(i) Redundancy can be broadly classified into Statistical redundancy and Psycho visual redundancy.
(ii) Statistical redundancy can be classified into inter-pixel redundancy and coding redundancy.
(iii) Inter-pixel can be further classified into spatial redundancy and temporal redundancy.
(iv) Spatial redundancy or correlation between neighboring pixel values.
(v) Spectral redundancy or correlation between different color planes or spectral bands.
(vi) Temporal redundancy or correlation between adjacent frames in a sequence of images in video applications.
(vii) Image compression research aims at reducing the number of bits needed to represent an image by removing the spatial and spectral redundancies as much as possible.
(viii) In digital image compression, three basic data redundancies can be identified and exploited: Coding redundancy, Inter-pixel redundancy and Psychovisual redundancy.
- Coding Redundancy:
- Coding redundancy is associated with the representation of information.
- The information is represented in the form of codes.
- If the gray levels of an image are coded in a way that uses more code symbols than absolutely necessary to represent each gray level then the resulting image is said to contain coding redundancy.
- Inter-pixel Spatial Redundancy:
- Interpixel redundancy is due to the correlation between the neighboring pixels in an image.
- That means neighboring pixels are not statistically independent. The gray levels are not equally probable.
- The value of any given pixel can be predicated from the value of its neighbors that is they are highly correlated.
- The information carried by individual pixel is relatively small. To reduce the interpixel redundancy the difference between adjacent pixels can be used to represent an image.
- Inter-pixel Temporal Redundancy:
- Interpixel temporal redundancy is the statistical correlation between pixels from successive frames in video sequence.
- Temporal redundancy is also called interframe redundancy. Temporal redundancy can be exploited using motion compensated predictive coding.
- Removing a large amount of redundancy leads to efficient video compression.
- Psychovisual Redundancy:
- The Psychovisual redundancies exist because human perception does not involve quantitative analysis of every pixel or luminance value in the image.
- It’s elimination is real visual information is possible only because the information itself is not essential for normal visual processing.