Digital Media
Dr. Jim Rowan
ITEC 2110
Chapter 3
Roll call
Computer Graphics...
• Arguably a very different viewing media
– Forced to look carefully at the processes that
move stuff from the real world to the computer...
AND BACK!
– Usually consumed on a fairly low resolution
monitor
• Graphic images work very differently on a
screen than when in print
–
–
–
–
can be seen with lights out
will be viewed from different resolution monitors
viewing angles are different
reflections off screen... magazine (gloss)
Computer Graphics
• Complex enough to be arguably an enabling
technology not just a single thing
• WWW
– fostered the shift from text based media
– has begun to develop its own visual vocabulary
• Ways to model computer graphics
– bitmapped images
– vector graphics
– each with their advantages and disadvantages
But first:
How to display a bunch of
data?
• This is a field of study all by itself that
includes computer graphics, cognitive
science and psychology
• For example: Different graphing forms
– pie charts
– bar charts
– use color for delineation?
Designing information display
• How to lie with statistics
• Edward Tufte, Yale University
– Visual Display of Quantitative Information
– Envisioning Information
– Visual Explanations
Computer Display types
• Now... all are rectangular arrays of
pixels
• Not always that way
– Early graphics (1976) used a “steerable”
electron gun, not raster graphics
– Since then...
– we have moved away from electron gun
Internal and External graphics
models
• Internally an application keeps a numeric
model
• Externally an application must project this
internal model onto a bitmapped display
– This process is called “rendering”
• An application must also project this internal
model onto a storage model to be able to
store and recall it later
Two approaches to graphic
modeling
• Bitmapped graphics
– grandfathered name... more like pixel
mapped graphics
• Vector graphics
– more like object graphics because you
describe objects using vectors (formulas)
• a line is defined by slope, length & position
• a circle is radius & position
• a square is length of side & position
Bitmapped graphics...
• Logical and physical pixels
– images are modeled internally as an array of pixel
values... the logical pixels
– physical pixels are the actual dots on screen
• Moving from logical and physical pixels
– called rendering
– may be different size and different resolution
– will probably require clipping and scaling to move
from logical to physical pixels
00011000000011110000010110100000111100000001100001
A true bitmapped image is black and white
Each logical pixel is represented by a single bit
When color came along it borrowed the idea...
except that each logical pixel became a 3 byte
RGB specification instead of a single bit
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
255
255
255
255
255
255
255
255
255
0
0
255
0
0
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
0
0
255
0
0
255
0
0
255
0
0
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
0
0
255
255
0
0
0
0
255
0
0
255
255
0
0
0
0
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
0
0
255
0
0
255
0
0
255
0
0
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
0
0
255
0
0
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
255
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
1111 1111 . 1111 1111 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
1111 1111 . 0000 0000 . 0000 0000
0000 0000 . 0000 0000 . 1111 1111
0000 0000 . 0000 0000 . 1111 1111
1111 1111 . 0000 0000 . 0000 0000
0000 0000 . 0000 0000 . 1111 1111
...
for 1080 more bit...
1111 1111 . 1111 1111 . 1111
255
255
255
0
0
255
72 bits in the color table
100 bits in the pixel map
255
0
0
172 bits total
00
00
00
01
01
00
00
00
00
00
00
00
01
01
01
01
00
00
00
00
00
01
10
01
01
10
01
00
00
00
00
00
01
01
01
01
00
00
00
00
00
00
00
01
01
00
00
00
00
00
Question:
With 2 bits encoding the color, if
we expanded the color table,
how many colors could be
represented?
Vector graphics
• Internal model is very different than
bitmapped graphics
• Images are described as mathematical
equations
• Rendering is very different
– must translate equations to physical pixels
– not simply clip or scale
– must compute the array of physical pixels from the
equations
bitmapped graphic
vector graphic
Here are two images, blue squares
Both are displayed at 72 pixels per inch
Both are 1024 X 1024 pixels in size
Each with 3 byte (24 bit, millions of colors) color encoding
Which would be bigger?
Why?
bitmapped graphic
vector graphic
Here are two images, blue squares
Both are displayed at 72 pixels per inch
Both are 318 X 318 pixels in size
Each with 3 byte (24 bit, millions of colors) color encoding
Which would be bigger?
Why?
Bitmapped/Vector Graphics
• Bitmapped image size is set by
– size, resolution and color resolution
– not affected by contents
• Vector graphics size is set by
– the contents of the image
• the more complex, the larger the file gets
– resolution does not affect the size of the file
Bitmapped/Vector Graphics
• Selection of objects in the image
– vector is easy, objects are described by
mathematical equations
– bitmapped, no objects, just pixels
• Special effect (like blur) differences
– bitmapped, easy
– vector, first convert to bitmapped, then blur
Bitmapped/Vector Graphics
• Scaling and Resize
– Vector? Simple... change formula
• Changes can be made BEFORE pixel values
are calculated
– Bitmapped? Complicated...
• frequently results in artifacts
• Why is bitmapped complicated?
Original image: 10 x 5
Now make it twice as big
[Draw on image]
Original image: 10 x 5
[Draw on image]
Now make it twice as big
What happens if there are
two colors next to one
another?
Strictly duplicate?
jagged edges
Interpolate them?
Original image: 10 x 5
To make it 50% larger...
What do you do?
Do you make it
15 x 7? or 15 x 8?
1 pixel => 1? 2?
There is no such thing as
1.5 pixels...
Bitmapped <==> Vector
• Vector can be converted to bitmapped
(rasterized)... this process already
exists since you RENDER vectors to
display them.
• Bitmapped to vector is complicated
– Vector is based on shapes but bitmapped
does not define any shapes
– Software must identify edges and find the
shapes.
Image layers
• Bitmapped and vector graphics use this
as an organizational device
• In bitmapped graphics
– layers are used like digital tracing paper to
isolate objects in the image
– colors can be separated and manipulated
individually
Questions?