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 PinkKhakiSteel Blue 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 RGB decimal0-255 hexadecimal00-FF Numbers in the range are represented in base 16, using two “digits” that each have 16 values, drawn from 0..9, A..F ABCDEF 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:  C9 16 (1)201/16 =  C 16 (2)remainder is 9 10  9 16 Example: A7 16  (1)A 16 is 10 10, 10*16 = 160 (2)7 16 = 7 10  = 167

Converting numbers In pairs: try the following conversions (you can use a calculator): DecimalHexadecimal

Hexadecimal Colors Using hexadecimal colors in CSS: R GB decimal0-255 hexadecimal00-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 indexcolor color paletteimage contents image indexcolor 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-depthmax colors

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) / * 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!