Page 31 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Unit 2: Graphics Systems

In 1991, S3 Graphics introduced the S3 86C911, which its designers named Notes
after the Porsche 911 as a suggestion of the performance increase it promised.
The 86C911 spawned a host of imitators: by 1995, all major PC graphics chip
makers had added 2D acceleration support to their chips.

2.5 Resolution

It refers to the sharpness and clearness of a picture. The word is most often used to explain
monitors, printers, and bit-mapped graphic images. In the case of dot-matrix and laser printers,
the resolution shows the number of dots per inch. For example, a 300-dpi (dots per inch) printer
is one that is able of printing 300 distinct dots in a line 1 inch long. This means it can print
90,000 dots per square inch.
For graphics monitors, the screen resolution signifies the number of dots (pixels) on the whole
screen. For example, a 640-by-480 pixel screen is capable of displaying 640 distinct dots on each of
480 lines, or about 300,000 pixels. This translates into different dpi measurements depending on the
size of the screen. For example, a 15-inch VGA monitor (640×480) displays about 50 dots per inch.
Printers, monitors, scanners, and other I/O devices are frequently classified as high resolution,
medium resolution, or low resolution. The real resolution ranges for each of these grades is
continually shifting as the technology improves.
The basic building block of any graphic image is a pixel, a reduction of “Picture Element”,
arranged in precise rows and columns. The number of rows and columns of pixels is referred
to as the “resolution” of the image and is typically expressed by the number of horizontal pixels
(rows) multiplied by the number of vertical pixels (columns), for example: 800*600, 1024*768,
1152*864. Note that these resolutions are sized at a 4:3 ratio. The motive for this is that monitors
are manufactured with a 4:3 aspect ratio, so for a pixel to show square it should appear at the
same 4:3 width to height aspect ratio.
However, the resolution alone defines nothing except the physical size of the image. Each of
these resolutions also has an associated “bit depth”, which defines the number of colours that
can be presented. The bit depth refers to how many bits of data are associated with each pixel
and are available to store a colour value. A 24 bit image, commonly referred to as “true colour”,
allocates for the storage of 24 bits of data per pixel. To clarify the measurement, there are of
course 8 bits in every byte. This raises the question of how colours are defined.
2.6 Colour Displays

Accepting how colours are defined in graphics data is important to understanding graphics file
formats. In this, we touch on a few of the many factors governing how colours are perceived.
We just want to make certain that you have an approval of some of the problems that come up
when people start to deal with colour.
2.6.1 How we See Colour
The eye has a finite number of colour receptors that, taken together, respond to the full range
of light frequencies (about 380 to 770 nanometres). As a result, the eye hypothetically supports
only the perception of about 10,000 different colours simultaneously (although, as we have
mentioned, many more colours than this can be perceived, though not resolved simultaneously).
The eye is also biased to the kind of light it detects. It is most sensitive to green light, followed
by red, and then blue. It is also the case that the visual perception system can sense contrasts
between adjacent colours more simply than it can sense complete colour differences, particularly
if those colours are physically divided in the object being viewed. In addition, the ability to
discern colours varies from person to person; it is been estimated that one out of every twelve
people has some form of colour blindness.
In addition, the eye is limited in its ability to resolve the colour of little objects. The size of a pixel
on a typical CRT display screen, for example, is less than a third of a millimetre in diameter.

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