JPEG - Multimedia Computing - Lecture Slides, Slides of Multimedia Applications

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JPEG

The JPEG Standard

• JPEG is an image compression standard which

was accepted as an international standard in

• Developed by the Joint Photographic Expert Group of the ISO/IEC
• For coding and compression of color/gray scale images
• Yields acceptable compression in the 10:1 range

The JPEG Standard

• JPEG is effective because of the following

three observations

• Image data usually changes slowly across an image, especially within an 8x8 block - Therefore images contain much redundancy
• Experiments indicate that humans are not very sensitive to the high frequency data images - Therefore we can remove much of this data using transform coding

The JPEG Standard

• Humans are much more sensitive to

brightness (luminance) information than to

color (chrominance)

• JPEG uses chroma subsampling (4:2:0)
• The following slide gives an overview of the

various steps in JPEG compression

JPEG Encoding Overview

• The main steps in JPEG encoding are the

following

• Transform RGB to YUV or YIQ and subsample color
• DCT on 8x8 image blocks
• Quantization
• Zig-zag ordering and run-length encoding
• Entropy coding

DCT on Image Blocks

• The image is divided up into 8x8 blocks
  • 2D DCT is performed on each block
  • The DCT is performed independently for each block
  • This is why, when a high degree of compression is requested, JPEG gives a “blocky” image result

Quantization

• Use larger entries in Q for the higher spatial

frequencies

• These are entries to the lower right part of the matrix
• The following slide shows the default Q ( u , v ) values for luminance and chrominance - Based on psychophysical studies intended to maximize compression ratios while minimizing perceptual distortion - Since after division the entries are smaller, we can use fewer bits to encode them

Quantization

Original and DCT coded block

Quantized and Reconstructed

Blocks

Same steps on a less homogeneous

block

• Steps 2 and

Preparation for Entropy Coding

• We have seen two main steps in JPEG coding:

DCT and quantization

• The remaining steps all lead up to entropy

coding of the quantized DCT coefficients

• These additional data compression steps are lossless
• Most of the lossiness is in the quantization step

Run-Length Coding

• We now do run-length coding
  • The AC and DC components are treated differently
  • Since after quantization we have many 0 AC components, RLC is a good idea
  • Note that most of the zero components are towards the lower right corner (high spatial frequencies)
  • To take advantage of this, use zigzag scanning to create a 64-vector