The
DCT Transform
Traditional JPEG compression uses the Discrete Cosine Transformation
(DCT), which compresses an image in 8x8 blocks and results
in visible artifacts at high compression rates. JPEG artifacts
include visible seams at the tile edges, dubbed as "blocking
artifacts".
The
Wavelet Transform
The wavelet transform encodes an image in a continuous stream
allowing it to avoid the artifacts that result from DCT's
division of an image into discrete compression blocks. Wavelet
artifacts take the form of blurring high contrast lines, merely
making the image look softer. The wavelet transform performs
what's called, multi-resolution compression-it stores image
information in a series of bands, with the most important
image information at the beginning of the file. Each band
contains a representation of the entire image, with the various
bands containing details of the image at every level, from
coarse resolution and textures to fine details.
An inherent benefit of the wavelet's multi-resolution architecture,
is the ability to progressively access the encoded image in
a smooth continuous fashion without having to download, decode
and/or print the entire file. Wavelet compressed images appear
first as an image with coarse resolution and then finer resolution
details are progressively filled in. Since the most important
details are stored at the front of the image file, users will
first see a blurry version of the image and the remaining
details appearing as the bitstream arrives. Usually with about
10% of the image data, the user can tell what the image will
be and can decide whether or not to wait for higher resolution.
The current JPEG is single-resolution, so with 10% of the
data, the user will have barely gotten a peek at the top of
the image and has wait for the entire download.
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