Hi David, and thanks for the great explanation! It has been very helpful
I will try to modify or instruct the import plugins of the file
formats I need forcing the association between the maps and the
texture unit meaning then.
Thanks again,
Davide
-- Forwarded message --
From: David Spilling david.spill...@gmail.com
To: OpenSceneGraph Users osg-users@lists.openscenegraph.org
Date: Tue, 12 Jan 2010 12:35:34 +
Subject: Re: [osg-users] 3d file formats supporting bump map
Hi Davide,
Certainly the obj loader supports bump maps, but let me be clear about what
it actually does. Apologies if this comes over as patronising; I have no idea
of your level of expertise.
In OSG/OpenGL land, textures (diffuse, bump map, specular map, etc.) are all
attached to a particular geometry via a texture ID - this is an integer index
ranging from 0 upwards. Shaders are then applied to this geometry that will
colour the pixel based on a particular understanding of what each texture ID
actually is. So for example, I can write a shader that assumes that texture
unit 0 is the diffuse colour, texture unit 1 is some sort of specularity map,
texture unit 2 is a bump map, etc. Or I can assume (for example as osgShadow
tends to) that texture unit 0 is the diffuser colour, texture unit 1 is the
shadow map.
However - and this is the key - there is nothing that enforces this mapping
between texture unit and meaning.
3DS, and the obj format, both explicitly include entries in the model for
diffuse texture and specular map and bump map and so on. However the
3DS or obj format loader doesn't know what shader you are going to apply to
this model, so has no idea which texture ID/unit to assign a loaded bump
map texture to. Should it be 1? 2? etc.
Some formats (e.g. .osg) allow you to save an objects shader within the
object definition itself, thereby guaranteeing that the textures that the
object loads are assigned to the right texture unit, and used in the right
way. 3DS and .obj don't.
AFAIK, there is currently no OSG-wide method for enforcing any consistency.
All that being said, the OBJ loader allows you to specify what texture units
you want which maps to go to when you load the model, via an options string.
The options are all listed in the first few lines of ReaderWriterOBJ.cpp. The
mapping between the flags in the .obj file and the name of the texture is in
lines 434 onwards of obj.cpp. (It's useful to have a look at that, because
the OBJ spec is not rigourously followed by all modelling tools - e.g. you
see map_opacity in some obj files, even though it should be map_d according
to the spec).
I'm not amazingly familiar with osgFX, but reading the source for the
BumpMapping technique, I notice that
a) you can define which texture units should be used for diffuse and normal
(note : not bump) textures, and
b) by default these are 0=normal, 1=diffuse.
I also don't know what Blender exports in terms of maps. When you say that
the blender export isn't exporting the bump map, have you looked in the OBJ
file to actually see whether there is a map_bump (or map_Bump, or just
bump) in it? Has it been called something else by the exporter?
Hope that helps,
David
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