Page 204 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes has four pixels all the same colour. Why not find a way to make those four pixels into one?
That would cut down the number of bytes by three-fourths, at least in the one corner. That is a
compression factor.
Bitmaps can be compressed to a point. The process is called “run-length encoding.” Runs of pixels
that are all the same colour are all combined into one pixel. The longer the run of pixels, the more
compression. Bitmaps with little detail or colour variance will really compress. Those with a
great deal of detail do not offer much in the way of compression. Bitmaps that use the run-length
encoding can carry either the common “.bmp” extension or “.rle”. Another difference between
the two files is that the common Bitmap can accept 16 million different colours per pixel. Saving
the same image in run-length encoding knocks the bits-per-pixel down to 8. That locks the level
of colour in at no more than 256. That is even more compression of bytes to boot.

12.2 image formats types

One of the initial decisions you have to make as you are creating graphics for your presentations or
pages is what files formats to use for your graphics. File formats break down into two categories:
bitmapped and structured or vector.

12.2.1 Bitmapped Graphics
Bitmapped graphics work by saving the colour information for every pixel of the image. As a result,
the larger the image, the larger the file size. In addition, the more colours the graphic can use, the
larger the image size, since more information has to be stored for each pixel (see Table 12.1).
table 12.1: Common Bitmap Colour Depths and the number of Colours supported

Bit Depth number of Colours
1 bit 2 colours
8-bit 256 colours
16-bit 65.5 thousand colours
24-bit 16.77 million colours
32-bit 16.77 million colours plus 8-bit alpha channel
The most common bitmapped graphic formats you will run across are:

• Windows Bitmap format (.bmp extension)
• Graphic Interchange Format (.gif extension), which is widely used for Web graphics
• Portable Network Graphics format (.png extension), implemented as an alternative to the
.gif format,
• Macintosh Picture format (.pic extension) and
• Tagged Interchange Format (.tif extension), which is often used for cross-platform (Macintosh
and Windows) graphics.
Another way of considering the basic formats is when you would typically use them (see Table 12.2).

table 12.2: typical Bitmapped formats organized by application

application typical formats
Presentation Software .BMP (Windows)
.PIC (Macintosh)
.JPG
Web Pages .GIF
.JPG
.PNG

198 LoveLy professionaL University