Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!watnot!watdcsu!brewster
From: brewster@watdcsu.UUCP
Newsgroups: comp.graphics
Subject: Re: Compressing Image Data
Message-ID: <2970@watdcsu.UUCP>
Date: Sun, 1-Feb-87 21:30:53 EST
Article-I.D.: watdcsu.2970
Posted: Sun Feb  1 21:30:53 1987
Date-Received: Mon, 2-Feb-87 03:44:32 EST
References: <505@hao.UCAR.EDU> <252@gssc.UUCP>
Organization: U of Waterloo, Ontario
Lines: 55
Keywords: Image processing



>Reply-To: jdm@gssc.UUCP (John D. Miller)
>Organization: Graphic Software Systems, Beaverton Or

>other, slightly newer and more sophisticated methods stem from digital
>signal processing.  i am refering to the numerous pulse-code-modulation (PCM)
>techniques, each of which have their own strengths and weaknesses in the DSP
>field.  

>for image processing, you might want to consider a form of PCM called PDM, or
>pulse-delta-modulation, where the delta is the difference from one pixel value
>to the next this makes sense when you are dealing with shades of related colors
>AND these shades are close in proximity to each other in your color map.  some
>trickyness is involved here: you can usually specify the delta from one pixel
>to another in less than 16-bits (which saves you space over storing the pixel
>values), but you must choose this quantity carefully.  if the spectral
>frequency content of your image is very high, or can be very high, you may
>easily saturate the delta quantity and the delta would have to be "overloaded",
>othat is, multiple deltas may sometimes equal the delta between two pixels, or
>suffer a high-frequency loss in the picture, effectively lowering the contrast.

     no guarantees as to the feasibility of the following, but in
     conventional dsp it is generally recognized that adaptive
     delta modulation is far superior to regular delta modulation.

     regular delta modulation can suffer from two problems :
     a) delta is too small so output signal can't track
	rapid changes in input, causing loss of frequency
	content as mentioned above
     b) delta is too large so that output signal has jaggies
	for small changes in input, which is equivalent
	to the addition of spurious high frequency noise.

     too avoid this problem adaptive delta modulation allows the
     size of delta to vary as a function of the input signal.
     obviously the demodulation problem is more complicated,
     but for a given compression factor (i.e. 16 bits to 8 bit
     delta values) the adaptive delta modulation will be able
     to represent your signal more accurately.

     i'm sure this approach to image compression has been studied
     extensively and is written up in the literature, but don't
     ask me where exactly.

     the thing to remember with images is that when introducing
     extraneous noise or artifacts (as any compression/decompression
     algorithm must do), it is better to introduce noise in 
     areas of high spectral content as the human vision system is
     least sensitive to noise in these regions.

						   Try not  to become  a  man
UUCP  : {decvax|ihnp4}!watmath!watdcsu!brewster    of success but rather  try
Else  : Dave Brewer, (519) 886-6657                to  become a  man of value.
                                                         Albert Einstein