COMSM2200

Multimedia Processing

Coursework Assignment I - 2006/07

To be completed by students working individually.

The Task

The task is to design and implement a program to investigate the use of the discrete cosine transform (DCT) in image compression. The program should do the following:

Read in a pgm format image.
Compute DCTs for contiguous square blocks in the image, where the block size is a power of 2 specified by the user (you can assume that the dimensions of the image are also a power of 2).
Apply uniform scalar quantisation to the DCT coefficients using:
- the same quantisation interval for all the coefficients; or
- different quantisation intervals defined by a quantisation matrix.
The user should select the type of quantisation and the parameters involved.
Output the number of non-zero quantised DCT coefficients.
Reconstruct a `decoded image' by applying the inverse DCT to each block of quantised coefficients.
Output the mean squared error (mse) between the original image and the decoded image.
Output the decoded image in pgm format.

The block size, type of quantisation and quantisation parameters should be entered via the command line by the user (you may want to specify a file containing the quantisation matrix). Write your program in C using functions from the IPLIB library. You should implement the forward and inverse DCTs directly using equations (9), (10) and (11) in section 7 of the course notes.

The Submission

You need to submit a listing of your program and a summary of experiments carried out to investigate the use of the DCT for image compression. The latter should include descriptions and analysis of:

The different quantisation strategies used.
The likely degree of compression achieved in each case indicated by the number of non-zero coefficients, including the validity of the compression estimate.
The quality of the decoded image, both in terms of the mse and perceptual criteria.
Examples of decoded images resulting from the experiments.

The completed report should be about 5-10 pages. Use `xv' to convert images into postscript form for inclusion in the report and the unix utility `a2ps' to produce a compact program listing. Contact me if you have problems with including images in reports.

No marks will be given for submissions consisting only of a program listing.

Assessment and Deadline

This coursework is worth 25% of the total mark for this unit. Reports should be submitted online in pdf format by 23.59 on Friday 23 March 2007.

Majid Mirmehdi, M.Mirmehdi@bristol.ac.uk,