From: Eric Durant <edurant@engin.umich.edu>
Subject: Re: JPEG and Compresion Ratio
Date: 03 May 2000 00:00:00 GMT
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omirf wrote:
>IMWRITE function in Image Procesing Toolbox of Matlab
[...]
>2. I am trying to assess the actual compression provied by this >function. I can compare the
file sizes for original and compressed
>images. However, this might be misleading due to the existance of
>file header, which might be independent of compression ratio. Can
>you suggest a method to estimate the actual compression acheived,
>assuming one has non-compressed and compressed images.

The other followups are generally correct.  To make it explicit -- there
is no simple relation between the quality factor and the compression
ratio; it depends upon the image.

You should be safe assuming the header sizes are approximately constant
for a given program.  Software can insert optional information, such as
the creator name, into the JPEG stream, so the bytes of side information
may vary significantly between programs.

If you want the true compression ratio, you could write a program to
parse through the JPEG file and add up the lengths of the image data
blocks.  I wrote a program in C++ a couple of years ago to extract the
image size block from a JPEG file; in the process, it finds blocks and
interprets their length fields.  This may be useful as a basis for such
a program.  It is available on my Website.

The following is a good article on the details of JPEG compression.  It
does not discuss the JFIF stream format, though.

@ARTICLE{Wal91,
    author = "Gregory K. Wallace",
    title = "The {JPEG} still picture compression standard",
    journal = "Communications of the ACM",
    year = 1991,
    volume = 34,
    number = 4,
    pages = "30--44",
    month = apr,
}

-- Eric Durant
   http://www.edurant.com/