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18442Re: White (Neutral) Balance in Panoramas

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  • Rik Littlefield
    Mar 4, 2008
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      --- In PanoToolsNG@yahoogroups.com, "William Donelson" <wd@...> wrote:
      > --- In PanoToolsNG@yahoogroups.com, Snowy Aldon <lists@> wrote:
      > > It seems strange that the velvet gives a neutral value when
      > > illuminated to be in the mid-tone. I would have have supposed
      > > that the colour bias of the dyes would become apparent
      > > and introduce a cast.
      > You have to be careful which patch of velvet you use.
      > Some are not black.
      > > Do you colour balance/correct/neutralise to the velvet
      > > and adjust the images to give an equal RGB value grey?
      > > And how does achieving a truly "black" black give any
      > > information regarding the colour balance
      > > of the image?
      > We do not use the box-of-velvet anymore, as our
      > "4 patches of velvet" at different orientations
      > works very well. Some of the patches look blacker
      > than others, and some can be used as gray-level samples.

      Using black velvet this way is a very clever trick. What happens, I
      believe, is that the "gray" appearance is caused by a myriad of
      specular reflections from the surfaces of the fibers. Specular
      reflections are always non-colored, so indeed this method should
      produce a very neutral gray as long as the body of the fibers is very
      dark in comparison.

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