Transcript Multimedia Communications

Image Compression
GIF, TIFF
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GIF: Graphics Interchange Format
Basic mode
A LZW method
Dynamic mode
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GIF Interlaced Mode
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TIFF: Tagged Image File Format
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Supports pixel resolution up to 48 bits (16x3)
Intended for transfer both images and digitized
documents in different formats
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Code #1: uncompressed format
Code # 2, 3, 4: digitized document
Code # 5: LZW-compressed
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Digitized Documents
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FAX:
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T4(Group 3)
T6 (Group 4) for ISDN
Use modified Huffman
codes
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Image Compression
JPEG
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What is the JPEG?
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JPEG: Joint Photographic Experts Group
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general-purpose compression standard of
for continuous-tone still image (gray scale and color)
jointly developed by ITU-T and ISO
Encoding modes
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Lossless mode
Sequential DCT-based mode (baseline mode)
Progressive DCT-based mode
Hierarchical mode
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JPEG Encoding
Sequential (baseline) mode
Hierarchical mode
Progressive mode
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Progressive Mode
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Each image component is encoded in multiple
scans
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Bit rate is same with that of sequential mode
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Successive refinement on a rough image untill
reaching the requested quality
a kind of data re-ordering
Preview image can be generated without he
need to completely decode the image
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Hierarchical Mode
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A progressive presentation
Encodes an image at multiple resolutions
For a low resolution display device a ‘best’ image is
available
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Hierarchical mode : without receiving unnecessary data for
higher resolution
Progressive mode : only after the total transmission time for the
full-resolution
Hierarchical mode is more effective in network
environment
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Major Steps of Image Compression
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Picture (Image) Preparation
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Picture (Image) Processing: source coding
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forward DCT
Quantization: lossy
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analogue-to-digital conversion through sampling
A picture is divided into blocks of 8x8 pixels
mapping of the real numbers into integers
Entropy Encoding: lossless
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Run length coding
Huffman coding or Arithmetic coding
Picture
Uncompressed
Preparation
Picture
Picture
Processing
Quantization
Entropy
Encoding
Compressed
Picture
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JPEG Encoder Schematic
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Image Preparation
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Separation of source image into color components
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1 ~ 255 components (or plane)
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Each component may have different horizontal resolution
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e.g.) 1 component for gray-scale image, 3 components for RGB, or
YCbCr
e.g.) YUV: 4:2:0 format
Subdivision of components into 8x8 pixel blocks
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noninterleaved date ordering: component by component
interleaved data ordering
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Image/Block Preparation
Block preparation
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JPEG Compression/Decompression
Compression Steps
Decompression Steps
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Discrete Cosine Transform
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One-dimensional DCT: used in audio
Two-dimensional DCT: used in image
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Spatial or temporal domain  frequency domain
Each 8x8 block of samples is transformed
Intuition
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Sampled values usually vary slightly from point to point
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Coefficient of the low frequency  high value
Coefficient of high frequencies  small or zero
Human eyes is highly sensitive at low-intensity levels, whereas
its sensitivity is greatly reduced at high-intensity levels
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A reduction of the number of high-frequency DCT coefficients
weakly affects image quality
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Forward DCT (1)
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In theory, DCT is lossless.
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In practice, information is lost because of truncation
errors in calculation
Forward DCT
i  0,1,...,7, j  0,1,...,7
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(2 x  1)i
((2 y  1) j
F [i, j ]  C (i)C ( j ) P[ x, y] cos
cos
,
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x 0 y 0
where C (i)  C ( j ) 
1
2 , for i, j  0
1, otherwise
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Forward DCT(2)
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Forward DCT (3)
v
y
64 DCT
coefficients
FDCT
Spatial domain
x
u
Frequency domain
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Quantization
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64 DCT coefficients are
quantized according to an
quantization table specified by
application.
Lossy
Quantization levels are
different for each frequency
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more precise level for low
freq.
determined by experiments
Quantization matrices are
suggested by JPEG
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Entropy Encoding
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DC coefficients from consecutive DCT blocks
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AC coefficients
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Differential encoding
Run-length encoding
Then, Huffman encoding is used
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Vectoring using zig-zag scan
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Data reordering: zigzag sequence
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low frequency comes first (larger values come first)
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Differential Encoding
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DC coefficients in consecutive quantized blocks
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12, 13, 11, 11, 10, …
 12, 1, -2, 0, -1, …
Encoded in the form (SSS, value), where SSS: # of
bits needed to encode the value
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Run-length Encoding
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Remaining 63 AC coefficients in the vector may
contain long strings of zeros
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Use RLE: (skip, value),
where skip: # of zeros in the run, value: next nonzero
coefficient
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(0,6)(0,7)(0,3)(0,3)(0,3)(0,2)(0,2)(0,2)(0,2)(0,0)
(0,0) indicates the end of the string for this block
The value field is encoded in the form SSS/value
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Huffman Encoding
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JPEG provides the Huffman code tables used
with DC and AC coefficients for both luminance
and chrominance
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Arithmetic encoding is also specified in JPEG, but
protected by patent
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Frame Building
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Frame header contains
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Width and height of a
complete image/picture
Number and type of
components
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Digitization format
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(CLUT, RGB, YCbCr)
(4:2:2, 4:2:0, etc)
Scan: contains a
component
Scan header contains
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Identity of components
(R/G/B)
# of bits used to digitize
each component
Quantization table
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JPEG Decoding
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In progressive mode, first the DC and low-frequency coefficients of
each block are sent
In hierarchical mode, the total image is first sent using a low
resolution – i.e. 320x240 – then at high resolution 640x480
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