Path: utzoo!utgpu!news-server.csri.toronto.edu!dgp.toronto.edu!mccool
Newsgroups: comp.graphics
From: mccool@dgp.toronto.edu (Michael McCool)
Subject: Re: Generating photographic grain imperfection in computer graphics?
Message-ID: <1991Apr12.120123.9763@jarvis.csri.toronto.edu>
References: <5644@media-lab.MEDIA.MIT.EDU>
Distribution: usa
Date: 12 Apr 91 16:01:23 GMT
Lines: 47

ereidell@media-lab.MEDIA.MIT.EDU (Evan A. Reidell) writes:

>can anyone "in the know" think of good SIGGRAPH papers that do their
>best to describe why film stock looks the way it does, and how that
>might be synthesized as a filter through which CGI rendering takes place?

Don't know of any Siggraph papers on the subject, but I think a good
start would be:

	1) Simulation of Grain Noise
	2) Simulation of Logarithmic Intensity Response.

Film grain is Poisson, wheras stochastic ray-tracing uses jittered
or Poisson-disk sampling, which have different characteristics.  In
particular, they introduce less low-frequency noise, and hence let
the Monte-Carlo integration technique converge with fewer samples.
To get the "photographic" look, you need to sample completely at
random.  You could probably do it by randomly sampling an image,
but make sure you interpolate between pixel values.  You should
use a different pattern for each frame.  Then average the samples
down.  I'm not sure off the top of my head if you can just add noise,
it may be image intensity-dependent.  There's probably some way to
figure this out and just add the noise without having to 
simulate the resampling.    The apparent amplitude of the 
noise interacts with the intensity response.

Film also has a logarithmic intensity response, and you would
have to simulate the correct "gamma" for film.  This is fairly
straightforward, just map the colours.  Most image processing
books talk about film response and then ignore it for the rest
of the book.  I think Wintz has a discussion of film response.

Also see Barrett and Swindell, "Radiological Imaging", for a 
really good discussion of this topic.  In general, I think the 
curve for film bends *opposite* TV monitor response, at least
for negative film.  This reference talks about both noise and
response.

Other effects are possible with film, such as "Solarization"... like
in (too many) music videos (Eurythmics comes to mind).  Flash some
light on the film just as it finishes developing, and you get
and edge-enhancement effect, and washing out of colour.  This 
should be pretty easy to simulate with a little image processing.

In the shadow of the sun,
Michael McCool@dgp.toronto.edu,
Dynamic Graphics Project, University of Toronto.