Um, yes... but you won't get the same result...

OK, I think I should stop bothering everyone after this... But I just want
to prove a little point first:

You CANNOT get the same effect of converting and image to greyscale later by
colouring the objects in your scene with greyscale equivalents.  It doesn't
work.

The reason is simple and I won't go on about it after this :)

If you plan to have only white lights in your scene, fine, it doesn't make
ANY difference (except for reflections) if you convert a colour image to
greyscale, or if you build your scene with the greyscale equivalents.
However, if you are going to use coloured lights, things get tricky.  The
reason it won't work properly is because different colours react in
different ways on other colours.  For example, the highlight or specular on
an object will look like it has a different exposure.  A good example is to
place a yellow sphere with a red light on one side and a blue light on the
other.  If you examine the highlights you'll find that the one produced by
the red light will be exposed more than the one from the blue light.  Now if
you were to convert your scene's colours to greyscale before rendering, this
effect isn't produced because you now only have intensities, so all the
highlights will have a linear progression of magnitude.  This means that you
can no longer produce scenes with realistic lighting (or reflections), and
that's why it is VERY hard to get the right contrast in a B&W scene if
you're going to make it by using intensities.  See?  So, ultimately, Peter's
solutions is the ONLY REAL solution to the problem, because that way you are
ACTUALLY rendering the colour, but it is being converted to greyscale later
(well, not later, still in POV-Ray) using the HF16G option, and then
converted to RGB still using POV-Ray by making a height_field from the
resulting PNG image and then colouring the height_field with a gradient from
black to white.

(Did anyone understand my point?)
OK, I'll shut up now...

--
Lance.


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