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Computer graphics
3nd stage
21-10-2011
Lec. (1)
College of sciences for women ((())) Dept. of computer sciences
Year 2014-2015
introduction
Computer graphics is concerned with all aspects of producing pictures or images
using a computer. The field began humbly almost 50 years ago, with the display
of a few lines on a cathode-ray tube (CRT); now, we can create images by
computer that are indistinguishable from photographs of real objects.
Computer graphics provides a set of tools to create pictures and to interact with
them in natural ways. The tools consist of both hardware and software, and
together they permit programmers to fashion programs with a strong graphics
capability. Data are presented visually through shapes, colors, and texture rather
than by tables of numbers. Words and numbers are replaced whenever possible by
pictures, because the eye- brain system is better at recognizing and interpreting
visual representations. With interactive graphics a person instructs the computer
using natural hand movements, such as pointing and drawing.
Cathode Ray Tubes (CRTs):
Contains a filament that, when heated, emits a stream of electrons are focused
with an electromagnet into a sharp beam and directed to a specific point of the
face of the picture tube. The front surface of the picture tube is coated with small
phospher dots When the beam hits a phospher dot it glows with a brightness
relative to the strength of the beam and how long it is hit
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How might one measure CRT capabilities?
• Size of tube
• Brightness of phosphers vs. darkness of tube
• Speed of electron gun
• Width of electron beam
• Pixels?
Terms and Concepts :
Here are a few informal definitions of the graphics field :
Image processing: (more precisely, digital image processing) is the field that
deals with methods, techniques, and algorithms for image manipulation,
enhancement, and interpolation.
Computer graphics: is the field concerned with the generation, manipulation, and
storage of digital graphical data. This includes still images (two- and threedimensional), animated graphics, and interactive images (virtual reality). In fact,
most digital data that is not text, software, or audio, is graphics data.
Computer Graphics vs. Image Processing
In computer graphics, a computer is used to create a picture. Image processing,
on the other hand applies techniques to modify or interpret existing pictures
A pixel: (from picture element) is the smallest unit of a digital image. In the
computer, a pixel is represented by its color code, which is either a gray scale
value or the three components of a color. We tend to think of a pixel as a small
dot, circular or square.
(PIX [picture] Element) The fundamental display element of an electronic screen
or bitmapped image. Screen resolution is rated by the number of horizontal and
vertical pixels; for example, 1024x768 means 1,024 pixels are displayed in each
row, and there are 768 rows (lines). Likewise, bitmapped images are sized in
pixels: a 350x250 image has 350 pixels across and 250 down.
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Pixel Structures
In storage, pixels are made up of one or more bits. The greater this "bit depth,"
the more shades or colors can be represented. The most economical system is
monochrome, which uses one bit per pixel (on/off). Gray scale and color typically
use from 8 to 24 bits per pixel, providing from 16 to 16 million colors.
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Displaying the Pixel
On a display screen, pixels are either phosphor or liquid crystal elements. For
monochrome, the element is either energized fully or not. For gray scale, the pixel
is energized with different intensities, creating a range from light to dark. For
color displays, the red, green and blue sub pixels are each energized to a
particular intensity, and the combination of the three-color intensities creates the
perceived color to the eye.
Raster Graphics
• A raster is defined by pixels. A pixel is the smallest display element that
makes up the images seen on a computer monitor or television. In raster
images, the more pixels an image contains, the higher its resolution. For
example, in a raster image a square is drawn as a grid of pixels (dots) and
each of those pixels will have a specific color value. A line is made up of a
row of pixels with each pixel having a color value, and you work with this
line by working with the group of pixels that makes up the line, not as a
single object as you would in a vector file. Programs such as Photoshop,
PaintShop, and PhotoPaint all work with pixels (raster images). A raster
image is resolution-dependant because it contains a fixed number of pixels
that are used to create the image. Since there is a fixed and limited number
of pixels, a raster image will lose quality if enlarged beyond that number of
pixels as the computer will have to 'make up' the missing information. This
is usually the cause of the image becoming fuzzy or "steppy".
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Vector Graphics
Vector-based designs produce the most clear and precise graphics possible. They
use mathematically-defined areas to produce shapes, lines and curves. These
graphic formats are created by using professional graphic design programs such
as Adobe Illustrator. Because vector-based images are not made up of a specific
number of dots, they can be scaled to a larger size and not lose any image quality.
If you blow up a raster graphic, it will look blocky, or "pixelated." When you
blow up a vector graphic, the edges of each object within the graphic stay smooth
and clean. This makes vector graphics ideal for logos, which can be small enough
to appear on a business card, but can also be scaled to fill a billboard. Common
types of vector graphics include Adobe Illustrator, Macromedia Freehand, and
EPS files. Many Flash animations also use vector graphics, since they scale better
and typically take up less space than bitmap images.
What is the difference between vector and raster graphics?
The difference between vector and raster graphics is that raster graphics are
composed of pixels, while vector graphics are composed of paths. A raster graphic,
such as a gif or jpeg, is an array of pixels of various colors, which together form an
image. A vector graphic, such as an .eps file or Adobe Illustrator? file, is composed
of paths, or lines, that are either straight or curved. The data file for a vector image
contains the points where the paths start and end, how much the paths curve, and the
colors that either border or fill the paths. Because vector graphics are not made of
pixels, the images can be scaled to be very large without losing quality. Raster
graphics, on the other hand, become "blocky," since each pixel increases in size as
the image is made larger. This is why logos and other designs are typically created
in vector format -- the quality will look the same on a business card as it will on a
billboard.
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Is it possible to convert a raster graphic into a vector graphic?
Answer: A raster graphic, which is made up of a specific number of pixels cannot
be translated into a vector graphic, an image made up of paths. That being said,
some programs like Macromedia Flash can create paths out of raster graphics so
that they can be manipulated as vector images. This conversion typically reduces
the image's size significantly, but different areas of the image are approximated,
giving the image a stained-glass look.
While it is possible to create a vector image (i.e. an EPS file) from a raster
graphic (i.e. a JPEG image), an exact conversion is not possible. Because of the
approximations made in the conversion process, a scaled up vector version of an
original raster graphic would most likely look worse than just scaling the raster
graphic itself.
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1.3 Applications of computer graphics
•Entertainment
• Computer-aided design
• Scientific visualization
• Training
• Education
• E-commerce
• Computer art
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