In the context of digital image processing compression techniques that often follow steps like Discrete Cosine Transform (DCT) and Zigzag scanning, shift coding refers to a specific step designed to reduce the amount of data needed to represent image coefficients.
Understanding Shift Codes
Based on the provided information, the core function of shift coding is data compression performed after the Zigzag scanning process.
- Purpose: To decrease the number of bits required to represent the data obtained from the Zigzag operation.
- Mechanism: It takes advantage of the characteristic distribution of values after the Zigzag scan, where a significant portion of the data consists of small numbers.
- Benefit: By using a more efficient representation for these frequent small values, shift coding measurably reduces the overall data size and saves storage space or bandwidth.
Where Shift Codes Fit in the Pipeline
Shift coding is typically applied as a coding step following the Zigzag operation. The Zigzag scan reorders the 2D block of coefficients (often resulting from a transform like DCT) into a 1D sequence, generally grouping low-frequency coefficients (usually larger values) before high-frequency coefficients (often smaller values or zeros).
After this scan, and potentially after quantization, the resulting 1D sequence contains many small integer values and long runs of zeros, especially towards the end.
Why Shift Coding is Effective
The effectiveness of shift coding stems from the observation that after Zigzag, most of the values are between 0 and 15.
- Without specific coding, representing these values individually might require a fixed number of bits, such as 8 bits per value.
- However, a value between 0 and 15 can be represented using only 4 bits ($2^4 = 16$ possible values).
Therefore, applying shift coding reduces the wasted bits by using a representation that aligns better with the actual range of the data. This allows the process to reduce the number of bits required to represent the data of Zigzag operation by a measurable amount.
In essence, shift coding optimizes the bit representation for the common, small values produced by the Zigzag scan, leading to a more efficient compressed data stream.
