COMPUTER GRAPHICS CS 482 FALL 2014 AUGUST 27, 2014
FIXED-FUNCTION 3D GRAPHICS MESH SPECIFICATION LIGHTING
SPECIFICATION REFLECTION SHADING HIERARCHICAL MODELING
Slide 3
MESH SPECIFICATION CS 482 FALL 2014 POLYHEDRAL MESHES AUGUST
27, 2014: FIXED-FUNCTION 3D GRAPHICSPAGE 42 MOST 3D MODELS ARE
STORED AS 3D MESHES OF VERTICES, EDGES, AND FACES. VERTICES ARE
USUALLY TRIPLES OF CARTESIAN COORDINATES. EDGES ARE USUALLY PAIRS
OF VERTICES. FACES ARE USUALLY CYCLES OF VERTICES OR EDGES. AS
MESHES BECOME MORE REFINED, WITH A LARGER NUMBER OF SMALLER FACES,
THE MODEL APPROXIMATES SMOOTHER CONTOURS WITH MORE DETAILED
FEATURES.
Slide 4
LIGHTING SPECIFICATION CS 482 FALL 2014 LIGHT SOURCES AUGUST
27, 2014: FIXED-FUNCTION 3D GRAPHICSPAGE 43 MODELING DIFFERENT
LIGHT SOURCES IN A SCENE MAY PRODUCE DIFFERENT LEVELS OF LIGHTING
VARIATION, RESULTING IN DIFFERENT LEVELS OF REALISM. NEARBY POINT
LIGHT SOURCE DISTANT POINT LIGHT SOURCE CYLINDRICAL SPOT LIGHT
CONICAL SPOT LIGHT AREA LIGHT PARALLEL LIGHT
Slide 5
LIGHTING SPECIFICATION CS 482 FALL 2014 COMPLEXITY OF
ILLUMINATION MODELS AUGUST 27, 2014: FIXED-FUNCTION 3D GRAPHICSPAGE
44 NUMEROUS CONSIDERATIONS FACTOR INTO EFFECTIVELY LIGHTING A
MODEL: HOW DOES THE LIGHT STRIKE THE OBJECT? WHAT MATERIAL
PROPERTIES DOES THE OBJECT HAVE? ARE THE ITEMS BETWEEN THE OBJECT
AND THE VIEWER TRANSPARENT? IF SO, WHAT REFRACTIVE QUALITIES DO
THEY HAVE? IS THE OBJECT REFLECTIVE? IF SO, WHAT NEIGHBORING ITEMS
REFLECT OFF THE SURFACE OF THE OBJECT? DO ANY ITEMS CAST SHADOWS
ACROSS THE OBJECT? ARE ANY NEIGHBORING ITEMS SO BRIGHTLY COLORED
THAT THE LIGHTING CAUSES THEIR COLORS TO BLEED ONTO THE
OBJECT?
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REFLECTION CS 482 FALL 2014 AMBIENT REFLECTION AUGUST 27, 2014:
FIXED-FUNCTION 3D GRAPHICSPAGE 45 TO AVOID HAVING THE PARTS OF AN
OBJECT FACING AWAY FROM A LIGHT SOURCE APPEARING COMPLETELY BLACK,
IT IS NECESSARY TO MODEL AMBIENT REFLECTION, LIKE THE LIGHT THAT IS
SCATTERED BACK ONTO THE OBJECT FROM ITS SURROUNDINGS. WHERE I a IS
THE INTENSITY OF THE AMBIENT LIGHT (ASSUMED TO BE CONSTANT FOR ALL
OBJECTS), AND k a IS THE AMBIENT REFLECTION COEFFICIENT FOR THE
PARTICULAR MATERIAL OF EACH OBJECT. IMAGE WITH AMBIENT REFLECTION
ONLY THE EQUATION FOR THE ILLUMINATION DUE TO AMBIENT LIGHT MAY BE
APPROXIMATED AS:
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REFLECTION CS 482 FALL 2014 DIFFUSE REFLECTION AUGUST 27, 2014:
FIXED-FUNCTION 3D GRAPHICSPAGE 46 TO ILLUMINATE AN OBJECT SO THAT
ITS BRIGHTNESS VARIES ACCORDING TO THE DIRECTION OF THE LIGHT
SOURCE, WE CAN MODEL DIFFUSE REFLECTION, USING , THE ANGLE BETWEEN
THE NORMAL VECTOR TO THE SURFACE AND THE VECTOR FROM THE SURFACE TO
THE LIGHT, AS A PARAMETER. WHERE I p IS THE INTENSITY OF THE LIGHT
SOURCE (ASSUMED TO BE CONSTANT FOR ALL OBJECTS), AND k d IS THE
DIFFUSE REFLECTION COEFFICIENT FOR THE PARTICULAR MATERIAL OF EACH
OBJECT. IMAGE WITH DIFFUSE REFLECTION ONLY THE EQUATION FOR THE
ILLUMINATION DUE TO DIFFUSE REFLECTION MAY BE APPROXIMATED AS:
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REFLECTION CS 482 FALL 2014 COMBINING AMBIENT AND DIFFUSE
REFLECTION AUGUST 27, 2014: FIXED-FUNCTION 3D GRAPHICSPAGE 47
DRAMATICALLY DIFFERENT IMAGES ARE PRODUCED BY COMBINING AMBIENT
ILLUMINATION WITH DIFFUSE REFLECTION, USING THE FORMULA: HIGH
DIFFUSE/LOW AMBIENT HIGH DIFFUSE/MED. AMBIENT MED. DIFFUSE/MED.
AMBIENT HIGH DIFFUSE/HIGH AMBIENT LOW DIFFUSE/LOW AMBIENT
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REFLECTION CS 482 FALL 2014 SPECULAR REFLECTION AUGUST 27,
2014: FIXED-FUNCTION 3D GRAPHICSPAGE 48 UNTIL NOW, OUR ILLUMINATION
MODEL IS NOT AFFECTED BY THE VIEWERS POSITION. THIS CHANGES WHEN WE
CONSIDER SPECULAR REFLECTION, THE DEGREE TO WHICH THE OBJECTS SHINY
SURFACE REFLECTS LIGHT INTO THE VIEWERS EYES. WHERE k s IS THE
SPECULAR REFLECTION COEFFICIENT FOR THE PARTICULAR MATERIAL OF EACH
OBJECT, n IS THE SPECULAR REFLECTION EXPONENT (WHICH INDICATES HOW
FOCUSED REFLECTED HIGHLIGHTS ARE ON EACH OBJECTS SURFACE), AND IS
THE ANGLE BETWEEN THE LIGHTS REFLECTION VECTOR OFF OF THE SURFACE
AND THE VECTOR FROM THE SURFACE TO THE VIEWER. IMAGE WITH SPECULAR
REFLECTION ONLY THE SPECULAR REFLECTION FORMULA IS:
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REFLECTION CS 482 FALL 2014 EFFECTS OF SPECULAR COEFFICIENT AND
EXPONENT AUGUST 27, 2014: FIXED-FUNCTION 3D GRAPHICSPAGE 49
INCREASING SPECULAR EXPONENT INCREASING SPECULAR COEFFICIENT NOTICE
THE SMALLER HOT SPOTS ON THE SHINIER OBJECTS.
Slide 11
REFLECTION CS 482 FALL 2014 LIGHT SOURCE ATTENUATION AUGUST 27,
2014: FIXED-FUNCTION 3D GRAPHICSPAGE 50 THE ILLUMINATION MODEL
DOESNT HANDLE ATTENUATION, THE TENDENCY FOR ILLUMINATION TO
DISSIPATE WITH THE DISTANCE FROM THE LIGHT SOURCE. AN ATTENUATION
FACTOR, f att, CAN BE INSERTED INTO THE FORMULA AS FOLLOWS:
QUADRATIC/NEA R QUADRATIC/FAR LINEAR/FARLINEAR/NEARCONSTANT
QUADRATIC/MEDIU M LINEAR/MEDIUM f att IS USUALLY CONSTANT, INVERSE
LINEAR IN THE DISTANCE FROM THE LIGHT, OR INVERSE QUADRATIC IN THE
DISTANCE FROM THE LIGHT.
Slide 12
SHADING CS 482 FALL 2014 FLAT SHADING AUGUST 27, 2014:
FIXED-FUNCTION 3D GRAPHICSPAGE 51 BY APPLYING THE ILLUMINATION
MODEL TO A SINGLE POINT WITHIN EACH POLYGONAL SURFACE AND THEN
USING THAT SHADING ACROSS THE ENTIRE POLYGON, THE SURFACE OF THE
MODEL RETAINS ITS FACETED APPEARANCE. NOTE THAT THE MODEL APPEARS
SMOOTHER AS THE NUMBER OF POLYGONS INCREASES AND THEIR SIZE
DECREASES.
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SHADING CS 482 FALL 2014 GOURAUD SHADING AUGUST 27, 2014:
FIXED-FUNCTION 3D GRAPHICSPAGE 52 THE GOURAUD SMOOTH SHADING
ALGORITHM: CALCULATES AVERAGE VERTEX NORMAL VECTORS. PERFORMS
ILLUMINATION CALCULATION TO GET VERTEX COLORS. INTERPOLATES VERTEX
COLORS ACROSS EACH POLYGONAL FACE. TENDS TO SUFFER FROM MACH
BANDING. MAY MISS HIGHLIGHTS WITHIN A POLYGONAL FACE. OUR
ILLUMINATION MODEL PERMITS US TO SHADE OBJECTS IF WE KNOW THE
SCENES LIGHTING CONDITIONS, THE OBJECTS MATERIAL PROPERTIES AND
LOCATION, AND (MOST DIFFICULT OF ALL) NORMAL VECTORS AT EVERY POINT
ON THE OBJECT.
Slide 14
SHADING CS 482 FALL 2014 GOURAUD SHADING ALGORITHM AUGUST 27,
2014: FIXED-FUNCTION 3D GRAPHICSPAGE 53 APPROXIMATE THE NORMAL
VECTOR AT EACH VERTEX BY AVERAGING THE NORMAL VECTORS AT EACH FACE
CONTAINING THAT VERTEX. COMPUTE THE SHADING AT EACH VERTEX USING
THE ILLUMINATION MODEL. FOR EACH SCANLINE, INTERPOLATE THE SHADING
FOR THE ENDPOINTS BY USING THE CALCULATED SHADING FOR THE VERTICES.
THEN INTERPOLATE THE SHADING FOR THE INTERNAL SCANLINE POINTS BY
USING THE ENDPOINT SHADING VALUES. CONTINUE THIS PROCESS FOR THE
ENTIRE FACE.
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SHADING CS 482 FALL 2014 PHONG SHADING AUGUST 27, 2014:
FIXED-FUNCTION 3D GRAPHICSPAGE 54 THE PHONG SMOOTH SHADING
ALGORITHM: CALCULATES AVERAGE VERTEX NORMAL VECTORS. INTERPOLATES
VERTEX NORMAL VECTORS ACROSS THE FACE. PERFORMS ILLUMINATION
CALCULATION AT EACH PIXEL. PRODUCES MORE ACCURATE HIGHLIGHTS.
YIELDS FEWER MACH BAND EFFECTS.
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SHADING CS 482 FALL 2014 PHONG SHADING ALGORITHM AUGUST 27,
2014: FIXED-FUNCTION 3D GRAPHICSPAGE 55 APPROXIMATE THE NORMAL
VECTOR AT EACH VERTEX BY AVERAGING THE NORMAL VECTORS AT EACH FACE
CONTAINING THAT VERTEX. FOR EACH SCANLINE, INTERPOLATE THE NORMAL
VECTOR FOR THE ENDPOINTS BY USING THE NORMAL VECTOR APPROXIMATIONS
FOR THE VERTICES. THEN INTERPOLATE THE NORMAL VECTORS FOR THE
INTERNAL SCANLINE POINTS BY USING THE ENDPOINT NORMAL VECTORS.
CONTINUE THIS PROCESS FOR THE ENTIRE FACE. USE THE ILLUMINATION
MODEL TO COMPUTE THE SHADING AT EACH POINT IN THE FACE.
Slide 17
HIERARCHICAL MODELING CS 482 FALL 2014 SCENE REPRESENTATION
AUGUST 27, 2014: FIXED-FUNCTION 3D GRAPHICSPAGE 56 A GRAPHICAL
MODEL CAN BE REPRESENTED AS A LIST OF OBJECTS, BUT EDITING (E.G.,
DELETING) WOULD REQUIRE UPDATING MULTIPLE NODES IN THE LIST.
GROUPING RELATED OBJECTS IN A MODULAR HIERARCHY FACILITATES EDITING
HIGH-LEVEL EDITING VIA A SINGLE NODE.
Slide 18
HIERARCHICAL MODELING CS 482 FALL 2014 SCENE GRAPHS AUGUST 27,
2014: FIXED-FUNCTION 3D GRAPHICSPAGE 57 A SCENE GRAPH IS A
HIERARCHICAL ORDERING OF THE COMPONENTS OF A GRAPHICAL SCENE SO
THAT PARENT NODES AFFECT CHILD NODES. BY ORGANIZING THE GRAPHICAL
COMPONENTS IN THIS MANNER, MODELING THE BEHAVIOR OF SECONDARY
COMPONENTS IS LESS COMPLICATED, SINCE CERTAIN ASPECTS OF THAT
BEHAVIOR ARE ESSENTIALLY INHERITED FROM THE CORRESPONDING PRIMARY
COMPONENTS.