Computer Graphics and Video Representation

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Questions and Answers

What is the purpose of RLC (Run-Length Coding)?

To compress video files
To encode the symbol and the length of the group (correct)
To compress images
To decode the code into its original form

What is the main idea behind Variable Length Coding?

Techniques based on the entropy ideas (correct)
Techniques based on the frequency of characters
Techniques based on the ASCII values of characters
Techniques based on the alphabetical order of characters

How do you construct a Shannon-Fano tree?

By adding a '1' when going right to a child and adding a '0' when going left
By adding a '1' when going left to a child and adding a '0' when going right
By adding a '0' when going right to a child and adding a '1' when going left
By adding a '0' when going left to a child and adding a '1' when going right (correct)

What is the formula to calculate the entropy (η) of characters?

η = -∑pi log2(pi) (C) Signup and view all the answers

What is the formula to calculate the average number of bits needed for each character?

Average bits = Total bits / Total characters (D) Signup and view all the answers

What is the Compression Ratio formula?

Compression Ratio = B0 / B1 (B) Signup and view all the answers

What is the purpose of drawing a Shannon-Fano tree?

To construct a binary tree for encoding characters (C) Signup and view all the answers

What is the distribution of characters in Shannon-Fano coding?

Symbol distribution (B) Signup and view all the answers

What is the primary advantage of digital representation of video signal?

It can be stored on digital devices or in memory (D) Signup and view all the answers

Which of the following statements about video scanning is true?

Progressive scanning is used in CRT monitors (C) Signup and view all the answers

What is the purpose of compression ratio?

To compare the number of bits before and after compression (A) Signup and view all the answers

What is entropy in the context of information theory?

A measure of the information contained in a source (C) Signup and view all the answers

What is the purpose of defining entropy?

To specify the lower bound for the average number of bits to code (B) Signup and view all the answers

What is the difference between lossless and lossy compression?

Lossless compression reduces the size of files without losing information, while lossy compression reduces the size of files with some information loss (A) Signup and view all the answers

What is the formula for compression ratio?

Compression ratio = B0 / B1 (D) Signup and view all the answers

What is the primary advantage of digital representation of video signal in terms of repeated recording?

Image quality does not degrade (C) Signup and view all the answers

What is the compression ratio for the input string 'ABABBABCABABBA' using LZW compression?

1.56 (C) Signup and view all the answers

What is the output code for the input string 'ZWZWWZW' using LZW compression, given the initial dictionary (Z=1, W=2)?

1 2 3 4 2 (B) Signup and view all the answers

What is the compression ratio for the input string 'ZWZWWZW' using LZW compression, given the initial dictionary (Z=1, W=2)?

1.4 (D) Signup and view all the answers

What is the purpose of the LZW decompression algorithm?

To decompress data (D) Signup and view all the answers

What is the output string for the input code '1 2 4 5 2 3 4 6 1' using LZW decompression, given the initial dictionary (A=1, B=2, C=3)?

ABABBABCABABBA (B) Signup and view all the answers

What is the output string for the input code '1 2 3 4 2' using LZW decompression, given the initial dictionary (Z=1, W=2)?

ZWZWWZW (B) Signup and view all the answers

How many bits are needed to encode the message '(BCA)' using Arithmetic coding, given the probability distribution of the alphabet (A, B, C)?

6 (B) Signup and view all the answers

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Study Notes

Video Scanning

Progressive scanning traces a complete picture (a frame) for each time interval.
CRT Monitor uses progressive scanning with a minimum refresh rate of 85Hz.
Interlaced scanning is used in TVs.

Digital Representation of Video Signal

Digital video is represented as a sequence of digital images.
Advantages of digital representation:
- Video can be stored on digital devices or in memory.
- Video is ready to be processed (noise removal, cut and paste, etc.).
- Video can be integrated into various multimedia applications.
- Repeated recording does not degrade image quality.
- Ease of encryption.

Compression

Compression: process of coding to reduce the number of bits needed for information.
Two main categories of compression:
- Lossless: input message = output message.
- Lossy: input message â‰ output message.

Data Compression Scheme

Compression ratio: B0 / B1, where B0 is the number of bits before compression and B1 is the number of bits after compression.

Entropy

Entropy (): a measure of the information contained in a source.
Entropy is used to find the optimal code.
â‰ specifies the lower bound for the average number of bits to code.

Run-Length Coding (RLC)

RLC: code one symbol and the length of the group.
Example: ABBCCDDDDDDDEEFGGGGG can be encoded as ABBCCD#9EEFG#5, reducing 22 characters to 14 characters (36% reduction).

Variable Length Coding

Techniques based on entropy ideas.
Two techniques: Shannon-Fano and Huffman coding.
Variable length coding creates a binary tree.
To read the codes, start from the root, adding '0' when going left and '1' when going right.

Shannon-Fano Tree

Used to encode characters based on their probability distribution.
Calculate the average number of bits needed for each character and the compression ratio.

LZW Compression

Dictionary-based LZW compression algorithm.
Shows the dictionary and output for LZW compression of an input string.
Compression ratio is calculated.

LZW Decompression

Dictionary-based LZW decompression algorithm.
Shows the dictionary and output for LZW decompression of an input code.
Output string is the original input string.

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