In article <UcQROZw2W1joV6Ajm0U7uS80aIp5@4ax.com>, Glen Berry 
<7no### [at] ezwvcom> wrote:

> For what it's worth, here is one example of an S-curve:
> 
>     Output = 0.5-(cos(Input*pi)/2)
> 
> For this to work, both input and output need to be constrained between
> 0 and 1. It should work with other ranges of values with appropriate
> scaling. The angular unit is expressed in radians, in this example.
> 
> This generates an S-Curve, but I'm not sure of an easy way to add
> additional parameters and make it more flexible, without losing the
> "S" shape.

This is certainly "a" S-curve, but I wanted to know if there is a 
formula for one which is considered the "standard", official or not...a 
possibly better solution would be a polynomial curve, or something 
similar to what I posted in .binaries.images a while back, where you 
could change it from an S-curve to a straight line to a 
backwards-S-curve, by modifying the exponent. This also makes a S shape, 
and the version I posted works with "out of range" values.


> This curve is also very similar to a generic representation of the
> response curve of photgraphic film. It would need some modification to
> work as a film response emulator, a subject I raised earlier.

This sounds like it would best be done with the "curves" filter I am 
planning, which takes a 3D spline or a color_map for the curve...

-- 
Christopher James Huff - Personal e-mail: chr### [at] yahoocom
TAG(Technical Assistance Group) e-mail: chr### [at] tagpovrayorg
Personal Web page: http://chrishuff.dhs.org/
TAG Web page: http://tag.povray.org/

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