Why image compression?

Uncompressed images take too much space, require larger bandwidth for transmission and longer time to transmit

For e.g. Image on the left is 1MB while the one on the right is 50KB. Do you find any difference visually?

Reduce the redundancy of the image data in order to be able to store or transmit data in an efficient form.

Classification of Compression

Lossless vs. Lossy Compression


  • Digitally identical to the original image
  • Only achieve a modest amount of compression


  • Discards components of the signal that are known to be redundant
  • Signal is therefore changed from input
  • Achieving much higher compression under normal viewing conditions no visible loss is perceived (visually lossless)

Introduction to image compression

For human eyes, the image will still seems to be the same even when the Compression ratio is equal 10 Human eyes are less sensitive to those high frequency signals Our eyes will average fine details within the small area and record only the overall intensity of the area, which is regarded as a lowpass filter.

DWT for Image Compression

2D discrete wavelet transform (1D DWT applied alternatively to vertical and horizontal direction line by line ) converts images into “sub-bands” Upper left is the DC coefficient Lower right are higher frequencysub-bands.

WT Compression



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