1. Dr. Jim Rowan ITEC 2110 Chapter 3
Digital Media
Dr. Jim Rowan
ITEC 2110
Chapter 3

2. About Chapter 3 This is an overview of the next three chapters
Vector graphics
Bitmapped graphics
Color
This material is covered in 5 lectures
Vector graphics part 1
Color part 1
Color part 2
Vector graphics part 2: 3D

3. 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

4. Computer Graphics
Complex… arguably graphics are so important that they are an enabling technology in themselves
Computer graphics on the WWW
fostered the shift from print based media
has begun to develop its own visual vocabulary
Inside the computer there’s a numeric model of a real-world phenomenon
Two ways to model computer graphics (images)
numeric model #1: bitmapped images
numeric model #2: vector graphics
each with their advantages and disadvantages

5. But first: How to display data?
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

6. Designing information display
How to lie with statistics
Edward Tufte, Yale University
Visual Display of Quantitative Information
Envisioning Information
Visual Explanations

7. 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

8. 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 bitmapped display
The internal model, the numbers, are in the computer
This process of projecting this model onto a computer display is called “rendering”

9. Two approaches to graphic modeling
Why two approaches?
drastic filesize differences
each was good for its type of image
Bitmapped graphics
grandfathered name... more like pixel mapped graphics
Vector graphics
It’s more like object graphics because you describe objects using vectors (formulas)

10. 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
for example…

11.
A true bitmapped image is black and white
Each logical pixel is represented by a single bit

12. 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

13.
...
for 1080 more bits...
255
2550

14. 72 bits in the color table 100 bits in the pixel map 172 bits total
Question:
With 2 bits encoding the color, if we expanded the color table, how many colors could be represented?
255 00 01 10

15. 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

16. 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?

17. 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?

18. Bitmapped/Vector Graphics
Bitmapped image size is set by
affected by size, resolution and color resolution
not affected by contents
Vector graphics size is set by
affected by the contents of the image
the more complex, the larger the file gets
size of the file is not affected by resolution

19. 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

20. 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?

21. Original image: 10 x 5
Now make it twice as big
[Draw on image]

22. 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?

23. 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...

24. Bitmapped <==> Vector
Vector can be easily converted to bitmapped... this process already exists since you must 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.

25. Image layers
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

26. Show Image: testPageImage.tiff
1574 pixels wide
1987 pixels tall
RGB encoded
No compression
No table
How Big?

27. Show Image: testPageImage.tiff
Inspect it with mac cmd-I
Open image with hexFiend
How big is it?
What is in it?
Mostly why?

28. HMMMMMmmm…
We’ll talk more about this size issue in the chapter dedicated to bitmapped graphics
We will also consider compression techniques other than the table method

29. And now for something completely different
And now for something completely different! FLASHBACK To audio-related calculations!

30. Size and download speed calculations
How big is the audio file?
44,100 samples per second
16 bit quantization
stereo
3m16 seconds in length
How long to load to youTube over
highest speed dial up connection?
adsl?
How long to download from youTube over

31. Size and download speed calculations
How big is the audio file?
44,100 samples per second
8 bit quantization
stereo
3m16 seconds in length
How long to load to youTube over
highest speed dial up connection?
adsl?
How long to download from youTube over

32. Size and download speed calculations
How big is the audio file?
8,000 samples per second
8 bit quantization
stereo
3m16 seconds in length
How long to load to youTube over
highest speed dial up connection?
adsl?
How long to download from youTube over

33. Questions?