Short note: Lossy compression
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Introduction to the brightness of a region as perceived by the eye does not depend on the absolute grey level values. This is because the human eye does not respond with equal sensitivity to all visual information. Some information is visually more important than the others. Information which is not visually important is called Psychovisual Redundancy. Psychovisual Redundancy exist in all images and can be eliminated without hampering the subjective quality of image. That means simply reducing the quantization, compress the image but also produce false contouring. I.G.S coding reduces the quantization but also reduces false contouring. Steps involved in I.G.S coding are:

  • Lower 4-bits on the preceding modified pixel are added to the present pixel.
  • The new 4MSB’s of the present pixel are taken as the I.G.S code.
  • Repeat step 1 and 2 after moving on to new pixel. If the MSB is 1111, then add 0000 instead of the 4 LSB’s of the previous sum.

For e.g. Construct the IGS code for the given grey level data set {100,110,124,124,130,110,200,210}

Solution: From the grey level values it is clear that we could require 8-bits for their representation. Let I be the first pixel. We start by adding 0000 to the 1st pixel and then follow the steps given,

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This type of coding reduces false contouring which is a pitfall of the uniform quantization. It also eliminates psychovisual redundancies.

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