Graphics Format

Two Methods for representing graphics:

Bitmap
Vector/Structured graphic

I. Bitmap

A bit-map representation stores the graphic/image data in the same manner that the computer monitor contents are stored in video memory. A monitor screen image is composed of small points, termed pixels (a contraction for picture element).The number of pixels that compose a monitor image determine the quality of the image (resolution). The more pixels (e.g., 640 X 480; listed as horizontal X vertical numbers), the higher the resolution.

Bit-Map Monochrome/Grayscale Images

Bit-Map Grayscale Example In a monochrome (black/white), image, (like the example at the left <--), each pixel is stored as a single 0 or 1 value (bit). A 640 X 480 monochrome image would require 38.4KB of storage.

A grayscale image, (like the example at the right-- >), usually requires that each pixel be stored as a value between 0 - 255 (byte). Where the value represents the shade of gray of the pixel. A 640 X 480 grayscale image would require 307.2KB of storage.

Bit-Map Color Images

In a color image each pixel is represented as three values, one for each of the primary colors (red, green and blue - RGB). {Actually these are the additive primary colors; while other color representation systems do exist this discussion will only concern itself with the RGB system.} The size of a color image depends upon the number of shades of each primary color that is stored.

8-Bit Bit-Map Color Images

One common semi-standard can store 256 different colors termed 8-bit color. Monitors are capable of displaying millions of colors. This requires that 8-bit color images have color look-up tables (CLUT), stored with them to represent which 256 colors, out of the millions possible, are to be used in the image.

A 640 X 480 8-bit color image would require 307.2KB of storage (the same as 8-bit grayscale). This yields acceptable color quality, but does not compare to 35mm photographic quality.

24-Bit Bit-Map Color Images

Another semi-standard that does yield photographic quality is 24-bit color. Each pixel value is represented as three bytes (one for each primary RGB color). Thus 256 different shades of red, green and blue is possible for each pixel; yielding 256 X 256 X 256 possible combined colors (16,777,216).

A 640 X 480 24-bit color image would require 921.6KB of storage. {Actually most 24-bit images are 32-bit images. The extra byte of data for each pixel is used to store an alpha value representaing special effect information.}

Popular Bitmap File Formats

Format	Maximum bits per pixel	Maximum number of colors	Maximum image size	Compression methods	Multiple images
.bmp	24	16,777,216	65,535 by 65,535 pixels	RLE (optional)	no
.gif	8	256	65,535 by 65,535 pixels	LZW	yes
.jpeg	24	16,777,216	65,535 by 65,535 pixels	JPEG	no
.pcx	24	16,777,216	65,535 by 65,535 pixels	RLE	no
.png	48	281,474,976,710,656	2,147,483,647 by 2,147,483,647 pixels	Deflation(LZW variant)	no
.tiff	24	16,777,216	4,294,967,295 pixels total	LZW, RLE, and others	yes

The BMP File Structure

Bitmap file header	14 bytes
BMP files signature	2 bytes
File size	4 bytes
Unused	2 bytes
Unused	2 bytes
Location of bitmap data	4 bytes
Bitmap info header	40 bytes
length of this header	4 bytes
image width	4 bytes
image height	4 bytes
Bits per pixel	2 bytes
Compression method	4 bytes
Length of bitmap data	4 bytes
Horizontal resolution	4 bytes
Vertical resolution	4 bytes
Number of colors in image	4 bytes
Number of important colors	4 bytes
Color table	8 to 1,024 bytes
bitmap data	varies

JPEG

standard created by the Joint Photographics Experts Group

utilizes compression techniques that are specifically designed for photographic images.

takes advantage of limitations in the human vision system to achieve high rates of compression.

lossy format which allows a user to set the desired level of quality maintained in the image.

GIF - Graphics Interchange Format

created by the UNISYS Corp. and Compuserve,
used to facillatate transmissionof graphical images over phone lines via modems.
file size was the over-riding design goal behind GIF.
compressed using the Lempel-Ziv Welch algorithm (a form of Huffman Coding),
limited to only storing 8-bit color images.

limited to 256 colors
colors are indexed to a series of CLUTs (Color Look Up Tables)
if two images with different CLUTS are displayed annoying pallete shifts occur.

II. Vector / Structured Graphic Images

composed of analytical geometry formula representations for basic geometric shapes (e.g., line, rectangle, ellipse, etc.)

used primarily for storing graphics produced with technical drawing pograms.

major advantage - the geometric formulas are scale and resolution independent.

graphic can be resized easily by simply changing the coefficients of the geometric formulas which negates the errors and side-effects introduced when scaling bit-maps (i.e. aliasing or jaggies).

Vector/Structured Graphic Example

For comparison purposes the following buttons can be used to view scaled versions of the previous image.