Re: [plib-devel] ASE load/save

[Steve B. <sjb...@ai...>]

Dave McClurg wrote:

> ASE supports vertex colors and ssgLoadASE passes them to ssgVtxTable
> if they are found.  But vertex colors are optional and may not be present.
> 
> So... is a fix needed and if so what should be done?

Well, it goes back to the various ways to represent colours in OpenGL
(and hence in SSG).

A quick review:

1) You can glDisable(GL_LIGHTING) and whatever glColor you put on
   the vertex will apply - anything put in glMaterial (or
   ssgSimpleState) will be ignored.

2) You can glEnable(GL_LIGHTING) and glDisable(GL_COLOR_MATERIAL)
   and whatever colours you put into the glMaterial will apply -
   anything put in glColor will be ignored.

3) You can glEnable(GL_LIGHTING) and glEnable(GL_COLOR_MATERIAL)
   and set glColorMaterial() to diffuse, ambient, specular, emission
   OR both diffuse and ambient - in which case glMaterial will be
   used for everything else and glColor will set the component named
   in glColorMaterial.

The last of these is the one *I* use pretty much all the time...but
that's not a certainty for all applications.

Now, to add to the complications, SSG uses LAZY state changes.
That means that if for some reason a particular state doesn't
care what setting it has for a particular OpenGL operation - then
ssgState won't bother to set it (or even initialise the variable).

Hence, if (for example) GL_LIGHTING is disabled, then we won't
bother to set the material colours.

So, I guess what your material output routine has to do is to
check if GL_LIGHTING is enabled for this state - and not emit
an ASE material lighting setup if it's glDisabled.

Similarly, if glColorMaterial is enabled and set to (say)
GL_SPECULAR - then there isn't going to be a valid specular
colour in the ssgState - and the ASE material can have any
old junk in it.

Since you told me that ASE has both material colours and
per-vertex colours, it must have a similar set of rules
to OpenGL.

This is all awfully familiar.  There is a complicated set
of OpenGL colour rules - and no way to represent them in
some file formats.  That makes loaders possible - but
writers almost impossible to get right.  So you do the
best you can and document the limitations.

-- 
Steve Baker                  http://web2.airmail.net/sjbaker1
sjb...@ai... (home)  http://www.woodsoup.org/~sbaker
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