Gilles Tran wrote:

> raytracing, here's a link to an engraving describing this very, very early 

> on the Art of Measurement).
> http://www.columbia.edu/cu/lweb/eresources/exhibitions/treasures/html/55.html
> This page doesn't explain much in English, but I have a translation in a 
> book (that I don't have here right now). IIRC, the string is attached from 
> the painted object to the wall, and the intersections between the string and 
> the screen are recorded on paper, forming a shape that the artist uses as 
> basis for the final painting or drawing. I don't remember exactly how the 
> "pixel" is recorded, but I think it's by using some sort of cross-hair 
> device attached to the frame (what the guy on the right is doing). I'll 
> check later.

Note this is more 'scanline rendering' than raytracing.  For raytracing 
you would position the 'string' to one pixel of the 'image' after the 
other and look into the 'scene' where it points at.  Here the string is 
positioned in the scene and from there you look where it intersects the 
image.  An actual 'scanlining' of course does not take place.

Still a nice examle of 'early CG'.

BTW there is a true early raytracing example (although quite 
unspectacular) in the work of Leonardo da Vinci:

http://www-public.tu-bs.de:8080/%7Ey0013390/files/leonardo.png

(excuse the bad quality - photographed from the paperback version of "La 
scoperta dell'ombra" - the discovery of the shadow by Roberto Casati)

It illustrates that shadows comply with the rules of ray optics and that 
by mentally substituting the light source and the eye/camera you can 
simplify a lot of things (which is the essence of raytracing).

Christoph

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