CS & SE 233.420 - ADVANCED GRAPHICS LECTURE 2 · 7 LECTURE 2 Photosensitivity • Ability to detect different intensities of diffuse ... cone pigment that are maximally sensitive

CS & SE 233.420 - ADVANCED GRAPHICS

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LECTURE 2

TOPICS in IT: ADVANCED GRAPHICS

(CS & SE 233.420)

Lecture 25 Aug 2004


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LECTURE 2

Topics

• Image Formation – Electromagnetic spectrum– Light – Human Visual Image processing– Color

• Programming in OpenGL– Background– Programming Structure and naming

conventions


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LECTURE 2

Electromagnetic Spectrum• The surface of Earth bathed in electromagnetic

radiation from sun• Electromagnetic energy is created by a vibration that

produces wave that carry the energy• Waves are ordered in the electromagnetic spectrum

according to their wave length (λ)– Longest (radio)– Shortest (gamma)

• Frequency (f) refers to the number of waves a vibration creates during a period of time

• Wavelength and frequency are inversely related– f = 1/ λ


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Electromagnetic SpectrumWavelength vs Frequency

Increasing in Wavelength (λ)

Increasing in Frequency (f)


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(Human) Visible Light Spectrum


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Light Detection Advantages• Travels rapidly (300,000 km/sec) and in

straight lines• Surfaces reflect and absorb various

wavelengths of light depending on physical and chemical properties– Light therefore contains a great deal of information

about environment• Useful for many survival needs

– Allows detection of predators, prey at a distance without direct physical contact


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LECTURE 2

Photosensitivity

• Ability to detect different intensities of diffuse illumination

• Present in many plants and most animals, • Found in single celled animals, skin of many

simple organisms, and in specialized visual organs,

• Light sensitivity underlies daily rhythm of activities


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Vision

• Sensing changes in illumination that are rapid in time and localized in space

• Image forming eyes found in – annelids (segmented worms)– mollusks (snails, scallops, squid etc.)– arthropods (spiders, crabs, insects etc.)– and vertebrates


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Visual Acuity and Sensitivity

• Acuity – Expressed as angular resolving power

• Humans can resolve 1' (one minute) of arc, full moon 30' arc diameter,

• Some birds of prey, such as eagles can resolve 20" of arc

• Sensitivity – how an eye responds to light– In complete darkness, human eye can respond to single

photon of light, stimulation of only 10-15 cells by single photons in a small patch of retina is sufficient to produce a conscious percept in a human observer


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LECTURE 2

Human (Vertebrate) Vision • Retina

– The photosensitive part of the eye

– Specialized layer of photoreceptor cells and neurons at back of eye

– Image formed on retina by refraction of light by cornea and lens

• Efferent control systems– Ciliary muscles stretch lens

to alter focus setting– Iris controls amount of light

admitted to eye (f-stop)


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Rods and Cones• The retina is largely composed

of two types of cells, called rods and cones

• Rods – Specialised for night vision can

be activated by a single photon but they produce lower acuity vision, however.

• Cones – Specialised for day vision, are

concentrated at the fovea and provide high acuity vision

– Confer color vision to primates, birds, reptiles, and most fish


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LECTURE 2

Human Color Wavelength Sensitivities

• Humans have 3 types of cone pigment that are maximally sensitive to either red, green or blue

• Color perception results from the simultaneous stimulation of the 3 cone types

• Colorblindness results from a deficiency of one cone type


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Additive and Subtractive Color• Additive color

– Form a color by adding amounts of three primaries

– Used by 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– Used in Printing and

Negative film


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Color Matching

• In order to define the perceptual 3D space in a "standard" way

– A set of experiments can (and have been) carried by having observers try and match color of a given wavelength, lambda,

– Done by mixing three other pure wavelengths, such as R=700nm, G=546nm, and B=436nm

– A problem exists, because sometimes the red light needs to be added to the target before a match can be achieved.

• This is shown on the graph by having its intensity, R, take on a negative value.


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Commission Internationaled'Eclairage (CIE)

• Defined three new hypothetical light sources, x, y, and z, which yield positive matching curves

• Given a spectrum and wish to find the corresponding X, Y, and Z quantities

• Done by integrating the product of the spectral power and each of the three matching curves over all wavelengths.

• The weights X,Y,Z form the three-dimensional CIE XYZ space


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CIE Chromaticity Diagram

• Defines 3 color index in 2 dimensions

• done by projecting the 3D color space onto the plane X+Y+Z=1


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Color Gamuts

• The chromaticity diagram can be used to compare the "gamuts" of various possible output devices – Such as monitors and

printers • Note that a color printer

cannot reproduce all the colors visible on a color monitor


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RGB Color Cube

• The additive color model used for computer graphics is represented by the RGB color cube, – R, G, and B represent

the colors produced by red, green and blue phosphours, respectively.


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CIE XYZ Color Space


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CMYK color model

dyecolor

absorbs reflects

Cyan Red Blue and Green

Magenta Green Blue and Red

Yellow Blue Red and Green

Black (K) All None

• Used by printers• Green paper is green

because it reflects green and absorbs other wavelengths

• To produce blue, one would mix cyan and magenta inks– both reflect blue while each

absorbing one of green and red

• Black ink is used to ensure that a high quality black can always be printed, and is often referred to as to K


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Other Color Models

• HSV – Hue, Saturation, Value• HLS – Hue, Luminosity, Saturation• Color conversion formula exist to convert

colors between respective color models


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Programming with OpenGL

• Background• Programming Structure and naming

conventions• Primitives• Attributes and States• Programming in three dimensions• Inputs and Interaction• Working with Callbacks


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SGI and GL

• Silicon Graphics (SGI) revolutionized the graphics workstation by implementing the pipeline in hardware (1982)

• To access the system, application programmers used a library called IRIS GL

• With GL, it was relatively simple to program three dimensional interactive applications


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OpenGL

• The success of GL lead to OpenGL (1992) a platform-independent API that was– Easy to use– Close enough to the hardware to get excellent performance– Available in a number of languages

• OS-independent and hardware-independent– Focus on rendering

• Aimed at 2-D/3-D scenes made of polygons (and lines and points).

– Omitted windowing and input to avoid window system dependencies

• Not as good for 2-D windows/text/GUI-style graphics


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OpenGL Evolution

• Controlled by an Architectural Review Board (ARB)– Members include SGI, Microsoft, NVidia, HP, 3DLabs, IBM,

Apple, Dell, Intel, Matrox, Sun, HP, ….• Relatively stable• Evolution reflects new hardware capabilities

– 3D texture mapping and texture objects– Vertex programs

• Allows for platform specific features through extensions


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OpenGL Design

• The interface with the graphics hardware.• Designed for efficient implementation in hardware.

– Particular OpenGL implementations may be partially or totally software.

• We deal with OpenGL via function calls (or commands).– No global variables.– Most OpenGL functions have few parameters.

• C/C++ header: <GL/gl.h>– But you make lots of function calls.– No complex data types.– OpenGL is function-call intensive.

• Think: advantages/disadvantages.


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X, Windows, Apple O/S

Software OrganizationApplication Program

GL

GLX, AGL, or WGL

OpenGL Motif

widget or similar

GLU

GLUT

Software and/or Hardware


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OpenGL is Procedural

• OpenGL is a procedural rather than descriptive interface. • In order to get a rendering of a red sphere the programmer must

specify the appropriate sequence of commands – Set up the camera view and modelling transformations– Draw the geometry for a sphere with a red color. etc.

• Disadvantage of using a procedural interface – The application must specify all of the operations in exacting detail

and in the correct sequence to get the desired result. • Advantage of this approach

– It allows great flexibility in the process of generating the image– Application is free to trade-off rendering speed and image quality by

changing the steps through which the image is drawn.


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OpenGL is Not Pixel Exact

• This means that two different OpenGL implementations are very unlikely to render exactly the same image.

• This allows OpenGL to be implemented across a range of hardware platforms. – If the specification were too exact, it would limit the kinds of

hardware acceleration that could be used; limiting its usefulness as a standard.

• In practice, the lack of exactness need not be a burden -- unless you plan to build a rendering farm from a diverse set of machines.


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OpenGL State

• OpenGL is a state machine• OpenGL functions are of two types

– Primitive generating (drawing)• Can cause output if primitive is visible• How vertices are processed and appearance of primitive are

controlled by the state– Set and return state values

• Transformation functions• Attribute functions

• So: all drawn objects are composed of primitives. The properties of these are attributes, which are determined by OpenGL states.


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OpenGL Libraries

• OpenGL Core Library– OpenGL32 on Windows– GL on most unix/linux systems

• OpenGL Utility Library (GLU)– Provides functionality in OpenGL core but avoids having to rewrite

code• Additional functions & types for various graphics operations.

– Designed to be implemented in software; calls GL.– C/C++ header: <GL/glu.h>.

• Links with window system– GLX or X window systems– WGL or Windows– AGL for Macintosh


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GLUT

• Provides functionality common to all window systems– Open a window– Get input from mouse and keyboard– Menus– Event-driven

• Code is portable but GLUT lacks the functionality of a good toolkit for a specific platform


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GLUT Functions

• glutInit allows application to get command line arguments and initializes system

• gluInitDisplayMode requests properties for the window (the rendering context)– RGB color– Single buffering– Properties logically ORed together

• glutWindowSize in pixels• glutWindowPosition from top-left corner of display• glutCreateWindow create window• glutDisplayFunc display callback• glutMainLoop enter infinite event loop


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OpenGL Extensions

• Functionality that anyone can add to OpenGL.

• OpenGL specifies rules that extensions are to follow.

• May be system-dependent. We will not use any extensions.


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OpenGL Naming Conventions

• Functions– Begin with “gl”, words capitalized & run together– Example: glClearColor– Can include type information. For example, the “2d” in

“glVertex2d” indicates two parameters of type GLdouble.• Constants

– Begin with “GL”, all upper-case, “_” between words– Example: GL_TRIANGLE_STRIP

• Types– Begin with “GL”, next word not capitalized, all words run together– Example: GLdouble


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LECTURE 2

Related Package Naming Conventions

• Related packages use similar conventions.– GLU

• Function: gluScaleImage• Constant: GLU_TESS_ERROR• Type: GLUtesselatorObj

– GLUT• Function: glutInitDisplayMode• Constant: GLUT_MIDDLE_BUTTON


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LECTURE 2

OpenGL Types

• OpenGL defines its own types, which have the same (minimum) precision on all systems. Some of these:– GLint: at least 32-bit integer– GLfloat: at least 32-bit floating-point– GLdouble: at least 64-bit floating-point– and others …

• So, for example, GLdouble is probably the same as double, but may not be.– Converting (say) a GLdouble to a double is fine.– But be careful when tossing around GLdouble * and double *. (Why?)


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Lack of Object Orientation

• OpenGL is not object oriented so that there are multiple functions for a given logical function

• As a result, some OpenGL commands have several forms allowing for different types.– For example, glVertex* can take 2, 3, or 4 parameters of

many different types.• Function glVertex2d takes 2 parameters of type GLdouble.• Function glVertex3f takes 3 parameters of type GLfloat.• Function glVertex3fv (“v” for “vector”) takes a single

parameter of type GLfloat * (should be a pointer to an array of 3 GLfloat’s).

• Underlying storage mode is the same


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OpenGL Function Format

3 float inputs are requiredbelongs to GL library

glVertex3f(x,y,z)function

glVertex3fv(p)

p is a pointer to a 3 Dimensional arrayof floats


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OpenGL #defines

• Most constants are defined in the include files gl.h, glu.h and glut.h– #include <glut.h> should automatically include

the others– Examples

• glBegin(GL_POLYGON)• glClear(GL_COLOR_BUFFER_BIT)

• include files also define OpenGL datatypes: GLfloat, GLdouble,….


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LECTURE 2

glFlush()

• It is important to note that OpenGL commands are not necessarily executed as soon as they are issued.

• It is necessary to call the command glFlush()to ensure that all previously issued commands are executed.

• glFlush() is generally called at the end of a sequence of drawing commands to ensure all objects in the scene are drawn.


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Program Structure

• Most OpenGL programs have a similar structure that consists of the following functions– main():

• defines the callback functions• opens one or more windows with the required properties• enters event loop (last executable statement)

– init(): sets the state variables• Viewing• Attributes

– callbacks• Display function• Input and window functions


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Event Loop

• Every glut program must have a display callback– The display callback is executed whenever

OpenGL decides the display must be refreshed,• for example when the window is opened

• The main function ends with the program entering an event loop


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References• The Electro Magnetic Spectrum Tutorial,

http://www.emk.gazi.edu.tr/EMS.htm• What is RF Radiation?,

http://www.rfsafe.com/research/rf_radiation/what_is_rf/emf_spectrum.htm

• The Physics Classroom, The Electromagnetic and Visible Spectra, http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/u12l2a.html

• Light Sources, http://graphics.lcs.mit.edu/~mcmillan/comp136/Lecture23/Lights.html

• The Visual System, http://www.geocities.com/medinotes/visual_system.htm

• The Visual System I & II, Dr. Paul Patton, http://soma.npa.uiuc.edu/courses/bio303.

• Computer Graphics Color Models, CSC 418: Colour Representation, Michiel van de Panne, University of Toronto, http://www.dgp.toronto.edu/people/van/courses/csc418/colour.html


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References• http://www.eecs.tulane.edu/• www/Terry/OpenGL/Changing_State.html• www.inf.pucrs.br/~flash/tcg/aulas/oglfaq/viewing.htm -• www.cs.uaf.edu/~cs381/slides/20030910ogl.ppt• http://www.dgp.toronto.edu/~ah/csc418/fall_2001/tut/ogl_draw.html• http://isg.cs.tcd.ie/dingliaj/3d4/Lab2_p2.html• http://www.opengl.org/about/overview.html• http://www.winnetmag.com/Article/ArticleID/3581/3581.html• http://www.csee.umbc.edu/~ebert/691/Au00/Notes/c1_viewing.html• www.inf.ufrgs.br/~comba/gh-files/talk1.pdf• http://www.dgp.toronto.edu/~hertzman/courses/csc418/winter_2004/not

es/lecture/pipeline.html• Ed Angel Lecture notes (Lectures 4, 5, 6, 7, 8 and 9)• Open GL web page – www.opengl.org

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CS & SE 233.420 - ADVANCED GRAPHICS LECTURE 2 · 7 LECTURE 2 Photosensitivity • Ability to detect different intensities of diffuse ... cone pigment that are maximally sensitive