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Image Quality Problems

Some Amateurish Hints

1998:   See also notes added (2000 Nov)

We have only 256 colors (shades of gray) available on our monitor, and have noticed that the image quality is poor on AOL 3.0's (Win 3.1) web browser. IF you encounter this problem, your best viewing strategy would be to minimize the AOL window, and run a fairly recent version of Internet Explorer or Netscape on the AOL connection. Both are available as free downloads, from Microsoft and Netscape websites, if not on your machine already.

If you are not happy with Netcape or IE's image quality, The Opera web browser (http://www.operasoftware.com) has an even better image viewer. However, Opera handles text formatting and some other things in non-standard fashion. Another option might be to use a shareware program like LVIEW or Paint Shop Pro as the default JPEG viewer for your browser. We haven't tried that, and worry that it might cause problems with page display. But we have noticed that the images look much better when opened with these programs - the fine shades of gray seem less coarsely dithered (approximated by interspersing coarser shades).

The Slipknot browser (which needs only a UNIX dial-up connection, with no need for PPP/SLIP - hence the pun) and uses the LVIEW image viewer, gives truly excellent image quality. Unfortunately, UNIX Shell accounts are becoming hard to get, and the SLIPKNOT version we had did not handle page formatting in the currently standard ways.


Notes Added (2000 Nov):

1) Improving Image Quality on AOL 4's Browser       2) Dithering & Color Depth

1) Getting Better Image Quality on AOL's Browser

If you must use AOL's web browser, and have a reasonably fast conection - a 56K modem or DSL/Cable or better, set web-graphics preferences to "uncompressed graphics". This will slow image delivery slightly but result in much clearer photographic images. In AOL 4, you can set this option by clicking:

My AOL -> Preferences -> WWW -> Web Graphics

and unchecking the "compressed graphics" check-box.

Please see also the instructions for full-screen viewing in the Internet Explorer, Netscape Navigator/Communicator, and Opera browsers. To our knowledge, full-screen viewing is not possible with AOL's browser - at least not for AOL 4.

2) Dithering, and Display Color-Depth

Dithering is the process by which certain unavailable colors (or shades of gray) are simulated by a intermixing of dots/pixels of other (available) colors (or shades of gray) so as to produce the semblance (from afar) of that particular unavailable color (or shade of gray).

Reproductions of "black and white" photographs in daily newspapers are fairly obvious examples of one-bit (2 1 = "2 color") dithering. A close examination of such reproductions will reveal a pattern of black dots, on a white or off-white newsprint background, with dot-spacings and/or dot-sizes adjusted to simulate different shades of gray.

Although grayscale images are usually just 8-bits deep (with 28 = 256 shades of gray), we find that it is necessary - unless you are using a special image viewer - to set monitor display settings to at least 16 bits ("High Color" in MS Windows systems) to get proper rendition of grays in jpeg (.jpg/.jpeg) images. Otherwise, certain finer shades of gray are simulated by "dithering". For greyscale images, this means that a pattern of dots of different coarser shades of gray will be used to simulate the required finer shade. This will ruin the smoothness of skin tones and other fine shades and gradients in the image.

We do not understand why a color-depth above 8 bits should be necessary for grayscale images, but this is what we find. It may have to do with the way jpeg images are stored and processed - or it may be that web browsers treat color and grayscale images the same way, so only part of the "256" combinations are reserved for grays, the rest being devoted to colors - or it may be something else, that we are ignorant about...

In MS Windows 98, display color depth can be adjusted by:

  • clicking Start
  • choosing Settings --> Active Desktop --> Customize my Desktop
  • clicking Customize my Desktop --> Settings
  • clicking on the Colors scroll box and choosing the desired color setting:
        -   256 Colors (8 bit)   /   High Color (16 bit)   /   True Color (24 bit)
  • clicking Yes to accept the adjustment

If Win 98 is unable to make the adjustment (blank screen or multiple images), or if you do not want to accept the adjustment, either click No or wait 15 seconds, doing nothing, and the system should revert to the old color depth.

Please be aware that the maximum color setting (at a given screen resolution) is limited by the available Video Memory of your system and by your monitor's capabilities. Choosing too high a setting may cause problems on older monitors and systems with low video memory capacities.

Please note also that the higher the screen resolution (pixels wide X pixels high) the more video memory is needed, leaving less room for color depth increases. So a system that functions well with 24 bit color depth when at 640 X 480 screen resolution may only be able to handle 16 bit color or 8 bit color when at 1024 X 768 resolution. So you may have to trade screen resolution for color depth. Try out combinations (carefully) and see for yourself.

The same is true by the way, for refresh rates: higher refresh rates* use up more video memory. One has to be even more careful experimenting with this, however, as setting refresh rates above a monitor's capacity may, I am told, DESTROY the monitor! This is second-hand information - but PLEASE check your monitor's specifications before playing with refresh rates!

* Higher refresh rates produce less flicker. To check your monitor for flicker, use your peripheral vision, which is more sensitive than your central vision to slight movements or changes in light/shadow. (The evolutionary reason for this seems pretty obvious - detecting a predator or enemy's approach from the side or behind - through slight changes in light or shadow - may have saved your ancestors' skins and so preserved your genes - but that's only a guess on my part. - AJ)

To do this, turn your head away from the monitor, in a darkened room, so the monitor light falls on your eyes sideways. Take care not to look at the monitor, as that will bring in central vision, which sees colors and (by focusing) fine details better, but is less sensitive to slight changes in brightness. So consciously keep your eyes pointed staright ahead, as long as you can, allowing your brain to register flicker (or its absence).

At a given refresh rate, flicker can also be reduced by "non-interlacing" the electron gun sweeps. "Interlacing" means that the sweeps skip alternate horizontal lines on the first go at "painting" a screen, then return and fill in those skipped lines on the second go. At low refresh rates, this is a trick that is useful, but leaves a detectable, residual flicker, causing fatigue and annoyance over long exposures. Most better modern monitors have higher refresh rates AND are non-interlaced.


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