CAP 4703 Computer Graphic Methods
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Presentation Transcript
CAP 4703Computer Graphic Methods Prof. Roy Levow Lecture 1
Computer Graphics • Broadly concerned with producing images by computer • Many applications • Still image generation and editing • Animation and film, Computer games • Design • Scientific and medical data visualization • Simulation and Virtual reality • Publishing, …
Graphical System • Main Components • Processor • Memory • Frame Buffer • Output Devices • Input Devices
Processor and Memory • Depending on the application, the system can range from • Capable personal computer to a • Super-computer
Image Output • Images may be • Vector graphics • Formed by drawing lines to create image • Raster graphics • Composed of individual picture elements in an array or raster • Individual elements are pixels • Stored in a frame buffer • Raster graphics is most common • Vector graphics provides better scalability
Frame Buffer • Normally • Video RAM (VRAM) – designed to support fast transfer of large amounts of data • Dynamic RAM (DRAM) • Number of bits per pixel is the depth of the frame buffer • Determines number of distinct color values • B&W=1 • Full color >= 24
Frame Buffer .2 • True color systems are also known as RGB • Number of bits per pixel is divided by 3 • Each group represents one of the colors • Red, Green, Blue • Resolution is the number of pixels per frame buffer • Usually reported as width x height
Frame Buffer .3 • To reduce number of bits per pixel, a color buffer may be used • Color buffer holds actual RGB values • Frame buffer holds index into color buffer • Reduces size when number of colors used is small relative to total number possible
Grapihic Processor • Processing of graphic information may be done by • Normal CPU • Specialized Graphic Processor • High performance graphic display systems generally use separate graphic processor • Optimized for image processing and display • Specialized operations and high data throughput
Output Devices • Previously, most common output device was the cathode-ray tube or CRT • Image is created by a moving electron beam that causes a coating on the tube face to glow • Most commonly CRT scans a line at a time across the screen • But may be random scan
CRT • The image is not retained but must be refreshed (recreated) periodically • Most commonly at least 50 times per second • This is the refresh rate • Scan lines may be produced consecutively, non-interlaced • Or half at a time, interlaced
Color CRT • Each screen pixel consists of three closely spaced color dots, a RGB triad • Shadow mask helps focus electron beam on a single dot
LCD Display • Each pixel is a layered solid state device that can be turned on or off to produce that color component • Generally cannot support as rapid refresh as CRT
Input Devices • Keyboard • Pointing devices • Mouse • Joystick • Game console • Data tablet
Images • Physical image is generated by physical object • Computer graphic systems generate synthetic image • Produced by program
Objects and Viewers • World is populated with three dimensional objects • How object is seen depends on relationship to viewer and other attributes • Generally a viewer can see only some parts of an object • Different relationships between object and viewer can produce different images
Light • Without light all would be uniformly black • Light makes images visible and changes features through • Intensity • Color • Direction • Interaction with light is complex
Light • Visible spectrum has wavelengths between 350 nm and 780 nm • Color depends on wavelength • Blue ~450 nm • Green ~520 nm • Red ~650 nm
Modeling Light • Geometric Optics • Light assumed to come from point source • Fixed intensity • Travels in straight lines • Assume monochromatic (single color) • More complex sources can be viewed as collection of point sources
Ray Tracing • Effect of light can be viewed by following ray from source to viewer • Light travels in straight line until it hits surface • Surface point then acts as new point source • Ray behavior determined by trig laws • Infinite possibilities but only those that reach viewer matter
Radiosity • When surface scatters incoming light in all directions, ray tracing does not produce accurate results • Energy conservation based calculation is more accurate but also more computationally intensive
Human Visual System • Light enters eye through lens • Focused on retina at back of eye • Retina contains light sensitive cells • Rods – brightness only • Cones – three kinds for three colors • RGB cones allow RGB displays to function • Sensitivity is not uniform
Pinhole Camera • A simple model of a camera • Assumes light enters through a pin hole • Small enough that only one ray enters in any direction
Pinhole Camera Image • Can calculate point where ray from point source is projected on back of camera yp = -y/(z/d), similar for x
Pinhole Camera Image .2 • Point at back of camera is called the projection of the source • Field or angle of view is angle made by triangle from lens to image plane • Θ=2 arctan(h/2d)
Synthetic Camera Model • Standard approach for three-dimensional computer graphics • Compute image that would be captured by bellows camera • Bellows allows depth of camera to be changed as desired • Image is formed on projection plane, where back of camera would be
Clipping Window or Rectangle • Determines the edge boundaries of the image
Programmer’s Interface • Visual interface • Allows image creation by arranging visual components • Programming interface • Function library • Application Programmer’s Interface (API) • Defines available operations • Images build combining operations
Pen-Plotter Model • Typical of early vector drawing systems • Move pen from point to point drawing lines to create image moveto(0, 0); lineto(1, 0); lineto(1, 1); lineto(0, 1); lineto(0, 0);
Pen-Plotter Image • Adding additional lines could produce the image of a cube
Pen-Plotter Model • Limited functionality • Difficult to produce three-dimensional images
Three-Dimensional APIs • Synthetic Camera model is common • OpenGL • PHIGS • Direct3D • VRML • JAVA-3D
3-D API Components • Objects • Viewer • Light Sources • Material Properties
Primitive Objects • Points • Line segments • Polygons • Text • Curves • Surfaces
OpenGL Object Definition • List of components bounded by function calls • A triangle 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();
Camera or Viewer • Specification requires a number of attributes • Position • Orientation • Focal length • Film plane
