| Re: Working with color, cursor, and png to extract information [message #58522] |
Mon, 04 February 2008 12:31  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Brian Larsen writes:
> What I want to do:
> - Start with a png image from old research (like
> http://www.dfanning.com/programs/docs/read_toms_aerosol.jpg)
> - read in the png (yes I know the example is a jpg, but it should be
> the same procedure)
> - extract the "value" at each pixel in the image based on the colorbar
The problem is that this image is not a reflection of "values",
it is a reflection of colors used to represent those values.
And those colors are not built into the palette, which as you
see is just strictly gray scale, but into the fabric of the
image itself. This is not the kind of image you can do science
with, I guess is what I am saying.
An image is encoded with a scheme that can represent 16.7 million
colors. A color table typically contains 256 colors. But, *which*
256!? Obviously, there is no one-to-one relationship.
Suppose you do a statistical analysis and find that out of
the 16.7 million possible triples, you only have 16 in your
image, representing 16 colors. You still don't know the foggiest
thing about the *values* those colors represent, unless you
have information you are not sharing with us. That is to say,
the "meaning" of those color triples is not encoded in the image
in any way. Presumably when you look for a "value", you are looking
for a physically meaningful number. There are no such numbers in
a 24-bit image.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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