3.13 Random Dot Stereograms (RDS)
Random Dot Stereograms (RDS) are a way of encoding stereo images on a
flat screen. Fractint can convert any image to a RDS using either the
color number in the current palette or the grayscale value as depth. Try
these steps. Generate a plasma fractal using the 640x480x256 video mode.
When the image on the screen is complete, press <ctrl-s> ("s" for
"Stereo"), and press <Enter> at the "RDS Parameters" screen prompt to
accept the defaults. (More on the parameters in a moment.) The screen
will be converted into a seemingly random collection of colored dots.
Relax your eyes, looking through the screen rather than at the screen
surface. The image will (hopefully) resolve itself into the hills and
valleys of the 3D Plasma fractal.
Because pressing the two-keyed <ctrl-s> gets tiresome after a while, we
have made <k> key a synonym for <ctrl-s> for convenience. Don't get too
attached to <k> though; we reserve the right to reuse it for another
purpose later.
The RDS feature has five and sometimes six parameters. Pressing <ctrl-s>
always takes you to the parameter screen.
The first parameter allows you to control the depth effect. A larger
value (positive or negative) exaggerates the sense of depth. If you make
the depth negative, the high and low areas of the image are reversed. If
your RDS image is streaky try either a lower depth factor or a higher
resolution.
The second parameter indicates the overall width in inches of the image
on your monitor or printout. The default value of 10 inches is roughly
the width of an image on a standard 14" to 16" monitor. This value does
not normally need to be changed for viewing images on standard monitors.
However, if your monitor or image hardcopy is much wider or narrower
than 10 inches (25 cm), and you have trouble seeing the image, enter the
image width in inches. The issue here is that if the widest separation
of left and right pixels is greater than the physical separation of your
eyes, you will not be able to fuse the images. Conversely, a too-small
separation may cause your eyes to hyper-converge (fuse the wrong pixels
together). A larger width value reduces the width between left and right
pixels. You can use the calibration feature to help set the width
parameter - see below. Once you have found a good width setting, you can
place the value in your SSTOOLS.INI file with the command
monitorwidth=<nnn>.
The third parameter allows you to control the method use to extract
depth information from the original image. If your answer "no" at the
"Use Grayscale value for Depth" prompt, then the color number of each
pixel will be used. This value is independent of active color palette.
If you answer "yes" and the prompt, then the depth values are keyed to
the brightness of the color, which will change if you change palettes.
The fourth parameter allows you to set the position of vertical stereo
calibration bars to the middle or the top of the image, or have the bars
initially turned off. Use this feature to help you adjust your eye's
convergence to see the image. You will see two vertical bars on the
screen. You can turn off and on these bars with the <Enter> or <Space>
keys after generating the RDS image. If you save an RDS image by
pressing <s>, if the bars are turned on at the time, they become a
permanent part of the image.
As you relax your eyes and look past the screen, these bars will appear
as four bars. When you adjust your eyes so that the two middle bars
merge into one bar, the 3D image should appear. The bars are set for the
average depth in the area near the bars. They should always be closer
together than the physical separation of your eyes, but not much less
than about 1.5 inches. About 1.75 inches is ideal for many images. The
depth and screen width controls affect the width of the bars.
At the RDS Parameters screen, you can select bars at the middle of the
screen or the top. If you select "none", the bars will initially be off,
but immediately after generation of the image you can still turn on the
bars with <Enter> or <Space> before you press any other keys. If the
initial setting of the calibration bars is "none", then if the bars are
turned on later they will appear in the middle. Hint: if you cycle the
colors and find you can't see the calibration bar, press <Enter> or
<Space> twice, and the bars will turn to a more visible color.
The fifth parameter asks if you want to use an image map GIF file
instead of using random dots. An image map can give your RDS image a
more interesting background texture than the random dots. If you answer
"yes" at the Use image map? prompt, Fractint will present you with a
file selection list of GIF images. Fractint will then go ahead and
transform your original image to RDS using the selected image map to
provide the "random" dots.
After you have selected an image map file, the next time you reach the
RDS Parameters screen you will see an additional prompt asking if you
want to use the same image map file again. Answering "yes" avoids the
file selection menu.
The best images to use as image maps are detailed textures with no solid
spots. The default type=circle fractal works well, as do the barnsley
fractals if you zoom in a little way. If the image map is smaller than
your RDS image, the image map will repeated to fill the space. If the
image map is larger, just the upper left corner of the image map will be
used.
The original image you are using for your stereogram is saved, so if you
want to modify the stereogram parameters and try again, just press
<ctrl-s> (or <k>) to get the parameter screen, changes the parameters,
and press <Enter>. The original image is restored and an RDS transform
with the revised parameters is performed. If you press <s> when viewing
an RDS image, after the RDS image is saved, the original is restored.
Try the RDS feature with continuous potential Mandelbrots as well as
plasma fractals.
For a summary of keystrokes in RDS mode, see RDS Commands (p. 39)