Data – How (Much of) It Is Stored

1CS 128/ES 228 - Lecture 7a

Data – How (Much of)It Is Stored


Outline

What Is an Image Really? Methods of Storing Images How to Make a Big File Small Compression Algorithms Conversion Algorithms

In theory In practice


What is an image? An image is anything we store on

the computer that we think of as a “picture”. It should look “the same” on any display.

Image file formats GIF, JPEG, TIFF, BMP NOT shapefiles


File Formats There are many image file formats

35 on the first page I hit looking for a list!

Each has advantages and disadvantages

CS 128/ES 228 - Lecture 7a 5

GIF Developed by

Compuserve in 1987

Particularly good for line drawings (anything with sharp edges)

VERY common on web


JPEG (or JPG) Product of the Joint

Photographers Experimental Group

Good for photos, images with subtle changes

Also popular on the web


GIF vs. JPEGJPEG GIF

Use For “Realistic” artwork Illustrations

Compres-sion

Lossy, but controllable Lossy, no control

Colors 24-bits 8-bits

Others No transparency Transparency


JPEG 2000 (aka JP2) “The JP2 and JPX file formats allow for

handling of color-space information, metadata, and for interactivity in networked applications as developed in the JPEG Part 9 JPIP protocol.”

Some imagery is now distributed as JP2 files – datum and projection included at no extra charge!


Portable Network Graphics (PNG) PNG also stands for “PNG’s Not GIF”

Loss-less compression using non-patented algorithm

Supports transparency, but not really animation

ISO standard since 2003


BMP Bitmap format –

Primarily for Windows (but not exclusively)

NO Compression means LARGE files

Standard Screen Snapshot is BMP


EPS, PICT, TIFF

Encapsulated PostScript (mostly for printing, some display)

PICTure format (Macs only)

Tag Interchange File Format (multi-platform, but less used these days)


Shapefiles and active software A running program may read from or

write to these formats, but generally uses its own memory management while running.

Shapefiles contain shape information and are not in any of these formats – and not truly image files They are vector layers, after all


Compression Algorithms Compression

algorithms “shrink” files

May do so by mathematical “tricks” or by discarding information


Two KEY Facts about Compression

NO LOSS-LESS compression algorithm can work all the time!

NO LOSSY compression algorithm can regenerate its original data.


An LOSS-LESS ExampleRun-length compression Count and record the length of the data

set and then each group of 0’s or 1’s

111010011100001000000

3 7 0 3 1 1 2 3 4 1 6


A LOSSY ExampleTruncation

12421449030293570214935210952172590275653048282535

124 029 935725 304

12400000000290000000935000000072500000003040000000


How much does compression affect image quality?

Original (32 MB)

Compressed(493 kB)


Converting Vector to Raster Must compute the equation of the

line

Then choose which pixels to highlight

Many algorithms, but differences are technical


Typical algorithm

Y = y0 + 1

Illuminate pixel (x, int(Y))

Y = Y + 1 X = X + 1 /m

Illuminate pixel (x, int(Y))

…

Until Y == y1

X = x0 Y = y0

Illuminate pixel (x, int(Y))(x1,y1)

(x0,y0)


Anti-aliasingBasic idea – Remove the “jaggies” by using color

variations


Conversion in practice


Converting Raster to Vector Basic idea

Find areas with sharp changes – these are your boundaries.

Adjust as topology indicates

Much harder in practice than the other way around

Alternative is hand-digitization

Documents

Data – How (Much of) It Is Stored