Page 202 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes If you are not using alpha transparency or do not need more than 256 colours then exporting as an
8-bit PNG is the way to go. On average, 8-bit PNG will be smaller in file size than GIF with absolutely
no difference in image quality. The PNG and GIF are also very similar in that they both work better
with large lines of the same pattern or colour. If you do not need alpha transparency but wish to use
a colour palette greater than 256 colours then you are looking at a 24-bit PNG. It will be important
to test between a 24-bit PNG and JPEG to see which achieves better results. The PNG still suffers the
same problem as GIF in that they cannot optimize photographs as well as JPEG can.

12.1 Basics of image format

Image file formats are standardized means of organizing and storing digital images. Image files are
composed of pixels, vector (geometric) data, or a combination of the two. Whatever the format, the
files are rasterized to pixels when displayed on most graphic displays. The pixels that constitute
an image are ordered as a grid (columns and rows); each pixel consists of numbers representing
magnitudes of brightness and colour.
12.1.1 image or Graphic

Technically, neither if you really want to be strict, computer pictures are files, the same way WORD
documents or solitaire games are files. They are all a bunch of ones and zeros all in a row. But
we do have to communicate with one another so let’s decide. We will use “image”. That seems
to cover a wide enough topic range.
That “graphic” is more of an adjective, as in “graphic format.” You see, we denote images on the
Internet by their graphic format. The GIF is not the name of the image. The GIF is the compression
factors used to create the raster format set up by CompuServe.
12.1.2 raster

Raster Image Formats (RIFs) should be the most familiar to Internet users. A Raster format breaks
the image into a series of coloured dots called pixels. The number of ones and zeros (bits) used to
create each pixel denotes the depth of colour you can put into your images.
Bump that up to 4 bits-per-pixel and you are able to set that colour dot to one of 16 colours. If you
go even higher to 8 bits-per-pixel, you can save that colour dot at up to 256 different colours.
Does that number, 256 sounds familiar to anyone? That is the upper colour level of a GIF image.
Sure, you can go with less than 256 colours, but you cannot have over 256.
That is why a GIF image does not work overly well for photographs and larger images. There
are a whole lot more than 256 colours in the world. Images can carry millions. But if you want
smaller icon images, GIFs are the way to go.
Raster image formats can also save at 16, 24, and 32 bits-per-pixel. At the two highest levels, the
pixels themselves can carry up to 16,777,216 different colours. The image looks great! Bitmaps
saved at 24 bits-per-pixel are great quality images, but of course they also run about a megabyte
per picture. There’s always a trade-off, is not there?
The three main Internet formats, GIF, JPEG, and Bitmap, are all Raster formats.
Some other Raster formats include the following:

CLP Windows Clipart
DCX ZOFT Paintbrush
DIB OS/2 Warp format
FPX Kodak’s FlashPic
IMG GEM Paint format
Contd....

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