Practical JPEG Compression

JPEG compression algorithms may fall in to one of several categories depending on how the compression is actually performed:

• Baseline/Sequential - the one that we described in detail

• Lossless

• Progressive

• Hierarchical

• "Motion JPEG" - Baseline JPEG applied to each image in a video.

Briefly, this is how each above approach is encoded:

1.

Lossless Mode

• A special case of the JPEG where indeed there is no loss

  

• Take difference from previous pixels (not blocks as in the Baseline mode) as a "predictor".

  Predictor uses linear combination of previously encoded neighbors.

  It can be one of seven different predictor based on pixels neighbors

  

• Since it uses only previously encoded neighbors, first row always uses P2, first column always uses P1.

• Effect of Predictor (test with 20 images)

  

  Note: "2D" predictors (4-7) always do better than "1D" predictors.

Comparison with Other Lossless Compression Programs (compression ratio):

-----------------------------------------------------------------
      Compression Program              Compression Ratio        
                                Lena  football    F-18   flowers 
-----------------------------------------------------------------
        lossless JPEG           1.45     1.54     2.29     1.26   
    optimal lossless JPEG       1.49     1.67     2.71     1.33   
       compress (LZW)           0.86     1.24     2.21     0.87   
      gzip (Lempel-Ziv)         1.08     1.36     3.10     1.05   
gzip -9 (optimal Lempel-Ziv)    1.08     1.36     3.13     1.05   
    pack (Huffman coding)       1.02     1.12     1.19     1.00     
-----------------------------------------------------------------

2.

Progressive Mode

• Goal: display low quality image and successively improve.

• Two ways to successively improve image:

  (a)

  Spectral selection: Send DC component, then first few AC, some more AC, etc.

  (b)

  Successive approximation: send DCT coefficients MSB (most significant bit) to LSB (least significant bit).

3.

Hierarchical Mode

A Three-level Hierarchical JPEG Encoder

(From V. Bhaskaran and K. Konstantinides, "Image and Video Compression Standards: Algorithms and Architectures", Kluwer Academic Publishers, 1995.)

• Down-sample by factors of 2 in each direction.

  Example: map 640x480 to 320x240

• Code smaller image using another method (Progressive, Baseline, or Lossless).

• Decode and up-sample encoded image

• Encode difference between the up-sampled and the original using Progressive, Baseline, or Lossless.

• Can be repeated multiple times.

• Good for viewing high resolution image on low resolution display.

4.

JPEG-2

• Big change was to use adaptive quantization