Notes on the color table and indexed color concept CSE 232 Spring 2003 Notes on the color table and indexed color concept 11/18/2018 Stockman CSE/MSU S03

General idea Images can consume a large amount of storage space – in main or disk memory Gray scale or labeled image needs only small integer pixels ([0, 255] range is good) RGB color image can be 3X as large Can use a color table to convert an integer label L to color code R[L], G[L], B[L] Just need to make a “table” so that color codes can be obtained using index L 11/18/2018 Stockman CSE/MSU S03

Color table: cheaper graphics image has 8-bit pixels each 8-bit number is an index into a color look up table can change colors without changing image 512 x 512 RGB image with 3x8-bit color values requires 750K Bytes. 512 x 512 8-bit codes = 256K Bytes + 256 x 3x8-bit color table =257K Bytes 11/18/2018 Stockman CSE/MSU S03

Color Table concept int pixel image (P2) Color table RGB pixel image (P3) 11/18/2018 Stockman CSE/MSU S03

GIF image format = image+table (plus other stuff) header information color table image pixels (up to 8 bits each) LZW compressed RGB data is stored in the color table and NOT in the image pixels themselves get good compression and ability to quickly change the colors there are only 256 different colors for any single image, but table RGB triples can be changed gif files are very good for line drawings 11/18/2018 Stockman CSE/MSU S03

ColorTable ADT model is an ADT array of up to 256 colors array index is an int in [0, 255], which we’ll call a “pseudo color” or “color index” operations are create, destroy, add new entry, (see P2 specs for others) 11/18/2018 Stockman CSE/MSU S03

Types of compression Lossless: no information is lost; every bit of the original image can be recovered. if an image has no more than 256 different color triples in it, then color table can exactly recreate it. Lossy: information is approximated; the original image cannot be recovered exactly. Suppose there are 1000 color triples in the original image but we replace each by the closest one of a set of 256 triples in color table; e.g. (218, 58, 150)  (220, 60, 150) 11/18/2018 Stockman CSE/MSU S03