RGB Tristimulus Graph: Negative Red Values Explained

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Hey guys,

I'm just trying to get my head around the RGB tristimulus graph, as follows:

[URL]http://cnx.org/content/m18092/latest/trist.png[/URL]

Why are there negative values for red? What does it mean?

I'd appreciate it if someone could help. And no, this isn't a homework question, it's part of my Computer Graphics course notes, and I'm having trouble understanding this particular concept.

Thank you!
 
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Refer to a http://www.tftcentral.co.uk/images/gamut/ciechart.jpg" .
'Pure' wavelengths are the convex edge of the diagram. So the tristimulus values (435.8nm, 546nm, 700nm) are three points on this edge, and the colors they can represent are the points inside that triangle. Not every color is inside it, which is why you can have negative values.

Note that the chart itself isn't really accurate for this reason. And your computer monitor only displays an even smaller gamut (see e.g. http://jkphoto.net/attach/1/1292499025.jpg" image showing some common color spaces)
 
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