CS 1308 Computer Literacy and the Internet

Creating Digital Pictures

A traditional photograph is an analog representation of an image.

Digitizing a picture is the act of creating picture elements (pixels) to represent the image.

The process is similar to the sampling of analog audio The more pixels, the better the quality The more bits for each pixel, the closer the

approximation The human eye can be tricked into filling in

the missing pieces

A 15x12 picture

One Pixel

15x12 = 170 total pixels

Resolution

The number of pixels The higher the resolution, the clearer

the picture Increased resolution allows you to zoom

in and see more detail Higher resolution increases the size of

the file in memory

Aliasing

Zoomed in 5 times

Notice the Pixelation

Colors

RGB (Red, Green, Blue) How many bits for one color in this

cube?

Image Colors

256 Colors45,000 Colors

Image Colors

16 Colors45,000 Colors

Image Colors

45,000 Colors 2 Colors

Image Colors

45,000 Colors 250 Grey scale tones

Pixel Densities

Resolution Example

How much space?

For each pixel you have to store a color or grey scale.

Resolution 300x200 = 60,000 pixels 8 bit greyscale (256 tones) – 60KB 24 bit true color – 180KB (3 bytes per

pixel)

Graphics File Formats

Raster Graphics Stores information on a pixel-by-pixel

basis BMP, GIF, JPEG, PNG

Vector Graphics Stores a description of shapes in the

picture Scalable Vector Graphics (SVG)

JPEG vs GIF

JPEG Joint Photographic Experts Group

Forgent Networks claims patent on key algorithm 24 bit color Lossy compression

Cleverly removes portions of picture that humans won’t see Very good for pictures, “realistic” scenes Allows use to select amount of compression

GIF (Graphics Interchange Format) Patented by Compuserve 256 colors Lossless compression Good for line drawings, clip art

Digital Cameras

Much like a normal camera in terms of optics

Has advantage of knowing what a picture looks like immediately

Digital Pictures

Image is acquired by absorbing reflected light and recording the wavelengths (very similar to sampling audio)

The sensor takes the place of the film

Stored in proprietary RAW format before download to computer

Sensor sends digital signals to be processed and stored in non-volatile memory

Megapixels?

What does it mean? Amount of sensor information Allows for more pixels at maximum

resolution Why is 5 megapixels better than 3 or

4? Can they really zoom in on pictures

like on CSI?

Displaying Images

Display types CRT LCD Gas Plasma

Typical resolutions 800x600 1024x768 1152x864 1280x1024

4 to 3 ratio Movies are 16:9

Refresh rate Interlacing

Graphics Card

15”

12”19”

Graphics Cards

Hardware that connects monitors to your PC Have processors, memory, clocks Almost sub-computers Controls resolution and refresh rate

Drivers Software that allows graphics cards to interface with

monitors Specific to Operating Systems

Linux often has worse support because fewer people use it

Some card include “tuners” for television NTSC

MPEG Compression MPEG-1

video resolution of 352-by-240 at 30 frames per second (fps) quality slightly below the quality of conventional VCR videos

MPEG-2 offers resolutions of 720x480 and 1280x720 at 60 fps full CD-quality audio sufficient for all the major TV standards, including NTSC, and even

HDTV MPEG-2 is used by DVD-ROMs. MPEG-2 can compress a 2 hour video

into a few gigabytes decompressing an MPEG-2 data stream requires only modest

computing power, encoding video in MPEG-2 format requires significantly more processing power

MPEG-4 graphics and video compression algorithm standard that is based

on MPEG-1 and MPEG-2 and Apple QuickTime technology Wavelet-based MPEG-4 files are smaller than JPEG or QuickTime

files designed to transmit video and images over a narrower bandwidth

and can mix video with text, graphics and 2-D and 3-D animation layers

Digital Animation

Programs used to describe desired output Maya

Images are “rendered” one screen at a time 24 fps More for slow motion Takes a lot of computer

time Graphics machines

need A LOT of memory, processor speed, and secondary storage