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.


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This page maintained by

Noel Giffin,
noel@triumf.ca