C O M P U T E R G R A P H I C S Guoying Zhao 1 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 1 / 46 Computer Graphics Introduction II

C O M P U T E R G R A P H I C S Guoying Zhao 2 / 46 Image Formation

C O M P U T E R G R A P H I C S Guoying Zhao 3 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 3 / 46 Objectives Fundamental imaging notions Physical basis for image formation –Light –Color –Perception Synthetic camera model Other models

C O M P U T E R G R A P H I C S Guoying Zhao 4 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 4 / 46 Image Formation In computer graphics, we form images which are generally two dimensional using a process analogous to how images are formed by physical imaging systems –Cameras –Microscopes –Telescopes –Human visual system

C O M P U T E R G R A P H I C S Guoying Zhao 5 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 5 / 46 Elements of Image Formation Objects Viewer Light source(s) Attributes that govern how light interacts with the materials in the scene Note the independence of the objects, the viewer, and the light source(s)

C O M P U T E R G R A P H I C S Guoying Zhao 6 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 6 / 46 Light Light is the part of the electromagnetic spectrum that causes a reaction in our visual systems Generally these are wavelengths in the range of about nm (nanometers)

C O M P U T E R G R A P H I C S Guoying Zhao 7 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 7 / 46 Ray Tracing and Geometric Optics One way to form an image is to follow rays of light from a point source finding which rays enter the lens of the camera. However, each ray of light may have multiple interactions with objects before being absorbed or going to infinity.

C O M P U T E R G R A P H I C S Guoying Zhao 8 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 8 / 46 Luminance and Color Images Luminance Image –Monochromatic –Values are gray levels: –Analogous to working with black and white film or television Color Image –Has perceptional attributes of hue, saturation, and lightness

C O M P U T E R G R A P H I C S Guoying Zhao 9 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 9 / 46 Three-Color Theory Human visual system has two types of sensors –Rods: monochromatic, night vision –Cones Color sensitive Three types of cones Only three values (the tristimulus values) are sent to the brain Need only match these three values –Need only three primary colors

C O M P U T E R G R A P H I C S Guoying Zhao 10 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 10 / 46 Shadow Mask CRT

C O M P U T E R G R A P H I C S Guoying Zhao 11 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 11 / 46 Additive and Subtractive Color Additive color –Form a color by adding amounts of three primaries CRTs, projection systems, positive film –Primaries are Red (R), Green (G), Blue (B) Subtractive color –Form a color by filtering white light with cyan (C), Magenta (M), and Yellow (Y) filters Light-material interactions Printing Negative film

C O M P U T E R G R A P H I C S Guoying Zhao 12 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 12 / 46 Pinhole Camera x p = -x/z/dy p = -y/z/d Use trigonometry to find projection of point at (x,y,z) These are equations of simple perspective Point(x p, y p, -d) : projection of the point (x,y,z) z p =-d

C O M P U T E R G R A P H I C S Guoying Zhao 13 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 13 / 46 Synthetic Camera Model center of projection image plane projector p projection of p

C O M P U T E R G R A P H I C S Guoying Zhao 14 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 14 / 46 Advantages Separation of objects, viewer, light sources Two-dimensional graphics is a special case of three-dimensional graphics Leads to simple software API –Specify objects, lights, camera, attributes –Let implementation determine image Leads to fast hardware implementation

C O M P U T E R G R A P H I C S Guoying Zhao 15 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 15 / 46 Image Formation Models Cannot compute color or shade of each object independently –Some objects are blocked from light –Light can reflect from object to object –Some objects might be translucent

C O M P U T E R G R A P H I C S Guoying Zhao 16 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 16 / 46 Why not ray tracing? Ray tracing seems more physically based so why don’t we use it to design a graphics system? Possible and is actually simple for simple objects such as polygons and quadrics with simple point sources In principle, can produce global lighting effects such as shadows and multiple reflections but ray tracing is slow and not well-suited for interactive applications

C O M P U T E R G R A P H I C S Guoying Zhao 17 / 46 Models and Architectures

C O M P U T E R G R A P H I C S Guoying Zhao 18 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 18 / 46 Objectives Learn the basic design of a graphics system Introduce pipeline architecture Examine software components for an interactive graphics system

C O M P U T E R G R A P H I C S Guoying Zhao 19 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 19 / 46 Image Formation Revisited Can we mimic the synthetic camera model to design graphics hardware software? Application Programmer Interface (API) –Need only specify Objects Materials Viewer Lights But how is the API implemented?

C O M P U T E R G R A P H I C S Guoying Zhao 20 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 20 / 46 Physical Approaches Ray tracing: follow rays of light from center of projection until they either are absorbed by objects or go off to infinity –Can handle global effects Multiple reflections Translucent objects –Slow –Must have whole data base available at all times Radiosity: Energy based approach –viewpoint independent –Very slow

C O M P U T E R G R A P H I C S Guoying Zhao 21 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 21 / 46 Practical Approach Process objects one at a time in the order they are generated by the application –Can consider only local lighting Pipeline architecture All steps can be implemented in hardware on the graphics card application program display

C O M P U T E R G R A P H I C S Guoying Zhao 22 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 22 / 46 Vertex Processing Much of the work in the pipeline is in converting object representations from one coordinate system to another –Object coordinates –Camera (eye) coordinates –Screen coordinates Every change of coordinates is equivalent to a matrix transformation Vertex processor also computes vertex colors

C O M P U T E R G R A P H I C S Guoying Zhao 23 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 23 / 46 Projection Projection is the process that combines the 3D viewer with the 3D objects to produce the 2D image –Perspective projections: all projectors meet at the center of projection –Parallel projection: projectors are parallel, center of projection is replaced by a direction of projection

C O M P U T E R G R A P H I C S Guoying Zhao 24 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 24 / 46 Perspective Projection Center of Projection (COP)

C O M P U T E R G R A P H I C S Guoying Zhao 25 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 25 / 46 Parallel Projection Direction of projection (DOP)

C O M P U T E R G R A P H I C S Guoying Zhao 26 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 26 / 46 Primitive Assembly Vertices must be collected into geometric objects before clipping and rasterization can take place –Line segments –Polygons –Curves and surfaces

C O M P U T E R G R A P H I C S Guoying Zhao 27 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 27 / 46 Clipping Just as a real camera cannot “see” the whole world, the virtual camera can only see part of the world or object space –Objects that are not within this volume are said to be clipped out of the scene

C O M P U T E R G R A P H I C S Guoying Zhao 28 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 28 / 46 Rasterization If an object is not clipped out, the appropriate pixels in the frame buffer must be assigned colors Rasterizer produces a set of fragments for each object Fragments are “potential pixels” –Have a location in frame buffer –Color and depth attributes Vertex attributes are interpolated over objects by the rasterizer

C O M P U T E R G R A P H I C S Guoying Zhao 29 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 29 / 46 Fragment Processing Fragments are processed to determine the color of the corresponding pixel in the frame buffer Colors can be determined by texture mapping or interpolation of vertex colors Fragments may be blocked by other fragments closer to the camera –Hidden-surface removal

C O M P U T E R G R A P H I C S Guoying Zhao 30 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 30 / 46 The Programmer’s Interface Programmer sees the graphics system through a software interface: the Application Programmer Interface (API)

C O M P U T E R G R A P H I C S Guoying Zhao 31 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 31 / 46 API Contents Functions that specify what we need to form an image –Objects –Viewer –Light Source(s) –Materials Other information –Input from devices such as mouse and keyboard –Capabilities of system

C O M P U T E R G R A P H I C S Guoying Zhao 32 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 32 / 46 Object Specification Most APIs support a limited set of primitives including –Points (0D object) –Line segments (1D objects) –Polygons (2D objects) –Some curves and surfaces Quadrics Parametric polynomials All are defined through locations in space or vertices

C O M P U T E R G R A P H I C S Guoying Zhao 33 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 33 / 46 Example glBegin(GL_POLYGON) glVertex3f(0.0, 0.0, 0.0); glVertex3f(0.0, 1.0, 0.0); glVertex3f(0.0, 0.0, 1.0); glEnd( ); type of object location of vertex end of object definition

C O M P U T E R G R A P H I C S Guoying Zhao 34 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 34 / 46 Camera Specification Six degrees of freedom –Position of center of lens –Orientation Lens Film size Orientation of film plane

C O M P U T E R G R A P H I C S Guoying Zhao 35 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 35 / 46 Lights and Materials Types of lights –Point sources vs. distributed sources –Spot lights –Near and far sources –Color properties Material properties –Absorption: color properties –Scattering Diffuse Specular

C O M P U T E R G R A P H I C S Guoying Zhao 36 / 46 What Kind of Math do We Need? Cartesian Coordinates Typically modeling space is floating point, screen space is integer Often, screen coordinates are measured top to bottom, based on raster scan (0,0) x, y Cartesian gridInteger Grid

C O M P U T E R G R A P H I C S Guoying Zhao 37 / 46 Example 3D Primitives Polyhedron Sphere Polyline Patch

C O M P U T E R G R A P H I C S Guoying Zhao 38 / 46 Graphics Display Hardware Vector (calligraphic, stroke, random-scan) –still used in some plotters Raster (TV, bitmap, pixmap), used in displays and laser printers Ideal Drawing Vector Drawing Raster Outline primitives Filled primitives

C O M P U T E R G R A P H I C S Guoying Zhao 39 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 39 / 46 Random-Scan Display(Vector Displays) Like an electronic pen Mainly represent the geometric attributes

C O M P U T E R G R A P H I C S Guoying Zhao 40 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 40 / 46 Raster-Scan Display Resolution –The maximum number of points that can be displayed without overlap on a CRT is referred to as the resolution –Number of pixels in the frame buffer Frame Buffer –Picture definition is stored in a memory area called Frame buffer or refresh buffer Refresh Rate –The frequency at which a picture is redrawn on the screen is referred to as the refresh rate

C O M P U T E R G R A P H I C S Guoying Zhao 41 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 41 / 46 Frame Buffer Storing the color of each pixel on screen Each cell of frame buffer has the corresponding pixel on screen

C O M P U T E R G R A P H I C S Guoying Zhao 42 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 42 / 46 The Size of Frame Buffer Resolution: 1024*768 Pixel color: Three components RGB, each component is 8 bits Question: How much storing space does the frame buffer need? (in Bytes, in MegaByte) 1 Byte = 8 bits. 1024*768*3 Bytes 1KB=1024B; 1MB=1024KB; 1024*768*3/1024/1024=2.25MB

C O M P U T E R G R A P H I C S Guoying Zhao 43 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 43 / 46 Refresh Rate The frequency at which a picture is redrawn on the screen is referred to as the refresh rate

C O M P U T E R G R A P H I C S Guoying Zhao 44 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 44 / 46 Raster-Scan Display Raster –A rectangular array of points or dots Pixel –One dot or picture element of the raster Scan Line –A row of pixels Scan conversion (rasterization) – Converting of geometric entities to pixel assignments in the frame buffer

C O M P U T E R G R A P H I C S Guoying Zhao 45 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 45 / 46 Raster

C O M P U T E R G R A P H I C S Guoying Zhao 46 / 46 C O M P U T E R G R A P H I C S Guoying Zhao 46 / 46 Rasterized Line Segment Discretization of continues points