Re: tangent maps

["Tim Leydecker" <BauerOink(at)gmx.de>]

Hey André,

nice proof of the limitations and caveats of Normalmaps you given.

But still, in a hands on approach to solves renderingissues they
are an option that may be favoured over "the other bumpmap nodes".
Actually, I´d love to finally see reflection and specular contribution
calculated base on the modified Normal, too. Would help quite
a bit in getting tesselated geometry reflect nicely or at least look
closer to the "idealized" NURBS surfaces  they could be derrived from...

In your example, many problems are due to a somewhat suboptimal
UVlayout - theroretically - most of those can be worked around by
creating Unique UV´s (per face), e.g. making sure there´s no distortion.

That part needs to be taken care of in general anyway, as UV distortions
pretty much defy painting any type of map anyway in 2D, like Photoshop.

But again, nice detallied description of the pros&cons.

Cheers

tim


----- Original Message ----- 
From: "André Adam" <a_adam(at)49games.de>
To: <XSI(at)Softimage.COM>
Sent: Thursday, January 25, 2007 11:20 AM
Subject: Re: tangent maps


Hi Bernard,

the tangent property in XSI is the so called tangent map, or tangent
space. Encoded in there you can find a given texture space's u-direction
(the reason why you have to reference a texture projection when creating
a tangent property) in 3d space (you could call this the "texture
mapped" u-direction) on a per sample basis, so we're talking of vectors
here. In absence of a more convenient property Softimage decided to
encode the tangents into vertex colors. That means you have a little bit
of math to do to revert them back to vector data: (VC-0.5)*2. This
formula stretches the RGB vertex color values, which have a 0-1 range,
back to a vector's -1 to +1 range.

The reason you need a tangent space when dealing with normal maps is
that a "normal map in tangent space" (there is also an object space flavour)
stores relative information as to where a given normal should be bent,
which after all is what bump mapping is all about (bump mapping and
normal mappinng are practically the same thing). The lila-blueish color
in a tangent space normal map means that the geometry's input normal
should not be altered, where cyan, magenta, etc colours indicated a
shift in normal orientation.
This shift, which is a relative bit of information as it does not point
to global world coordinates, needs a reference on the object itself from
which the relative shift should be calculated - the tangent space. The
tangent space offers a predefined vector for each sample point and is
directly generated from the texture projection the normal map is mapped
with onto the object.

I've written a small utility that creates little pointers from the
tangent space right on the object's surface to better show the vectors
encoded there. I've attached two screenshots which might clearify that a
bit. The first one shows a straight spherical mapping, the second one is
twisted. As soon as a texture space shows distortions (or even worse:
seams), just like the second one, tangent spaces become tricky; the
samples for a single vertex point in different directions, which finally
would create artifacts during normal mapping. To cope with this issue
the tangent operator in XSI has a smoothing value, which basically
middles the different sample's tangent vectors using an angle-based
approach.

Since the normal map stores its data relative to the tangent space,
generation, as done with the ultimapper for example, therefore always
needs to reference it during generation to provide correct results.
Ultimapper basically marries referenced object curvature from a high
resolution source object with the normals of the low resolution source
object and the tangent space of the low resolution object.

The Photoshop plugins and tools provided by graphics hardware vendors
don't have this kind of 3d information available when generating normal
maps from heightmaps. They assume that the object the generated normal
map might get mapped onto shows an ideal tangent space, which usually is
nonsense. The more distorted the texture space on the mapped object is,
the wronger (is that english?!) the bump mapping results with such a
textures are. I'd stay away from these tools for production work.

Hope that makes some sense, cheers!

   -André



Bernard Lebel wrote:

-snip-
> - you need to use a "Tangent" property. If someone could tell me why
> I'd be more than happy. I suspect this is due to the fact that tangent
> normal maps are heavily used in realtime rendering, and the Tangent
> property is a color at vertices property (which also makes it an
> absolute nightmare scripting-wise... why not having a new cluster
> property for this one, please oh God?) so there must be something
> related here.
-snip


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