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KALASALINGAM UNIVERSITY
Image Compression
Under the guidance of B.Perumal Presented by
H.Murugan(9915107001) ECE(DCN)
Image:• An image is a two dimensional signal. It is defined by the mathematical function f(x,y)
• where x and y are the two co ordinates horizontally and vertically.Pixel:• The value of f(x,y) at any point is gives the pixel value at that point of an image.Image Resolution:• It refers to number of pixels in an image
Types of imageBinary image: it contain only two pixel value o or 1Gray scale image:it has 256 different shades of colors in it, and ranges from 0 to 255
RGB Image/color image: for 8 bit format – it ranges from 0 to 255For 16 bit format – it ranges from 0 to 65535
What is Compression Reduce the size of data. Compression is a way to reduce the number of bits in a frame but retaining its meaning.
Image compression can benefit users by having pictures load faster and web pages use up less space on a Web host.
Image compression does not reduce the physical size of an image but instead compresses the data that makes up the image into a smaller size.
Need for compression• To understand the need for compact image representation, consider the amount of data required to represent a 2 hour Standard Definition (SD) using 720 x 480 x 24 bit pixel arrays.
• A video is a sequence of video frames where each frame is a full color still image.
• Because video player must display the frames sequentially at rates near 30fps, SD video data must be accessed at
• 30fps x (720x480)ppf x 3bpp = 31,104,000 bps
• Thus a 2 hour movie consists of • 31,104,000 bps x (602) sph x 2 hrs ≈ 2.24 x 1011 bytes. OR 224GB of data• sph = second per hour• Twenty seven 8.5GB dual layer DVDs are needed to store it.
• To put a 2hr movie on a single DVD, each frame must be compressed by a factor of around 26.3.
• The compression must be even higher for HD, where image resolution reach 1920 x 1080 x 24 bits/image
• Web page images & High-resolution digital camera photos also are also compressed to save storage space & reduce transmission time.
• Residential Internet connection delivers data at speeds ranging from 56kbps (conventional phone line) to more than 12mbps (broadband).
• Time required to transmit a small 128 x 128 x 24 bit full color image over this range of speed is from 7.0 to 0.03 sec.
• Compression can reduce the transmission time by a factor of around 2 to 10 or more.
• Similarly, number of uncompressed full color images that an 8 Megapixel digital camera can store on a 1GB Memory card can be increased.
formats
Compression Methods
Lossless Compression: When a file that has been compressed can be decoded back into its original form with zero loss of information, the compression is said to be a Lossless Compression
Lossy Compression: If, after compression, the original file cannot be brought back again then the compression is said to be Lossy
Method 1:Run-length encoding• Simplest method of compression.• How: replace consecutive repeating occurrences of a symbol
by 1 occurrence of the symbol itself, then followed by the number of occurrences.
• The method can be more efficient if the data uses only 2 symbols (0s and 1s) in bit patterns and 1 symbol is more frequent than another.
Method 2:Huffman CodingAssign fewer bits to symbols that occur more frequently and more bits to symbols appear less often.
There’s no unique Huffman code and every Huffman code has the same average code length.
Algorithm:① Make a leaf node for each code symbol
Add the generation probability of each symbol to the leaf node② Take the two leaf nodes with the smallest probability and connect them into
a new nodeAdd 1 or 0 to each of the two branchesThe probability of the new node is the sum of the probabilities of the two connecting nodes
③ If there is only one node left, the code construction is completed. If not, go back to (2)
Huffman Coding:
Original Source Source reductionSymbol Probability Code 1 2 3 4
a2 0.4 1 0.4 1 0.4 1 0.4 1 0.6 0 a6 0.3 00 0.3 00 0.3 00 0.3 00 0.4 1 a1 0.1 011 0.1 011 0.2 010 0.3 01 a4 0.1 0100 0.1 0100 0.1 011 a3 0.06 01010 0.1 0101 a5 0.04 01011
How the encoding and decoding process takes place?
• Encoding:
• Decoding:
Method 3:Lempel Ziv Encoding
• It is dictionary-based encoding
• Basic idea: Create a dictionary(a table) of strings used during communication.
If both sender and receiver have a copy of the dictionary, then previously-encountered strings can be substituted by their index in the dictionary.
Contd...,This compression has 2 phases:
Building an indexed dictionary Compressing a string of symbols
• Algorithm for lempel ziv encoding: Extract the smallest substring that cannot be found in the remaining uncompressed string.
Store that substring in the dictionary as a new entry and assign it an index value
Substring is replaced with the index found in the dictionary
Insert the index and the last character of the substring into the compressed string
Lossy Compression• Lossy compression is the converse of lossless data
compression.
• It is Used for compressing images and video files.
• Methods of lossy compression: JPEG: compress pictures and graphics MPEG:compress video MP3: compress audio
Method 1:JPEG Encoding• Used to compress pictures and graphics.• In JPEG, a grayscale picture is divided into 8x8 pixel blocks to decrease the number of calculations.
• Basic idea: Change the picture into a linear (vector) sets of numbers that reveals the redundancies.
The redundancies is then removed by one of lossless.
JPEG Encoding- DCT• DCT: Discrete Concise Transform• DCT transforms the 64 values in 8x8 pixel block in a way that
the relative relationships between pixels are kept but the redundancies are revealed.
• Example: A gradient grayscale
Quantization & CompressionQuantization:
After T table is created, the values are quantized to reduce the number of bits needed for encoding.
Quantization divides the number of bits by a constant, then drops the fraction. This is done to optimize the number of bits and the number of 0s for each particular application.
• Compression: Quantized values are read from the table and redundant 0s are
removed. To cluster the 0s together, the table is read diagonally in an zigzag
fashion. The reason is if the table doesn’t have fine changes, the bottom right corner of the table is all 0s.
JPEG usually uses lossless run-length encoding at the compression phase.
JPEG Encoding
Method 2:MPEG Encoding• Used to compress video.
• Basic idea: Each video is a rapid sequence of a set of frames. Each frame is a spatial combination of pixels, or a picture.
Compressing video = spatially compressing each frame + temporally compressing a set of frames.
Types of MPEG compression• Spatial Compression
Each frame is spatially compressed by JPEG.
• Temporal Compression Redundant frames are removed. For example, in a static scene in which someone is talking, most frames are the same except for the segment around the speaker’s lips, which changes from one frame to the next.
Advantages
• Used in remote sensing• In space exploration• Geological surveys for detecting mineral resources In industries
Applications
• Medical field• Internet• Astronomy• Defense• Machine/robot vision• Face recognition • Biometrics
Thank you