From: Angus Dorbie (dorbie++at++sgi.com)
Date: 06/12/2001 17:36:00
I think what was being said is that since both are a description of the
same physical surface property (approximately). The should reflect the
lighting being applied to the scene in equal ammounts. The variation in
appearance should be caused by the lighting description not the material
description. There is no physical parallel in the majority of materials
(especially with a Lambertian approximation) for having a surface
reflect direct illumination differenty from ambient illumination. Sure
you might want to do this for lots of reasons, but the DEFAULTS should
be to reflect both in equal ammounts and have the lights and lighting
model supply the variation in the terms.
It's easy to agree with this argument, but it's just too late, this
convention is there and it's now up to individual applications to
implement a change if they so choose.
Cheers,Angus.
Lawrence bertoldi wrote:
>
> So I believe that by "shading" the defuse and ambient components of a color
> you get a more realistic look. one could argue that they are the same color but
> of different intensity, giving the object a more 3d(ish) quality.
>
> Also the page(197) you quoted did not say that they were by default exactly the
> same color. It said there was an easy method to set them IE:
> "OpenGL provides you with a convenient way of assigning the same value to both
> simultaneously with glMaterial*()"
>
> In fact if you look in the (dare I say it)" The Inventor Mentor" (also ogl based)
> page 117 states that the default "value" for material ambient is .2 .2 .2
> and the default value for diffuse is .8 .8 .8. And if you think about what
> deffuse is, (the base color of an object) one would expect it to be of
> differing intensity then the ambient color (reflected color). The reflected
> color would naturally have a component of the base color but not exactly
> the same value.
>
> See Ya ~;-}>
>
> Lawrence Bertoldi Graphicg Software Engineer
> lberto++at++md.prestige.net
> 410.781.7286
>
> Bram Stolk wrote:
>
> > pfHello,
> >
> > It looks like the default ambient component of a material
> > is not what you would expect.
> >
> > >From the manpage:
> > After createing a pfMaterial, the default colors are:
> >
> > Light Component | Red Green Blue
> > ________________|___________________
> > PFMTL_AMBIENT | 0.2 0.2 0.2
> > PFMTL_DIFFUSE | 0.8 0.8 0.8
> > PFMTL_EMISSION | 0.0 0.0 0.0
> > PFMTL_SPECULAR | 0.0 0.0 0.0
> >
> > More logical would be:
> > ambient and diffuse both to 1,1,1
> >
> > At first, I thought that this was a bug in Performer, but it
> > turns out that OpenGL uses the same defaults, even though
> > in the OpenGL Programming Guide (red-book), it is stated that
> > "Ambient and diffuse reflectances define the color of the
> > material and are typically similar if not identical"
> >
> > This last statement makes a lot of sense:
> > The color of an object when lit with ambient light is highly
> > likely to be exactly the same as the color of the object
> > when it is lit with diffuse light. The red-book gives a good
> > example of a situation when you do want to use different values:
> > if the object is in a room with red walls, the object is likely
> > to have different ambient characteristics.
> > Note that in this case you require knowledge on other objects
> > in the proximity of the object you are specifying.
> >
> > The .2,.2,.2 ambient component that OpenGL and Performer use
> > per default, was probably inspired by the idea that in outdoor
> > scenes you want more diffuse light than ambient light.
> > HOWEVER! this fact should be expressed in the color settings
> > of the lightsources, not in the color settings of the materials.
> >
> > More evidence that the default ambient and default diffuse
> > components of a material should be identical, is on page 197
> > of the redbook:
> >
> > "For real-world objects, diffuse and ambient reflectance are
> > normally the same color. For this reason, OpenGL provides
> > you with a convenient way of assigning the same value to both
> > simultaneously with glMaterial*()"
> >
> > How confusing: why providing this convenience, without having
> > a convenient default in the first place ????
> >
> > A lot of modeling packages and modelers abuse ambient
> > settings, so in projects with a lot of data conversions, we
> > experience quite a few problems with this. To sanatize our
> > models, we try to adhere to the recommendations in the red-book,
> > unfortunately this seems to clash with default pfMaterials, and
> > even with the default OpenGL materials.
> >
> > So, how are other people on this list treating ambient light and
> > the ambient component of material colors?
> >
> > Bram Stolk
> >
> > --
> > ------------------------------------------------------------------------------
> > Bram Stolk, VR Specialist.
> > SARA Academic Computing Services Amsterdam, PO Box 94613, 1090 GP AMSTERDAM
> > email: bram++at++sara.nl Phone +31-20-5923059 Fax +31-20-6683167
> >
> > "I heard if you play the NT-4.0-CD backwards, you get a satanic message."
> > "Thats nothing, if you play it forward, it installs NT-4.0"
> > ------------------------------------------------------------------------------
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