This is a JPEG (JFIF) file, a very popular format which boasts incredible compression rates, thanks to a cunning 'lossy' algorithm involving Fourier transforms (*.jpg, *.jpeg). You can view such pictures with programs such as ChangeFSI, Translator, FYEO2 or Thump (freewares). Modern commercial bitmap manipulation programs all save to/read from JPEG: Photodesk, ProArtisan24, ...
MIME types: image/jpeg, image/pjpeg
Filetype &600 is deprecated.
How JPEG images are compressed
JPEG files encode colour pictures as YUV (Y = intensity, U and V are colour) data. Compressing involves the following steps:
· Convert RGB data to YUV
· Throw away 3 out of 4 of the U and V pixels.
· Convert 8x8 tiles of Y, U and V values through a Discrete Cosine Transform, into an 8x8 square of frequency coefficients.
· Discard coefficients which are zero, or close to zero. This will tend to change the visual appearance to the picture very little.
· Reduce the accuracy with which the remaining coefficients are held (known as 'quantisation'). Again, this changes the appearance very little. The amount by which this is done, controls the compression factor of the image. By now, most of the coefficients will be zero.
· Reorder the 64 coefficients in a zig-zag order, which increases the average length of runs of zeros in the coefficient block.
· Huffman-encode the resulting stream of values.
(Incidentally, decompression involves reversing these steps.)
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