Re: [Algorithms] negative UV coords

[Tony B. <ton...@kr...>]

In other words, yes.


> There's a number of issues involved in mapping a texture to a triangle,
> and given the nature of your question, I have to assume you may not be
> aware of some of them, so I'll try to explain the very fundamental
> components.
>
> The UV coordinates of a vertex are just that... a location in a texture
> mapped from 0..1 in each axis.  Graphics hardware typically have
> different settings to handle what happens when UV values are outside of
> that range.  The most basic are Clamp, Wrap, and Mirror.  If you set it
> to Clamp, any value outside 0..1 becomes either 0 or 1, so you can't
> exceed the given map on a model.  On a typical cube with default UV
> mapping, that means you'll have one face with a texture on it and all
> the other faces will have the perimeter color stretched through them.
> If you set it to Mirror, every odd modulus is frac(U) and every even
> modulus is 1-frac(U), so 1.1 is remapped to 0.9... thus you can
> essentially tile a texture across a plane and have it unfold the texture
> forward and backward forever.  With Wrap set, you get the texture
> repeating as frac(U), so 1.1 is the same as 0.1.  Each axis is
> configured separately.
>
> There's also a texture matrix that can be applied to the UVs, which I
> believe (correct me if I'm wrong) is applied before the
> wrap/clamp/mirror operation, so you can scale or rotate the UVs on a
> model without actually modifying the input data.
>
> Now, what you were asking about is the direction of a mapping.  A
> geometric triangle has a face normal that can be computed by cross
> product, or using the right-hand rule from edges 0-1 to 0-2.  Let's
> pretend the texture is in the same plane as the triangle.  The three UVs
> of the triangle vertices form a triangle in the texture plane.  Since
> it's a triangle, it has a facing as well.  Computing the sign of the
> cross product, if it's positive (the right hand rule of edge 0-1 to 0-2
> leaves you with your thumb pointing the same direction as the triangle's
> face normal), the triangle's texture will be mapped as you see it in the
> texture.  However, if you have a negative sign, or your thumb points
> opposite of the face normal, it will appear to be mirrored/backwards.
> It's not wrong, just the way it works.
>
> One thing is certain, if you are just poking at UV values and trying to
> figure it out, it will seem complicated.  Here's a few things to think
> about.  If you map a triangle, translating the UVs will never cause the
> direction of the mapping to flip (unless you set Mirror mode).  But
> simply negating them will, because that reverses the face normal.
> Negating either the positions or UVs will make the texture appear
> mirrored/backward.  So, if you negate both the vertex positions and UVs,
> you have flipped both triangle normals, and it will appear to be mapped
> correctly, but facing the opposite direction.  :-)
>
> I hope that helps.
>
> JH
>
> skunkwerk wrote:
>> hi,
>>    i'm looking for an explanation of what negative UV coordinates do
>> when using texture mapping.  my initial thought was that negative
>> coordinates are just a continuation in the same U-V plane of the tiled
>> texture, so a value of -1.6 would be equivalent to a positive value of 1.4
>>
>> however, based on tests i did opening a simple 3d model of a plane
>> with positive, and then negative uv coordinates, it seems like the
>> negative sign is actually flipping the texture around the u/v axis.
>> is this correct?
>>
>
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