![Quantization Tables
• In JPEG, each F[u,v] is divided by a constant
q(u,v).Table of q(u,v) is called quantization
table.
• Eye is most sensitive to low frequencies (upper
left corner), less sensitive to high frequencies
(lower right corner)JPEG Standard defines 2
default quantization tables, one for luminance
(below), one for chrominance. E.g Table below](https://image.slidesharecdn.com/jpegstandards-190812143849/85/Jpeg-standards-10-320.jpg)
Jpeg standards
- 2. WHAT IS JPEG STANDARDS??? • JPEG is sophisticated lossless compression method for color or grey-scale still images. • It works best on continuous tone images where adjacent pixels have similar colors. • One advantage of JPEG is the use of many parameters, allowing the use to adjust the amount of data lost over a wide variety range.
- 4. Differential coding of DC coefficient: DPCM using previous quantized DC coefficient as predictor Zig-zag scan of AC coefficients
- 5. Main Goals of JPEG: • High Compression Ratio in such cases where images quality is judged as very good to excellent. • The use of many parameters allowing user to experiment and achieve the desired level of compression. • Obtaining good results any kind of continuous tone images. • Sophisticated but not too complex compression method, allowing software and hardware implementation on many platform . Several modes of operations.
- 6. Modes of JPEG: • Sequential Mode: each image component is compressed in a single left to right, top to bottom scan. • Progressive Mode: the images is compressed in multiple blocks to be viewed from coarse to final detail. • Lossless Mode: important for cases where the user decides no pixel should be lost. • Hierarchical Mode: the image is compressed at multiple resolution allowing lower resolution blocks to be viewed without first having to decompressed the following higher-resolution blocks.
- 7. Basic JPEG Compression • JPEG compression involves the following: • Color Space Transform and sub sampling (YIQ) • DCT (Discrete Cosine Transformation) • Quantization Zigzag Scan DPCM on DC component • RLE on AC Components Entropy Coding — Huffman or Arithmetic
- 8. JPEG Encoding • Colour Space Transform and sub-sampling (YIQ): • The images values are transformed from RGB to luminance/chrominance color space. • The eye is sensitive to small changes in luminance but not in chrominance. So chrominance values can be used to compress the data. • This step is optional but important since the remainder of the algorithm works on each color component seperately. • Without transforming the color space, none of the three color components will tolerate the much loss leading to worse compression.
- 9. DCT (Discrete Cosine Transformation) • The DCT is applied to each data unit to create an 8x8 map of frequency component. • They represent the average pixel value and successive higher frequency changes within the group. • This prepares the images data for the crucial step of losing information.
- 10. Quantization Tables • In JPEG, each F[u,v] is divided by a constant q(u,v).Table of q(u,v) is called quantization table. • Eye is most sensitive to low frequencies (upper left corner), less sensitive to high frequencies (lower right corner)JPEG Standard defines 2 default quantization tables, one for luminance (below), one for chrominance. E.g Table below
- 11. Quantization Table • Zig-zag Scan What is the purpose of the Zig-zag Scan: • To group low frequency coefficients in top of vector.• Maps 8 x 8 to a 1 x 64 vector
- 12. Quantization • Each of the 64 frequency component in a data unit is divided by a separate number called its quantization coefficient (QC) and then rounded to an integer. • This is the step where the information is lost. • Large QCs cause more loss. • So high frequency components typically have larger QCs.
- 13. Quantization Why do we need to quantise: To throw out bits from DCT. • Example: • 45 (6 bits). • Truncate to 4 bits: 1011 = 11.Truncate to 3 bits: 101 = 5. • Quantization error is the main source of Lossy Compression. • DCT itself is not lossy • How we throw away bits in Quantization Step is lossy
- 14. Quantization Methods Uniform quantization • Divide by constant N and round result (N = 4 or 8 in examples on previous page). • Non powers-of-two gives fine control(e.g., N = 6 loses 2.5 bits)
- 15. Differential Pulse Code Modulation (DPCM) on DC Component • Another encoding method is employed• • DPCM on the DC component at least.• • Why is this strategy adopted : DC component is large and varies, but often close to previous value (like lossless JPEG) • .Encode the difference from previous 8x8 blocks – DPCM
- 16. Run Length Encode (RLE) on AC Components • Yet another simple compression technique is applied to the AC component:1x64 vector has lots of zeros in it Encode as (skip, value) pairs, where skip is the number of zeros and value is the next non-zero component.• Send (0,0) as end-of-block sentinel value.
- 17. Huffman (Entropy) Coding • DC and AC components finally need to be represented by a smaller number of bits (Arithmetic coding also supported in place of Huffman coding) • Huffman coding: Each DPCM-coded DC coefficient is represented by a pair of symbols : (Size, Amplitude) where Size indicates number of bits needed to represent coefficient and Amplitude contains actual bits.
- 18. UNIVERSITY QUESTIONS: 1.EXPLAIN JPEG STANDARDS?(4 MARKS) 2.WHAT IS JPEG? WHAT IS ITS STANDARDS? 3. EXPLAIN THE DCT IMAGE COMPRESSION SCHEME?(8 MARKS)
- 19. Thank You