[argyllcms] Re: Apply transmission / absorption spectrum to image

  • From: Richard Kirk <richard@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 1 Sep 2014 20:58:33 +0100

On 1 Sep 2014, at 17:41, Martin Lederhilger wrote:

> does anybody know if there is a tool to apply a transmission spectrum
> to an image (eg. sRGB -> XYZ -> spectrum -> apply transmission spectrum
> -> convert back for each pixel in the image). I would like to calculate
> colour correctly, how an image would look like, if looked through float
> glass (green).

Hi, Martin.

I have done a lot of this sort of thing. There is not a unique answer, and  
then you can get some unexpected effects: if you look at tungsten light through 
cobalt blue glass and amber glass, the two combined look red. If you are 
wanting to see how a broad spectrum scene would look thorough a particular 
green glass.  The best I have done is to calculate the smoothest spectra for 
each real (x,y) colour and transform that. This still will not necessarily fit 
your particular spectrum, but unless you are using something mad like yttrium 
glass, most reflection spectra are pretty smooth. I can then generate a 
transform list from these values, and interpolate a cube from the list values. 
But I cannot produce a formula.

If you need an approximate formula This is a bit specialized. If you want 
something that just gives the right effect, you could take your RGB channels 
and yellow and cyan bands between R&G and between G&B. If you have dense 
combinations of CMY dyes, the transmission often peaks in these two bands, so 
they can have a significant effect.

Richard Kirk
FilmLight Ltd, Artists House, 14-15 Manette Street, London W1D 4AP
Tel: +44 (0)20 7292 0400  Fax: +44 (0)20 7292 0401

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