Lossy Compression Examples

1
Lossy Compression Examples

• JPEG
• MPEG

2
JPEG

• Joint Photographic Experts Group
• Defines three different coding systems
• Lossy baseline coding system
• Extended coding system for greater compression
• Lossless independent coding system for reversible
  applications
• Baseline performed in three sequential steps
• DCT (Discrete Cosine Transform) computation
• Quantization
• Variable-length code assignment
• To start, the image is divided into pixel blocks
  of size 8x8 (left to right, top to bottom)

3
JPEG (cont.)

• Gray levels are shifted to center L/2 lt gl lt
  L/2
• DCT is run on the block producing frequency
  components centered on upper left corner.
• The DCT 2D block is converted into a 1D string by
  using a zig-zag pattern.
• The array is quantized.
• A selected number of coordinates is selected
  depending on the quality desired.
• These values are then encoded using a variable
  length coding system.
• The binary codes are transmitted.

4
JPEG Coding Example
8x8 block of original image
52 55 61 66 70 61 64 73 63 59
66 90 109 85 69 72 62 59 68 113
144 104 66 73 63 58 71 122 154 106
70 69 67 61 68 104 126 88 68 70 79
65 60 70 77 68 58 75 85 71 64
59 55 61 65 83 87 79 69 68 65
76 78 94
5
JPEG Coding Example
Level Shifted
-76 -73 -67 -62 -58 -67 -64 -55 -65 -69
-62 -38 -19 -43 -59 -56 -66 -69 -60 -15
16 -24 -62 -55 -65 -70 -57 -6 26 -22
-58 -59 -61 -67 -60 -24 -2 -40 -60
-58 -49 -63 -68 -58 -51 -65 -70 -53 -43
-57 -64 -69 -73 -67 -63 -45 -41 -49 -59
-60 -63 -52 -50 -34
6
JPEG Coding Example
DCT
-415 -29 -62 25 55 -20 -1 3 7 -21
-62 9 11 -7 -6 6 -46 8 77 -25
-30 10 7 -5 -50 13 35 -15 -9 6
0 3 11 -8 -13 -2 -1 1 -4
1 -10 1 3 -3 -1 0 2 -1 -4
-1 2 -1 2 -3 1 -2 -1 -1 -1
-2 -1 -1 0 -1
7
JPEG Coding Example
DCT Normalized
-26 -3 -6 2 2 0 0 0 1 -2
-4 0 0 0 0 0 -3 1 5 -1
-1 0 0 0 -4 1 2 -1 0 0
0 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0
8
JPEG Coding Example
DCT Normalized
-26 -3 -6 2 2 0 0 0 1 -2
-4 0 0 0 0 0 -3 1 5 -1
-1 0 0 0 -4 1 2 -1 0 0
0 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0
EOB indicates remainder are zeros.
-26 -3 1 -3 -2 -6 2 -4 1 -4 1 1 5 0 2 0 0 -1 2 0
0 0 0 0 -1 -1 EOB
9
JPEG Coding Example

• Now using a version of Huffman coding categories
  and DC differences, you have the sequence
• 1010110 0100 001 0100 0101 100001 0110 100011 001
  100011 001 001 100101 11100110 110110 0110
  11110100 000 1010
• Total number of bits in completely coded
  reordered array (and thus the number of bits
  necessary to represent the entire 8x8, 8-bit
  example sub image) is 92.
• The resulting compression ration is 512/92 or
  about 5.6 1

10
JPEG Dcompression

• Recreate the normalized transform coefficients
  using a lookup table.
• Denormalize that array.
• Reverse Discrete Cosine Transform.
• Level shift back to normal 0-255 range.
• If you do that, you have something like the
  original image, but not exact!

11
JPEG 2000

• Extends the initial JPEG standard to provide
  increased flexibility in both the compression of
  continuous tone still images and access to the
  compressed data.
• E.g. portions of a JPEG 2000 compressed image can
  be extracted for retransmission, storage,
  display, and/or editing.
• The standard is based on the wavelet coding
  techniques.