CS110: Computers and the Internet

Color and Image Representation

Today

• More information representation:• Hexadecimal numbers• Color representation• Image representation• Image compression

Standard Colors

All standards-compliant browsers should handle these color names

These color names can be used with the CSS properties of color and background-color:

For other colors, it’s safest to use a numerical representation…

RGB Color The human retina has three kinds of color-sensitive photoreceptors that were traditionally called red, green and blue cones

Visible colors can be created by adding different amounts of the three primary colors, red, green and blue

Color monitors display colors by adding different amounts of red, green and blue light

RGB color components are usually defined on a scale from 0 to 255

Over 16 million colors can be represented this way!

Explore RGB Color

Deep Pink Khaki Steel Blue

http://cs.wellesley.edu/~cs110/lectures/M01-color/NewColorPad/ColorPad.html

1) What shades are created when all three primary colors are equal?

2) Look at the following colors: red, yellow (=red+green), green, aqua (=green+blue), blue, fuchsia(=red+blue)

How can you make a color brighter or darker?

3) Create the following colors:

Color in CSS

Specifying turquoise using rgb in CSS:

Hexadecimal Notation

R G B

decimal 0-255 0-255 0-255

hexadecimal 00-FF 00-FF 00-FF

Numbers in the range 0-255 are represented in base 16, using two “digits” that each have 16 values, drawn from 0..9, A..F

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0 1 2 3 4 5 6 7 8 9 A B C D E F

decimal

hexadecimal

Converting decimal to hexadecimal:

(1) Left digit: divide by 16, convert quotient to 0..9,A..F

(2) Right digit: convert remainder to 0..9,A..F

Converting hexadecimal to decimal:

(1) Convert left digit to 0..15, multiply by 16

(2) Convert right digit to 0..15, add to (1)

Example: 20110 C916

(1) 201/16 = 1210 C16

(2) remainder is 910 916

Example: A716 16710

(1) A16 is 1010, 10*16 = 160

(2) 716 = 710 160+7 = 167

Converting numbers

• In pairs: try the following conversions (you can use a calculator):

Decimal Hexadecimal

7

26

240

100

255

Hexadecimal Colors

Using hexadecimal colors in CSS:

R G Bdecimal 0-255 0-255 0-255

hexadecimal 00-FF 00-FF 00-FF

blockquote { color: #9400D3; background-color: #E6E6FA; }

Image Representation

Problem: large image files can take a long time to download!

Example: On a 300 kbps (300k bits per second) cable modem, a 450 kB image takes about seconds to download

Solutions:

(1) Downsample images to make them smaller (fewer pixels)

(2) Compress the images, using “lossless” compression (no information is lost) or “lossy” compression (less important information is lost) techniques

RGB color for each pixel is stored in 24 bits (3 bytes)

How much memory is needed to store a 300x500 pixel image?

Indexed Color

0 1 0 1 0

1 2 3 2 1

0 3 0 3 0

1 2 3 2 1

0 1 0 1 0

index color

0

1

2

3

color paletteimage contentsimage

index color

00 #0000FF

01 #00FF00

10 #FFFF00

11 #00FFFF

How large is a file that stores a 300x500 pixel indexed color image with 4 colors, and its color palette?

(1) 300x500 pixels, with 2 bits per pixel (why?)

300 x 500 x 2 = 300,000 bits

(2) 4 colors, with 24 bits per color

4 x 24 = 96 bits

~37.5kB (compared to the 450kB uncompressed file!)2 bits per pixel

24 bits per color

Indexed Color

How large is a file that stores a 300x500 pixel indexed color image with 16 colors, and its color palette?

(1) What is the number of bits used to store the image pixels?

(2) What is the number of bits used to store the color palette?

Total file size (in bytes):

“bits per pixel” is also called the “bit depth”

bit depth must be large enough to store all of the colors

bit-depth max colors

1 2

2 4

3 8

4 16

5 32

6 64

7 128

8 256

GIF Indexed Color

The GIF file format is an indexed color format that allows 256 colors (out of a possible 16 million colors!)

Computing the file size for a GIF image, in bytes:

(1) number of bytes to store the image pixels: (width * height * bit-depth) / 8

(2) number of bytes to store the color palette: num_colors * 3

(3) total number of bytes*:

(width * height * bit-depth) / 8 + 3 * num_colors

(4) divide by 1,000 or 1,000,000 to convert to kilobytes or megabytes

*There’s also a small amount of fixed overhead for storing file type, dimensions, etc.

Other Image Formats

JPEG: wellesley.jpg

- millions of colors, compressed, nice for photos, does not allow transparency

PNG: cs110-logo.png

- millions of colors, compressed, nice for line art and graphics, allows 8-bit transparency

GIF: palette.gif

- limited to 256 colors, nice for line art and graphics, allows 1-bit transparency, supports animation

Generate all formats and compare with Fireworks!