Digital Media
Dr. Jim Rowan
ITEC 2110
Up Next!
• In the next several lectures we will be covering these
topics:
– Vector graphics
– Bitmapped graphics
– Color
• It will be presented in this order
– Bitmapped graphics
– Vector graphics part 1
– Color parts 1 & 2
– Vector graphics part 2: 3D
Computer Graphics...
• A very different viewing media than print
– Usually consumed on a fairly low resolution
monitor
– Forcing us to look carefully at the processes that
move stuff from the real world to the computer...
AND BACK!
• 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
Computer Graphics
• Computer graphics on the Internet
– fostered the shift away from print based media
– has begun to develop its own visual vocabulary
• Inside the computer there’s a numeric model of a realworld phenomenon
• Two ways to model computer graphics (2D images)
– bitmapped images
– vector graphics
But First,
More on “Telling the story”
• The way you represent data affects how
it is interpreted…
• In words
• In graphs
• In images
Heard as an ad for a legal firm:
Has your disability claim been denied?
We win 75% of all the disability claims we
contest!
How is that possible?
Car Insurance Ads
• Several different insurance ads make
some claim:
• People who switch to us save $400 a
year
• How is that possible?
Which hospital would you
choose?
• You need heart bypass surgery
• You have a number of hospitals to
choose from
• You make your decision based on
statistics
• Do you choose the hospital with the
highest or lowest death rate?
The way you display data
affects how it is understood
• This is a field of study all by itself that
includes computer graphics, cognitive
science and psychology
• The way data is displayed affects how people
interpret the data
– how color is used
– the numeric scales used
• Different graphing forms emphasize different
aspects of the numbers
– pie charts
– bar charts
– line graphs
Want more?
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
– Inside a computer it’s all numbers
• To display the internal model so we can see it, an
application must project this internal model onto a
display
– The internal model, the numbers, are in the computer
– This process of projecting this model onto a display is
called “rendering”
Two approaches to
internal graphic modeling
• Why two approaches?
– drastic filesize differences
– each is good for its type of image
– each has its own unique advantagess
• Bitmapped graphics
– grandfathered name... more like pixel
mapped graphics
• Vector graphics
– It’s more like object graphics because you
describe objects using vectors (formulas)
With bitmapped graphics...
• There are 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, shape and different
resolution
– will probably require clipping and scaling to move
from logical to physical pixels
– So let’s look at how this happens… starting with a
bitmapped image
00011000000011110000010110100000111100000001100000
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 color 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 bits...
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
– Simple to 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 displayed as 1024 X 1024 pixels in size
Each with 3 byte (24 bit, millions of colors) color encoding
Which would have the larger (in terms of file size) internal model?
Why?
bitmapped graphic
vector graphic
Here are two more complex images
Both are displayed at 72 pixels per inch
Both are displayed as 1024 x 720 pixels in size
Each with 3 byte (24 bit, millions of colors) color encoding
Which would have the larger (in terms of file size) internal model?
Why?
Bitmapped/Vector Graphics
• Bitmapped image file size is…
– affected by dimensions, resolution and
color resolution
– not affected by contents
• Vector graphics file size is…
– affected by the contents of the image
• the more complex, the larger the file gets
– size of the file is not affected by resolution
Bitmapped/Vector Graphics
• Access to objects found in the image
– vector is easy, objects are described by mathematical
equations
– bitmapped, no objects, just pixels… this is very difficult
• Special effect differences(like blur, which
requires access to surrounding pixels)
– Bitmapped?
• Easy the pixels are stored in the model
– Vector?
• Not so much…
• 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 scaling and resizing
complicated? ==>
Original image: 10 x 5
Now make it twice as big
Original image: 10 x 5
Now make it twice as big
Original image: 10 x 5
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
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...
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
GIMP <==> Inkscape
• Vector can more easily be converted to
bitmapped...
– in fact, this process already exists since you must
RENDER vectors to even 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.
Bitmapped
Image Manipulation
• Why?
– Correct deficiencies (i.e. flash red eye)
• encapsulated sequence of operations to perform a
particular change
– Create images that are difficult or impossible to
create in nature
• special effects
Image layers
• Both bitmapped and vector graphics
use layers 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
Image Manipulation Tools
• Selection tools
– for regular shapes
• rectangular and elliptical marquee tools
• why is it called marquee?
– for irregular shapes
• lasso (polygon, magnetic, magic wand...)
– magnetic snaps to an enclosed object
using edge-detection routines
Selection tools...
• Allow the application of filters to only the
selected parts of the image
• The unaffected area is called a mask...
can be thought of as a stencil
• A 1-bit mask is either transparent or
opaque
• An 8-bit mask allows 256 levels of
transparency... AKA alpha channel
Selection tools...
• Making the mask with a gradient produces a
softer transition... a feathered edge.
• Can use anti-aliasing along the edge more
effectively hides the hard edge visually
• Layers can have masks associated with them
• Allows interesting compositing of image parts
• We’ll talk more about size issues later
when we discuss bitmapped graphics in
more detail
• We will also consider compression
techniques other than the table method
Questions?