Department of Communication

Seton Hall University

Paint Systems

Computers use a binary system of notation for remembering numbers. That is, computers can only count from zero to one instead of zero to ten. The
computer tracks the thousands of pieces of information by assigning a binary code to each character we type, or dot of color we create on the computer
graphics screen. The code collects a series of 0's and 1's in an eight digit series called a Byte. Technically, the zeros indicate that there is no electronic
charge at a given spot in the computer memory, and ones indicate that there is an electronic charge present. Each digit in the code is called a binary digit
(bit). Electronically, the computer must be able to store the code in its Random Access Memory (RAM). To do so it assigns to each memory location
the condition of being "on" or "off." If the location is on, it has an electrical charge (signified by "1"), if it is off it does not (signified by "0").

The eight bit code (one Byte of information) tells us which bits are on and which are off. If the computer checks its memory at a given location and finds
01000011, this binary code translates into the number 67. Sixty-seven on the ASCII (pronounced "Ask-ee," an acronym for American Standard Code for Information Interchange) conversion chart of alpha/numeric characters equals the letter "C."  The computer display adapter (card connected to the
monitor) translates the code into the letter we see on the screen. On the same ASCII table we find that the number 64 (01000000) represents the
character "@". Other characters use different combinations of 1's and 0's. It is important to remember that a Byte can represent anything we want it to
represent. The number 67 (0100011) can be a letter of the alphabet or a color value at a given spot on the color monitor. The images we see on the
screen are stored in a frame buffer.

Frame - As in film or movie frame

Buffer - A temporary place to store computer information going to a peripheral. A frame buffer is specifically reserved for holding screen information,.
Printer buffers hold text and other information waiting to be printed, and keyboard buffers temporally hold key strokes.

The graphics boards we use in the computer graphics laboratory are set to display 800 pixels across the screen per line by 600 lines. A number is
assigned to each pixel representing a color. A pixel eight bits deep can hold any of 256 different colors, a numeric value from 0 to 255 for each pixel. The complete screen image, called a "bit map," is stored in a place in computer memory known as the screen buffer, a storage place in RAM where the screen image is kept while it is being sent to the monitor. Buffers temporarily hold data en route to a peripheral so that the data can be quickly accessed when it is needed.  A screen buffer is specifically reserved for holding screen information.  Printer buffers hold text and other printer information waiting to be printed, and keyboard buffers temporally hold key strokes.

Pixels may have several levels of color intensity depending on how much information is stored in RAM for each pixel. For example, if the amount of
memory available for each pixel is only one bit deep the pixel can only be on or off­­like the letters on a monochrome text monitor­­there are no shades
of gray. If there are two bits of memory for each pixel, the pixel can be one of four shades. Four bits of memory per pixel allow 16 shades or colors.
The amount of information doubles for each additional bit of memory.

2 bits allow 4 colors
3 bits allow 8 colors
4 bits allow 16 colors
5 bits allow 32 colors
6 bits allow 64 colors
7 bits allow 128 colors
8 bits allow 256 colors
9 bits allow 512 colors
10 bits allow 1024 colors
11 bits allow 2048 colors
12 bits allow 4096 colors
16 bits allow 65,536 colors
24 bits allow 16,777,216 colors
32 bits allow 4,294,967,296 colors

Most paint systems use the "HLS" color modifying system which lets you independently control the hue, luminosity, and saturation of each color on the screen. Hue is the actual color you select from the spectrum of colors available­­red, orange, yellow, green, blue, indigo, violet and shades inbetween. Luminosity (also called "brightness" in PhotoShop or "value") is the brightness of the color. Saturation is the amount of color in the shade that you have selected. Fully saturated colors are vivid. Colors with less saturation show more gray and look colorless. With the HLS (HSB in Photoshop)  you ustomize colors by mixing the amount of hue, luminosity and saturation. In some cases the software will allow you to "ramp" a series of  eight or more colors at once by selecting the first and last colors. The palette editor then automatically creates the inbetween shades and stores the new color values in the look-up table. Once stored you can use this special palette with later images.

Pixel-based images, sometimes called raster images because of the "raster" of lines that make up a television screen, they can be saved to disk by recording the numeric value for each pixel  in the image. Recall the image from disk or from computer RAM and the numeric values are converted back into the actual color by the graphics board.

Look-Up Tables-- (Used on systems witch display 256 colors or less per pixel.)
A pixel with eight bits of color information stored in the frame buffer can be set to any of 256 different colors. However, if these colors are referenced in a "look-up" table, you are not limited to a palette of 256 colors. Your palette can be customized from a rainbow of color possibilities. Many systems
offer you 16.8 million theoretical colors, 256 of which can be displayed at one time using an eight bit frame buffer. Look-up tables hold the RGB (red,
green, blue) information for each color. The numbers in the frame buffer reference the 256 places in the look-up table. Each place holds information
about a different color. Changing the colors in the look-up table means changing the colors on the computer screen. Systems that use a look-up table are called "pseudo-color" systems because the colors are indexed to the look-up table. If a different look-up table is substituted after a picture is drawn, all of the colors in the drawing change to match the colors in the new look-up table. Systems with 15 bits or more of color per pixel do not use look-up tables because each pixel has enough memory in the frame buffer to store an ample variety of colors. They are called "high color" systems and no look-up table is needed. Systems with a minimum of 24 bits of color per pixel are "true color" systems because they generate photo-realistic color images. Note that if a different look-up table has been substituted for the original, then a different color will appear on the screen. For this reason look-up tables are also called color maps. An artist can create a different color map for each picture­­or many color maps for a single picture, each color map being a different interpretation.  If you want to change a spring scene into a winter scene, no need to redraw it, simply create a new color map with winter colors replacing the spring colors.
 

Grid Assignment