[photo-3d] misconception of matching photoshop colours to glasses colours

[Andrew Woods <A.Woods@xxxxxxxxxxxxxxxxxx>]

> Anaglyphs have their own complications due to the colors chosen for 
> the pairs.  Not only do you have to match the colors in Photoshop or 
> Paint Shop Pro to the filters in the lenses, but viewing the result
> on a computer monitor or laptop, or LCD projector, or color printer will 
> all render slightly different colors.

The "process" of matching the colors in photoshop to the colours
of the filters in the lenses is a common misconception for computer
viewing applications.

Apart from the fairly rudimentary step of choosing either red, green
or blue for the left or right anaglyph colors to match the glasses
you are using (e.g. Red left eye, Blue/Green right eye) there's
very little color fine tuning you can do at this end to minimise
ghosting.

In computer displays, the full spectrum of colours in the rainbow
are simulated by mixing red, green and blue light.
For a particular display, the spectrum of these red, green
and blue colors is set in manufacture either by the phosphors
(for CRT displays) or the colour filters (for LCD displays).

An example of the color spectrum of a CRT display is illustrated below.

WARNING - ASCII ART - use a monospaced font to view correctly...

intensity
    ^         **       ++        .  
    |        *  *     +  +      . . 
    |       *    *   +    +     .R.      . 
    |      * BLUE * + GREEN+    .E.     . .
    |     *        *       .+.  .D.     . . 
    |    *        + *   ...  +..  ......   . 
    ---------------------------------------------->wavelength (nm)
          |        |         |     |
         400      500       600   650

   Figure 1.  Spectral output of a CRT based computer monitor

So, don't pick a particular orange color cast for your left eye
image to match the glasses you are using because this will this
will just result in a mixture of red, green and blue to achieve
orange and will produce lots of ghosting.

On the contrary, you should be choosing your anaglyph glasses
to match the spectral response of the display device you are using.
Using the illustration given in Figure 1, it would be optimum to
choose a pair of anaglyph glasses which have a red filter which
only passes light in the spectrum range 610-700nm.  And a cyan filter
(mixture of green and blue) filter which passes light in the range
400-590nm.  This combination would result in the least amount
of Red/Cyan anaglyph ghosting for this particular display.

Unfortunately, there are a number of ifs and buts at this point.
1. Glasses manufacturers rarely publish the spectral response of their
   glasses.  (we could help in this regard if anyone's interested).
2. It isn't a simple matter to get the particular filter spectral
   response that you want.  
and...
3. It's not easy for the home user to measure the spectral response
   of their display or their 3D glasses.  For the display you need
   a device called a spectro-radiometer and for the glasses you need
   a device called a spectro-photometer.

So, in summary, don't put too much effort into matching photoshop
colours to your glasses.  Just use the pure RED, BLUE and GREEN channels.
But certainly try using different 3D glasses from different suppliers
to see whether they better match your display.  I've found the IMAX
"we are born of stars" glasses to have a generally good response.

Caviet: This only applies to computer displays.  Printing is a completely
different kettle of fish since it is possible to select the inks that
are used for printing.

Cheers,
Andrew.

p.s. If anyone's interested, we have a spectro-photometer here at the
university which can be used to measure the spectral response of 
colour filters with fine detail.

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