Page 207 - DCAP303_MULTIMEDIA_SYSTEMS

Unit 12: Image Format

may look bad at 16 colours might look very good at say 48 colours (or it may require 256 colours if notes
photo-like). But often 16 colours are fine for graphics, with the significance that the fewer number
of colours, the smaller the file, which is extremely important for Web pages.
The GIF optionally offers transparent backgrounds, where one palette colour is declared
transparent, so that the background can show through it. The GIF File - Save As dialog box usually
has an Option Button to specify which one GIF palette index colour is to be transparent.

Interlacing is an option that quickly shows the entire image in low quality, and the quality sharpens
as the file download completes. Good for Web images, but it makes the file slightly larger.

The GIF files use a palette of indexed colours, and if you thought 24 bit RGB colour was kind a
complicated, then you are not seen nuthin’ yet (next page).
For GIF files, a 24 bit RGB image requires conversion to indexed colour. More specifically, this
means conversion to 256 colours, or less. Indexed colour can only have 256 colours maximum.
There are however selections of different ways to convert to 256 colours.

self assessment
Choose the correct answer:

1. Image file formats are not standardized means of organizing and storing digital images.
(a) True (b) False
2. ...................... should be the most familiar to Internet users.
(a) Image formats (b) Raster image formats

(c) Pixels (d) Meta/Vector Image Formats
3. ……………….. work by saving the colour information for every pixel of the image.

(a) Image formats (b) Raster image formats
(c) Pixels (d) Bitmapped graphics
4. ................................ is a computer term that represents a variety of mathematical formats
used to compress an image’s byte size.
(a) Compression (b) Raster image formats
(c) Pixels (d) Bitmapped graphics

12.4 Joint photographic experts Group (JpeG)

The JPEG standard defines four variations for compression of images. These are known as follows
Sequential DCT based mode. This is the JPEG baseline format.
sequential Lossless Mode: This format is defined for applications in which it would be desirable
to have no image detail lost in the compression process. This is not widely used as the compression
statistics for this method is modest compared to the other styles.
progressive DCt Based Mode: This format is very similar to the Baseline mode; however instead
of processing information in one scan of the image, this mode processes in a serious of scans, each
one better defining the image from the last scan that was processed.
Hierarchical Mode: This format allows a number of different resolution images to be held within
one file. This means that only one image file needs to be made for a number of resolutions it could
be viewed at. Again it uses more than one scan to define the image, the decoder simply uses scans
in order until it has reached the resolution it requires.

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