INTRODUCTION. Slides modified from Angel book 6e

Transcription

1 INTRODUCTION Slides modified from Angel book 6e

2 Fall 2012 COSC4328/5327 Computer Graphics 2 Objectives Historical introduction to computer graphics Fundamental imaging notions Physical basis for image formation Light, Color, Perception Synthetic camera model Other models Basic design of a graphics system Introduce a graphics pipeline architecture Examine software components for interactive graphics

3 Fall 2012 COSC4328/5327 Computer Graphics 3 Computer Graphics Computer graphics deals with all aspects of creating images with a computer Hardware Software Applications

4 Fall 2012 COSC4328/5327 Computer Graphics 4 Example Where did this image come from? What hardware/software did we need to produce it?

5 Fall 2012 COSC4328/5327 Computer Graphics 5 Preliminary Answer Application: The object is an artist s rendition of the sun for an animation to be shown in a domed environment (planetarium) Software: Maya for modeling and rendering but Maya is built on top of OpenGL Hardware: PC with graphics card for modeling and rendering

6 Fall 2012 COSC4328/5327 Computer Graphics 6 Basic Graphics System Input devices Output device Image formed in frame buffer

7 Fall 2012 COSC4328/5327 Computer Graphics 7 CRT Can be used either as a line-drawing device (calligraphic) or to display contents of frame buffer (raster mode)

8 Fall 2012 COSC4328/5327 Computer Graphics 8 Computer Graphics: Computer graphics goes back to the earliest days of computing Strip charts Pen plotters Simple displays using A/D converters to go from computer to calligraphic CRT Cost of refresh for CRT too high Computers slow, expensive, unreliable

9 Fall 2012 COSC4328/5327 Computer Graphics 9 Computer Graphics: Wireframe graphics Draw only lines Sketchpad Display Processors Storage tube wireframe representation of sun object

10 Fall 2012 COSC4328/5327 Computer Graphics 10 Sketchpad Ivan Sutherland s PhD thesis at MIT Recognized the potential of man-machine interaction Loop Display something User moves light pen Computer generates new display Sutherland also created many of the now common algorithms for computer graphics

11 Fall 2012 COSC4328/5327 Computer Graphics 11 Display Processor Rather than have the host computer try to refresh display use a special purpose computer called a display processor (DPU) Graphics stored in display list (display file) on display processor Host compiles display list and sends to DPU

12 Fall 2012 COSC4328/5327 Computer Graphics 12 Direct View Storage Tube Created by Tektronix Did not require constant refresh Standard interface to computers Allowed for standard software Plot3D in Fortran Relatively inexpensive Opened door to use of computer graphics for CAD community

13 Fall 2012 COSC4328/5327 Computer Graphics 13 Computer Graphics: Raster Graphics Beginning of graphics standards IFIPS GKS: European effort Becomes ISO 2D standard Core: North American effort 3D but fails to become ISO standard Workstations and PCs

14 Fall 2012 COSC4328/5327 Computer Graphics 14 Raster Graphics Image produced as an array (the raster) of picture elements (pixels) in the frame buffer

15 Fall 2012 COSC4328/5327 Computer Graphics 15 Raster Graphics Allows us to go from lines and wire frame images to filled polygons

16 Fall 2012 COSC4328/5327 Computer Graphics 16 Vector vs. Raster Which draws better pictures? Which draws better shaded pictures? Which draws better color pictures? Which draws better lines? Which requires more memory? Which has a constant display time?

17 Fall 2012 COSC4328/5327 Computer Graphics 17 PCs and Workstations Although we no longer make the distinction between workstations and PCs, historically they evolved from different roots Early workstations characterized by Networked connection: client-server model High-level of interactivity Early PCs included frame buffer as part of user memory Easy to change contents and create images

18 Fall 2012 COSC4328/5327 Computer Graphics 18 Computer Graphics Pong 1972 The Black Hole 1979

19 Fall 2012 COSC4328/5327 Computer Graphics 19 Computer Graphics: Tron 1982 Wrath of Kahn 1982 Last Starfighter 1984

20 Fall 2012 COSC4328/5327 Computer Graphics 20 Computer Graphics: Realism comes to computer graphics smooth shading environment mapping bump mapping

21 Fall 2012 COSC4328/5327 Computer Graphics 21 Computer Graphics: Special purpose hardware Silicon Graphics geometry engine VLSI implementation of graphics pipeline Industry-based standards PHIGS RenderMan Networked graphics: X Window System Human-Computer Interface (HCI)

22 Fall 2012 COSC4328/5327 Computer Graphics 22 Computer Graphics: Babylon TV Jurrasic Park 1993 Titanic 1997 (linux rendering) The Matrix 1999

23 Fall 2012 COSC4328/5327 Computer Graphics 23 Computer Graphics: OpenGL API Completely computer-generated feature-length movies (Toy Story) are successful New hardware capabilities Texture mapping Blending Accumulation, stencil buffers

24 Fall 2012 COSC4328/5327 Computer Graphics 24 Computer Graphics: Photorealism Graphics cards for PCs dominate market Nvidia, ATI Game boxes and game players determine direction of market Computer graphics routine in movie industry: Maya, Lightwave Programmable pipelines

25 Fall 2012 COSC4328/5327 Computer Graphics 25 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

26 Fall 2012 COSC4328/5327 Computer Graphics 26 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)

27 Fall 2012 COSC4328/5327 Computer Graphics 27 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) Long wavelengths appear as reds and short wavelengths as blues

28 Fall 2012 COSC4328/5327 Computer Graphics 28 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.

29 Fall 2012 COSC4328/5327 Computer Graphics 29 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 Do we have to match every frequency in visible spectrum? No!

30 Fall 2012 COSC4328/5327 Computer Graphics 30 Three-Color Theory Human visual system has two types of sensors Rods: monochromatic, night vision Cones Color sensitive Three types of cones: S, M, L Name after wavelength they respond to Only three values (the tristimulus values) are sent to the brain Need only match these three values Can get by with three primary colors

31 Fall 2012 COSC4328/5327 Computer Graphics 31 What is This?

32 Fall 2012 COSC4328/5327 Computer Graphics 32 Color Perception

33 Fall 2012 COSC4328/5327 Computer Graphics 33 Light Source What does the power spectrum from varying sources look like? Fluorescent solid Tungsten - dash

34 Fall 2012 COSC4328/5327 Computer Graphics 34 Color Perception How is a red Cortland apple perceived under fluorescent lighting? This is called the dominant wavelength It is the perceived hue

35 Fall 2012 COSC4328/5327 Computer Graphics 35 Color Terms Hue Dominant wavelength (spectral color) Brightness/Luminance Brightness increases until colors wash out Luminance is total power of the light. Purity/Saturation How close to a spectral pure color How many wavelengths make up a color Percent of luminance in dominant wavelength Chromaticity Combination of hue and purity Metamer Two colors (objects) that appear the same under one set of conditions and different under another.

36 Fall 2012 COSC4328/5327 Computer Graphics 36 Optical Illusions unding.htm olourperception/colourperception.html

37 Fall 2012 COSC4328/5327 Computer Graphics 37 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

38 Fall 2012 COSC4328/5327 Computer Graphics 38 CIE In 1931, Commission Internationale de Eclariage created three primaries They are imaginary primaries Adding different amounts of them will produce all visible colors These three colors form a 3- space All perceived hues in diagram Plus others

39 Fall 2012 COSC4328/5327 Computer Graphics 39 CIE Colors on a line emanating from the origin have the same hue and saturation, but different luminance Boundary colors correspond to maximum saturation of a spectral color Except on purple line Triangle is a gamut for RGB All colors that can be matched by RGB

40 Fall 2012 COSC4328/5327 Computer Graphics 40 CIE Color Regions

41 Fall 2012 COSC4328/5327 Computer Graphics 41 Other Gamuts matched to CIE

42 Color Terms Spectral Color Color of the rainbow Non-spectral color A perceivable color that doesn t have a wavelength (not part of rainbow) Colors on the Purple line. What about white? Dominant wavelength Line from W through the color C will intersect the dominant wavelength White Spot (W) Equal parts of all primaries Complimentary color Line from color through W will intersect this Gamut Colors that can be represented by a group of primaries

43 Fall 2012 COSC4328/5327 Computer Graphics 43 CIE Some Relationships W White spot Achromatic point B Complimentary to A Equal quantities of A & B give white Non-spectral No dominant wavelength C Dominant wavelength for A Mix with White to get A Complimentary wavelength for B D Complimentary color to A Non-spectral (on purple line) D C A W B

44 Fall 2012 COSC4328/5327 Computer Graphics 44 CIE 3D not 2D Sometimes a color appears in the gamut but is not

45 Fall 2012 COSC4328/5327 Computer Graphics 45 Shadow Mask CRT

46 Fall 2012 COSC4328/5327 Computer Graphics 46 Liquid Crystal Display Liquid Crystal Not solid, not liquid Can transmit and change polarized light Change alignment with a current Place liquid crystals between two polarized panels Applying charge to crystals will untwist them allowing light to pass through (or stop) Can be backlit or reflective

47 Fall 2012 COSC4328/5327 Computer Graphics 47 LCD Arrange pixels in a grid with row and column oriented conductive substrates A charge on the correct row and column will light up pixel Amount of charge controls how much light passes through Generally 256 levels Alternating Red, Green, and Blue columns allows for color

48 Fall 2012 COSC4328/5327 Computer Graphics 48 Pinhole Camera Use trigonometry to find projection of point at (x,y,z) x p = -x/z/d y p = -y/z/d z p = d These are equations of simple perspective

49 Fall 2012 COSC4328/5327 Computer Graphics 49 Synthetic Camera Model projector p image plane projection of p center of projection

50 Fall 2012 COSC4328/5327 Computer Graphics 50 Advantages Separation of objects, viewer, light sources Two-dimensional graphics is a special case of threedimensional graphics Leads to simple software API Specify objects, lights, camera, attributes Let implementation determine image Leads to fast hardware implementation

51 Fall 2012 COSC4328/5327 Computer Graphics 51 Global vs Local Lighting 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

52 Fall 2012 COSC4328/5327 Computer Graphics 52 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 Ray tracing with GPUs is close to real time

53 Fall 2012 COSC4328/5327 Computer Graphics 53 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?

54 Fall 2012 COSC4328/5327 Computer Graphics 54 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 Very slow

55 Fall 2012 COSC4328/5327 Computer Graphics 55 Practical Approach Process objects one at a time in the order they are generated by the application Can consider only local lighting Pipeline architecture application program display All steps can be implemented in hardware on the graphics card

56 Fall 2012 COSC4328/5327 Computer Graphics 56 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

57 Fall 2012 COSC4328/5327 Computer Graphics 57 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

58 Fall 2012 COSC4328/5327 Computer Graphics 58 Primitive Assembly Vertices must be collected into geometric objects before clipping and rasterization can take place Line segments Polygons Curves and surfaces

59 Fall 2012 COSC4328/5327 Computer Graphics 59 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

60 Fall 2012 COSC4328/5327 Computer Graphics 60 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 bufffer Color and depth attributes Vertex attributes are interpolated over objects by the rasterizer

61 Fall 2012 COSC4328/5327 Computer Graphics 61 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

62 Fall 2012 COSC4328/5327 Computer Graphics 62 The Programmer s Interface Programmer sees the graphics system through a software interface: the Application Programmer Interface (API)

63 Fall 2012 COSC4328/5327 Computer Graphics 63 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

64 Fall 2012 COSC4328/5327 Computer Graphics 64 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

65 Fall 2012 COSC4328/5327 Computer Graphics 65 Example (GL 2.X) type of object 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( ); location of vertex end of object definition

66 Fall 2012 COSC4328/5327 Computer Graphics 66 Example (GL > 3.X - GPU based) Put geometric data in an array vec3 points[3]; points[0] = vec3(0.0, 0.0, 0.0); points[1] = vec3(0.0, 1.0, 0.0); points[2] = vec3(0.0, 0.0, 1.0); Send array to GPU Tell GPU to render as triangle

67 Fall 2012 COSC4328/5327 Computer Graphics 67 Camera Specification Six degrees of freedom Position of center of lens Orientation Lens Film size Orientation of film plane

68 Fall 2012 COSC4328/5327 Computer Graphics 68 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