> > On closer inspection, neither do I. Is the Chrominance the RGB and the
> > Luminance the Diffuse?
> 
> That's what I think it is.
> 
> > What about the other numbers?
> 
> I think it's the "luminance" broken down by wavelength (400 nm being the
> wavelength of red light)
> 
> Could someone who understands this enlighten us (no pun intended)?

To me it looks like the Chromaticity values are the CIE x, y and z
tristimuli values, and Luminance is the CIE Y value.
To convert these values to RGB, I think you have to multiply each of the
chromaticity values by the luminance value.

The long list of numbers are the reflection values for each wavelength.

I don't think any of these values has anything to do with "diffuse
coefficients".

/Ib

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> To me it looks like the Chromaticity values are the CIE x, y and z
> tristimuli values, and Luminance is the CIE Y value.
> To convert these values to RGB, I think you have to multiply each of the
> chromaticity values by the luminance value.

I tried that and ended up with some very very weird colors. Leaving them
as-is works fine.

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> 
> Francois Labreque wrote:
> >
> > I think it's the "luminance" broken down by wavelength (400 nm being the
> > wavelength of red light)
> >
> 
> Sorry, it's the otherway round: red light is about 700 nm,
> bluelight at about 400 nm,
> bigger than 700 nm are infra-red (then the whole radio thing)
> smaller than 400 nm are ultraviolet (sp) then X-ray and so.

I knew that.

Another very fine example of why I shouldn't post before getting my
morning coffee.

-- 
Francois Labreque | And a four year old carelessly banging on a toy
    flabreque     | piano is not only 'music', it's probably the last
        @         | moment of 'artistic purity' they'll ever enjoy
   videotron.ca   | before outside influences start corrupting their
                  | expression.    - Chris R.

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"Tony[B]" wrote:
> 
> > To me it looks like the Chromaticity values are the CIE x, y and z
> > tristimuli values, and Luminance is the CIE Y value.
> > To convert these values to RGB, I think you have to multiply each of the
> > chromaticity values by the luminance value.
> 
> I tried that and ended up with some very very weird colors. Leaving them
> as-is works fine.

You are right, multiplying by Y yields too dark colours, but on the
other hand, using the xyz-value gives too little difference between the
colours to justify names like Light Blue, Dark Blue or Strong Blue
(which are the colours I have used for my tests).

I have reviewed the CIE formula, and come to the conclusion, that it is
better to convert back from xyz to XYZ values and use them as RGB. That
is done by multiplying x and z by Y divided by y: X = x * (Y/y), Z = z
*(Y/y). Y we already have as the luminance. 

/Ib

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> I have reviewed the CIE formula, and come to the conclusion, that it is
> better to convert back from xyz to XYZ values and use them as RGB. That
> is done by multiplying x and z by Y divided by y: X = x * (Y/y), Z = z
> *(Y/y). Y we already have as the luminance.

Actually, right now, I'm dividing each chromaticity value by the largest
value of the triplet, and using luminance as diffuse. It works as expected,
most of the time. Some colors are a bit dark.

Post a reply to this message

"Tony[B]" wrote:
> 
> > I have reviewed the CIE formula, and come to the conclusion, that it is
> > better to convert back from xyz to XYZ values and use them as RGB. That
> > is done by multiplying x and z by Y divided by y: X = x * (Y/y), Z = z
> > *(Y/y). Y we already have as the luminance.
> 
> Actually, right now, I'm dividing each chromaticity value by the largest
> value of the triplet, and using luminance as diffuse. It works as expected,
> most of the time. Some colors are a bit dark.

That brings the question to mind: how do you know what to expect?

Anyway, I have tried your technique, and compared it to my suggestion,
and we get very similar colours. There is however a subtle difference in
the shading of the spheres, that I used for my test. I think that is
because you change the ratio between the amount of diffuse and ambient
light, that is reflected from the surface, and that changes the
difference between highlights and shadows along with the apparent
colour. I prefer to control the colour with the rgb-values, and leave
ambient and diffuse alone.

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> That brings the question to mind: how do you know what to expect?

Because I have a rough idea what the materials look like... you need to get
out more. ;)

> Anyway, I have tried your technique, and compared it to my suggestion,
> and we get very similar colours.

Excelent.

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