Re: [Algorithms] negative UV coords

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

True, but the original post was a very simple question, so I thought it
deserved a simple answer. :)


> In this one specific case, where the U or V has been negated (but not
> both), and the texture addressing mode is set to wrap, yes the texture
> is mirrored.  However, I suggest that it is a poor mental model,
> thinking about stepping as a property of certain coordinate regions, as
> if the coordinate system defines directionality of a mapping--it does not.
>
> Flow is a function of the geometry triangle, UV triangle, and the
> texture addressing modes--not the property of any one component in
> isolation...
>
> After fighting to generate really solid tangents (which depends on all
> these things), I began to appreciate the way it all works together.  :-)
>
> Thanks,
> JH
>
> Tony Ball wrote:
>> Another way of thinking about it that might help some people is to realize
>> that only the fractional part of the UV coordinates are used to look up the
>> texture map. So going stepping from -1 to 0 the fractional component is
>> moving
>> the opposite direction to when stepping from 0 to 1.
>>
>>
>>
>>> Assuming you don't have texture clamping turned on, negating uv's will
>>> cause them to be mirrored along the negated axis.  If you think about
>>> it, by negating u, you will be walking backwards along the u axis, which
>>> means those texels will be encountered in reverse order, which is the
>>> order they will be mapped onto the triangle.
>>>
>>>
>>> Tom Plunket wrote:
>>>
>>>>> 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?
>>>>>
>>>>>
>>>> Based on the variety of answers you're getting, I'd suggest that it
>>>> depends how it's implemented.
>>>>
>>>> Flipping when going negative seems bizarre to me, and I'm trying to
>>>> remember what happened on the PS2 (which is the last platform where
>>>> I even thought about it), however it had enough challenges with
>>>> tiling in the first place that there had to be significant magic in
>>>> the data conditioner to make the data look like something that the
>>>> machine could handle at all.
>>>>
>>>> -tom!
>>>>
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>>
>>
>>
>>
>>
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