Re: One big texture on many small instances?

["peter boeykens" <peter_b(at)skynet.be>]

> I was talking about the shading on phong, lambert, blinn... shaders, as 
> well as reflection, dirtmap, etc... all of this is evaluated for each 
> instance, however, a texture projection, even if the support is outside the 
> master model, even if it's a spatial projection, is just evaluated once, 
> for the master model.

think of it this way:
a master object is a block of data, with a description of the geometry, 
texture coords, shader,..
an instance is just a null, a set of transforms.
the master block gets translated to mental ray, as well as the collection of 
transforms.
Mental ray then goes and populates the scene, creating duplicates of this 
block of data, using the transforms.
Once done, it all exists as real geometry in mental rays memory - so for 
shading tasks, it shouldnt be too different from regular geometry.
with just one block of data this is a minimal amount of data to handle - for 
scene size, manipulation and transferring to mental ray. Thats the advantage 
of instances: minimal amount of memory needed.
If texture coordinates were unique per instance, the advantage of using them 
would be largely ruled out - uv's represent a lot of data.

texturing methods that dont rely on xsi to give the coordinates to mental 
ray, but rather have mental ray create them on the fly, will work with 
instances and can be used to make them unique.

especially the vector_state set to intersection point is usefull for this.
This can feed most 3d textures (fractals, clouds, veins..)
Instances textured this way will all look unique, and it also works for 
displacement.
"One big texture on many small instances" is exactly what you get this way.
Often I plug large scale textures into gradients to create subtle color 
variations for the instances.





> I'll try what you described though, looks interesting...
>
> Thanks!
>
> Antonin
>
> peter boeykens wrote:
> > > You're right, rendering is as long, but scene interaction is much faster 
> > > using instances. Plus, if the model changes they're much easier to 
> > > update...
> > >
> > > I'll probably use duplicates, however I'm still wondering why texture 
> > > UV's are evaluated only once when everything else is evaluated for each 
> > > instance... Oh well...
> >
> > thats not really true.
> > apart from the transforms almost nothing is evaluated "per instance".
> > true enough, textures that are spatial will be evaluated properly, but 
> > thats not per instance, but rather per sample - the texture itself is one 
> > and the same for all instances, it just looks different at different 
> > points in space.
> >
> > if you use an image implicit and feed that proper coordinates it should 
> > also stretch through all the instances.
> > something like making a floor with tiles and having one big painting on 
> > them?
> > It would be fixed in space though, and if your instances move they would 
> > "swim" through the texture.
> >
> > Making the coordinates for this is not so trivial though.
> > If I remember correctly, you need a vector state set to intersection 
> > point, then plug that into 2 B/W gradients that use the vector x and the 
> > vector z respectively. you can do the placement through the min and max. 
> > those gradients converted to scalars could be fed into a vector again 
> > that would in turn feed the image implicit.
> >
> > A lot of hassle to get it to work and quite limited, since you'd be 
> > restricted to either xy, xz or yz plane.
> >
> >
> >
> >
> >
> > > Thanks for the reply.
> > >
> > > Antonin
> > >
> > > Bernard Lebel wrote:
> > > > Any particular reason why you're using instances? The only situation
> > > > where I think instances are useful is for static, unchanging objects.
> > > > Even then, I now resort more on particle instances than instances.
> > > > They take long to save, and they have to be populated when you open
> > > > the scene.
> > > >
> > > > It is my understanding that render-wise, instances won't save you
> > > > anything as well. Real geometry would allow you to do what you want.
> > > >
> > > >
> > > > Bernard
> > > >
> > > >
> > > >
> > > >
> > > > On 8/2/07, Antonin Messier-Turcotte <amessier(at)flystudio.com> wrote:
> > > >
> > > > > Thanks for the reply... I'm not sure if it would work in my case 
> > > > > though.
> > > > > Seems like the switcher has only 16 inputs, where I've got about 500
> > > > > instances. Plus, my texture doesn't repeat, it's one big texture
> > > > > covering the entire instance formation. So I don't think it would 
> > > > > work,
> > > > > or maybe I'm misunderstanding your idea...?
> > > > >
> > > > > What's weird is that all shaders (for example phong or lambert, or
> > > > > reflection or even dirtmap) will be evaluated for each instance, while
> > > > > texture maps are only evaluated for the master model. Why is that?
> > > > >
> > > > > Antonin
> > > > >
> > > > > Sam Cuttriss wrote:
> > > > >
> > > > > > probably not instances,
> > > > > > but i imagine with the ba colourswitcher shader set to random you
> > > > > > ought to be able to offest the image uv remap parameters.
> > > > > >
> > > > > > just make your uvs the size of one repeat (on your image) then offset
> > > > > > by integers and you should be able to do it cleanly.
> > > > > >
> > > > > > there may be a cleaner way?
> > > > > > _sam
> > > > > >
> > > > > > *Sam Cuttriss
> > > > > > * Janimation 3D Aficionado
> > > > > >
> > > > > >
> > > > > > Antonin Messier-Turcotte wrote:
> > > > > >
> > > > > > > Hi,
> > > > > > >
> > > > > > > Let's say I've got a big object made up of little cubes. Is there 
> > > > > > > any
> > > > > > > way the cubes can be instances, yet they would grab a different part
> > > > > > > of a big texture that is projected on the whole big object? I can't
> > > > > > > seem to find a way to do this, even if the texture support is 
> > > > > > > outside
> > > > > > > the model that was instanced. Shouldn't it be possible?
> > > > > > >
> > > > > > > Thanks!
> > > > > > >
> > > > > > > Antonin
> > > > > > >
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