All you have to do is: unwrap the model not smoothed, then smooth it (for uvw purposes only), export uvw snapshot smoothed to know exactly how your mapping will be after smoothing, and use it over non-smoothed model.
EDIT: after reading again, what is “smoothed” in your meaning? I thought you’re talking about subdivision for precise texture painting.
If your painting on an object which is going to be subdivided at render time - either as a subd surface or via a polysmooth - then in order to correctly paint textures for that object then you do need to subdivide the object according to the subdivision rules used by your 3D app. How much you subdivide it depends on the type of geometry and it’s limit surface.
There are a few common rules used for subdividing UVs…
[li]Smooth All - All the UVs are smoothed. Works well although smooths off the corners of UV shells which results in UVs which don’t match the subdivided geometry.[/li][li]Smooth All, Hard Corners - The default used by Pixar’s Renderman and 3Delight. Smooths All UVs except for border CV and UVs with a valence of 2 (valence = number of connected edges). Limited support for this method - currently only Maya2011 and Cyslice can subdivide geometry using this rule. Personally this is what I always try to use as it results in the least amount of texture distortion and artefacting (even when painting solely in 2D) - although the hard corners thing can trip up modellers who aren’t aware of it.[/li][li]Smooth Internals, Hard Borders - Smooths the internal UVs and keeps the border UVs hard. Can result in overlapping UVs and heavy distortion on border edges. Used by Mudbox 2011 and ZBrush, now supported by Renderman sigh. I’m not a big fan of this method.[/li][li]Linear - Will result in distortion and artefacts, you can kind of get away with using this on organic forms where the faces don’t vary too much in size.You can also get away with this if you subdivide the geometry with smooth UVs on once and then flattening out the UVs again - although doing this negates one of the benefits (less geometry) of using SubD surfaces in the first place.[/li][/ul]
This is a pet hate of mine in 3ds max which i have been on at the developers about for ever. When you apply a turbosmooth on top and an unwrap the distortion that results across open UV edges is immensely annoying. Especially noticeable around fillets on technical models. Its not so prevelant on organic shapes. You can get around this with a really long winded UV hack which involves copying the UV’s from the same model but with the edges being broken along the UV seems, however its less than ideal.
In short i would always work on smoothed UV’s if i could.
It can be a rather complex topic. If it was for characters you were asking the wrong choices can pose some big problems.
Example, you have a low res base mesh that you subdivid at render time, without smoothing uv’s.
In your texturing app, when you paint an arc shape (like an eyebrow), you would have to intentionally paint your arc shape as a series of straight lines following the shape of your low res mesh. Otherwise when you render the final output, you would experience that your lines gets distorted. So in this case, I think you want smooth uv. However only internal borders (this is possible as an option in maya at least)
With normal maps, and even more importantly with displacement maps. You need to be very careful that all your softwares and methods work with the settings of your final render. I know that pixologic for example have a series of tutorials for a lot of different applications working with zbrush. Even if you were not using zbrush, these tutorials still provide a good insight on how your software works.
@ashrafazlan = care to elaborate why you feel so strongly about non-smoothing? I’m leaning toward that as well but I haven’t devoted enough time to studying the problem to fully know which way I want to commit to. Perhaps it’s not an either or question, however.
@hakanpersson = It’s a good point, though with the plethora of 3d painting capabilities that we have these days, shouldn’t that be a moot issue? Perhaps that’s the variable that determines which is the right answer here. If you employ 3d painting techniques then non-smoothing is fine, whereas if you only paint your textures in 2D, smoothing is the way to go.
Painting in 3D isn’t going to help you - your still going to suffer from distortion. Perhaps a visual example will help…
This was done in 3D (the same problems would exist if I had painted it in 2D as well) - The red was painted on the original geometry and the green was painted on the non-smoothed version. You’ll notice the difference between the original geometry and the smoothed-uv version is much less than the non-smoothed version - you could probably get away with this in production, but you’d never get away with the type of distortion seen on the non-smoothed version.
You don’t even need to do this as the options for Do Not Smooth/Smooth Internal/Smooth All are only used when Smooth UVs is on.
If you want a proper texturing workflow for SubD surfaces (or polysmoothed geometry) then you do need to paint on an object which has been subdivided using the same rules as those used by the renderer.