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« Image manipulation | Main | Compression »
Saturday
Oct272007

Image formats

Compuserve developed the Graphics Interchange Format (GIF). There are 2 versions; the 89A version is more useful, allowing for animated GIFs and transparent backgrounds. GIFs work on a restricted palette, so are best for a restricted colour range. They are great for line art or icons. However, be wary of using animated GIFs. They can look really naff and add nothing.

JPEG was formed by a working group from ISO and CCITT (which has apparently been renamed) called the Joint Photographic Experts Group. This is teh dominant format for true-colour images. They can achieve 15:1 to 30:1 compression rates, but compression and unpacking is relatively slow. There are many incompatible coding schemes, but standardisation has occurred.

The stages of JPEG compression are as follows:

  1.  Conversion from RGB to luminance/chrominance colour space (because the human eye is less sensitive to chrominance change than brightness).
  2. Colour information is then sub-sampled (lossy). Codes in 2x2 cells are stored as 4 intensity values and 2 colour difference values. Requires 6 values per group instead of 12.
  3. The image is then split into blocks of 8x8 and a Discrete Cosine Transform (DCT) is applied. Intensity data is transformed to frequency data. This process is reversible, so this stage is lossless.
  4. The frequency information is then quantized, which is lossy. Particularly applied to high frequency chrominance information, as the eye can't detect this. May remove some values and increase the occurance of others. E.g. the values 78, 79, 80, 81, 82 could all be replaced by 80.
  5. The data is then subjected to Huffman coding (as mentioned in the compression entry!).

It is possible to equate the quality setting to the compression rate. The default is a 75% quality setting, at a 12:1 compression rating. As compression rating gets higher, the 8x8 blocks become increasingly noticeable in the final image.

There is now a (relatively) new bitmapped graphics format called Portable Network Graphics (PNG). This is similar to the GIF format, and has been approved by the WWW consortium as a standard. GIF is subject to patent rules because the LZW compression that it uses is under patent. PNG uses lossless compression and handles full colour images, giving better quality than GIF. It doesn't support animation (but that may not be a big problem!). Apparently there may be a new version that does support animation.

 

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