origin of the•a view of the scene->a plane through the origin of theGaussian sphere•Every arc intersected by this plane is a silhouette edge –No need to check if the face is front or back facing for each frameHybrid SilhouettingH b id Silh tti•Use both image and geometric elementsUse both image and geometric elements–4 stepsStep 1: finds a list of silhouette edges•Step1:finds a list of silhouette edges•Step 2: render all the object’s triangles and silhouetteedges, assigning each a unique ID number•Step 3: the ID buffer is read back and extract the visiblesilhouette edges to form smooth stroke paths•Step 4: render stylized strokes along these reconstructedStep4:render stylized strokes along these reconstructedpathsH b id Silh ttiHybrid Silhouetting•The strokes themselves can be stylized in many different ways The strokes themselves can be stylized in many different waysOther StylesStylized rendering techniqueStylized rendering technique•Pencil sketch shading–Extend 1D texture method–Instead of selecting texels, uses L▪n to select a texture of appropriate densityappropriate density•The textures are crated by randomly selecting from thepencil strokes and placing them on the texture with randomspacing to avid coherence–If a Polygon requires more than one texture•Split the polygonSplit the polygonStylized rendering techniqueStylized rendering technique–The texture is applied with screen space coordinates to give a hand drawn lookhand-drawn look•Problem: as objects are animated, they “swim”through the textureStylized rendering technique Stylized rendering techniqueStroke texturesOther Styless c a l e←tone →←→Real-Time HatchingReal-Time Hatching•Maintaining both spatial and temporal coherence–Temporal: Tonal Art Maps (TAMS)•Automatic generationR d l l d did t t k d l t th–Randomly-placed candidate strokes and select the“best-fitting”。