Indexed Color A browser may support only a certain number of specific colors, creating a palette from which to choose Figure 3.11 The Netscape color palette 1
QUIZ How many bits are needed to represent this palette? Show your work. 2
How to digitize a picture Sample it Represent it as a collection of individual dots called pixels Quantize it Represent each pixel as one of 2 24 possible colors (TrueColor) Resolution = The # of pixels used to represent a picture 3
Digitized Images and Graphics Whole picture Figure 3.12 A digitized picture composed of many individual pixels 4
Digitized Images and Graphics Magnified portion of the picture See the pixels? Hands-on: paste the high-res image from the previous slide in Paint, then choose ZOOM = 800 Figure 3.12 A digitized picture composed of many individual pixels 5
QUIZ: Images A low-res image has 200 rows and 300 columns of pixels. What is the resolution? If the pixels are represented in True-Color, what is the size of the file? Same question in High-Color 6
Two types of image formats Raster Graphics = Storage on a pixel-by-pixel basis Vector Graphics = Storage in vector (i.e. mathematical) form 7
Raster Graphics GIF format Each image is made up of only 256 colors (indexed color similar to palette!) But they can be a different 256 for each image! Supports animation! Example Optimal for line art PNG format ( ping = Portable Network Graphics) Like GIF but achieves greater compression with wider range of color depth No animations 8
Bitmap format Contains the pixel color values of the image from left to right and from top to bottom Great candidate for run-length compression! Lossless, but files are large! JPEG format Averages color hues over short distances Lossy compression Optimal for color photographs 9
Vector Graphics A format that describes an image in terms of lines and geometric shapes A vector graphic is a series of commands that describe a line s direction, thickness, and color The file sizes tend to be smaller because not every pixel is described Example: Flash 10
Vector Graphics The good side: Vector graphics can be resized mathematically and changes can be calculated dynamically as needed The bad side: Vector graphics are not good for representing real-world images 11
3.6 Representing Video 12
Representing Video Video codec COmpressor/DECompressor Methods used to shrink the size of a movie to allow it to be played on a computer or over a network Almost all video codecs use lossy compressions to minimize the huge amounts of data associated with video 13
QUIZ video A 10-minute videoclip is shot in a resolution of 768x1024 True-Color pixels, and 24 frames/second. What is the uncompressed size of the video file? How long would it take to transmit it over a 100 Mbps Ethernet connection? 14
Representing Video Temporal compression A technique based on differences between consecutive frames: If most of an image in two frames hasn t changed, why should we waste space to duplicate all of the similar information? Spatial compression A technique based on removing redundant information within a frame: This problem is essentially the same as that faced when compressing still images 15
Chapter Review Questions Distinguish between analog and digital information Explain data compression and calculate compression ratios Explain the binary formats for negative (two s complement), fractional, and floating-point values Describe the characteristics of the ASCII and Unicode character sets Perform various types of text compression with pencil and paper: Keyword, Run-length, Huffman 16
Chapter Review Questions Explain the nature of sound and its representation Explain how RGB values define a color Distinguish between raster and vector graphics Explain temporal and spatial video compression 17
For next time: --Read pages 69-75 of text --Re-solve all today s quizzes Homework: 8,9,29,30,31,41,47,49,51,53, 61 --Due Friday, Feb.19 EOL6 18