csc 461: lecture 3 1 csc461 lecture 3: models and architectures objectives –learn the basic...

CSC 461: Lecture 3CSC 461: Lecture 3 11

CSC461 Lecture 3: CSC461 Lecture 3: Models and ArchitecturesModels and Architectures

ObjectivesObjectives

– Learn the basic design of a graphics systemLearn the basic design of a graphics system– Introduce pipeline architectureIntroduce pipeline architecture– Examine software components for an Examine software components for an

interactive graphics systeminteractive graphics system


Image Formation RevisitedImage Formation Revisited

Can we mimic the synthetic camera model Can we mimic the synthetic camera model to design graphics hardware software?to design graphics hardware software?

Application Programmer Interface (API)Application Programmer Interface (API)–Need only specify Need only specify

ObjectsObjects MaterialsMaterials ViewerViewer LightsLights

But how is the API implemented?But how is the API implemented?


Physical ApproachesPhysical ApproachesRay tracingRay tracing: follow rays of light from center : follow rays of light from center of projection until they either are absorbed of projection until they either are absorbed by objects or go off to infinityby objects or go off to infinity–Can handle global effectsCan handle global effects

Multiple reflectionsMultiple reflections Translucent objectsTranslucent objects

–SlowSlow–Need whole data baseNeed whole data base

RadiosityRadiosity: Energy based approach: Energy based approach–Very slowVery slow


Practical ApproachPractical ApproachProcess objects one at a time in the order they are Process objects one at a time in the order they are

generated by the applicationgenerated by the application–Can consider only local lightingCan consider only local lighting

Pipeline architecturePipeline architecture

All steps can be implemented in hardware on the All steps can be implemented in hardware on the graphics cardgraphics card

application program display


Vertex ProcessingVertex ProcessingMuch of the work in the pipeline is in converting Much of the work in the pipeline is in converting

object representations from one coordinate system object representations from one coordinate system to anotherto another

–Object coordinatesObject coordinates–Camera (eye) coordinatesCamera (eye) coordinates–Screen coordinatesScreen coordinates

Every change of coordinates is equivalent to a Every change of coordinates is equivalent to a matrix transformation matrix transformation

Vertex processor also computes vertex colorsVertex processor also computes vertex colors


ProjectionProjectionProjection Projection is the process that combines the is the process that combines the 3D viewer with the 3D objects to produce the 3D viewer with the 3D objects to produce the 2D image2D image–Perspective projections: all projectors meet at Perspective projections: all projectors meet at

the center of projectionthe center of projection–Parallel projection: projectors are parallel, center Parallel projection: projectors are parallel, center

of projection is replaced by a direction of of projection is replaced by a direction of projectionprojection


Primitive AssemblyPrimitive Assembly

Vertices must be collected into geometric Vertices must be collected into geometric objects before clipping and rasterization can objects before clipping and rasterization can take placetake place–Line segmentsLine segments–PolygonsPolygons–Curves and surfacesCurves and surfaces


ClippingClippingJust as a real camera cannot “see” the whole Just as a real camera cannot “see” the whole world, the virtual camera can only see part of world, the virtual camera can only see part of the world or object spacethe world or object space–Objects that are not within this volume are said to Objects that are not within this volume are said to

be be clippedclipped out of the scene out of the scene


RasterizationRasterization If an object is not clipped out, the appropriate If an object is not clipped out, the appropriate

pixels in the frame buffer must be assigned colorspixels in the frame buffer must be assigned colorsRasterizer produces a set of fragments for each Rasterizer produces a set of fragments for each

objectobjectFragments are “potential pixels”Fragments are “potential pixels”

–Have a location in frame buffferHave a location in frame bufffer–Color and depth attributesColor and depth attributes

Vertex attributes are interpolated over objects by Vertex attributes are interpolated over objects by the rasterizerthe rasterizer


Fragment ProcessingFragment ProcessingFragments are processed to determine the Fragments are processed to determine the color of the corresponding pixel in the frame color of the corresponding pixel in the frame bufferbuffer

Colors can be determined by texture Colors can be determined by texture mapping or interpolation of vertex colorsmapping or interpolation of vertex colors

Fragments may be blocked by other Fragments may be blocked by other fragments closer to the camera fragments closer to the camera –Hidden-surface removal Hidden-surface removal


The Programmer’s InterfaceThe Programmer’s InterfaceProgrammer sees the graphics system Programmer sees the graphics system through an interface: the Application through an interface: the Application Programmer Interface (API)Programmer Interface (API)


API ContentsAPI ContentsFunctions that specify what we need to Functions that specify what we need to form an imageform an image–ObjectsObjects–ViewerViewer–Light Source(s)Light Source(s)–MaterialsMaterials

Other informationOther information–Input from devices such as mouse and Input from devices such as mouse and

keyboardkeyboard–Capabilities of systemCapabilities of system


Object SpecificationObject SpecificationMost APIs support a limited set of Most APIs support a limited set of primitives includingprimitives including–Points (1D object)Points (1D object)–Line segments (2D objects)Line segments (2D objects)–Polygons (3D objects)Polygons (3D objects)–Some curves and surfacesSome curves and surfaces

QuadricsQuadricsParametric polynomialParametric polynomial

All areAll are defined through locations in space defined through locations in space or or verticesvertices


ExampleExample

glBegin(GL_POLYGON)glBegin(GL_POLYGON)glVertex3f(0.0, 0.0, 0.0);glVertex3f(0.0, 0.0, 0.0);glVertex3f(0.0, 1.0, 0.0);glVertex3f(0.0, 1.0, 0.0);glVertex3f(0.0, 0.0, 1.0);glVertex3f(0.0, 0.0, 1.0);glEnd( );glEnd( );

type of objectlocation of vertex

end of object definition


Camera SpecificationCamera Specification

Six degrees of Six degrees of freedomfreedom–Position of center of Position of center of

lenslens–OrientationOrientation

LensLensFilm sizeFilm sizeOrientation of film Orientation of film planeplane


Lights and MaterialsLights and MaterialsTypes of lightsTypes of lights

–Point sources vs distributed sourcesPoint sources vs distributed sources–Spot lightsSpot lights–Near and far sourcesNear and far sources–Color propertiesColor properties

Material propertiesMaterial properties–Absorption: color propertiesAbsorption: color properties–ScatteringScattering

DiffuseDiffuseSpecularSpecular

csc 461: lecture 3 1 csc461 lecture 3: models and architectures objectives –learn the basic...

Documents

d objectspolygons

d viewer

graphics hardware software

geometric objects

d objectline segments

d objectssome curves

d imageperspective projections

real camera