Re: [Algorithms] negative UV coords

[Jason H. <jas...@di...>]

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!
>>>
>>> -------------------------------------------------------------------------
>>> This SF.Net email is sponsored by the Moblin Your Move Developer's challenge
>>> Build the coolest Linux based applications with Moblin SDK & win great
>>> prizes
>>> Grand prize is a trip for two to an Open Source event anywhere in the world
>>> http://moblin-contest.org/redirect.php?banner_id=100&url=/
>>> _______________________________________________
>>> GDAlgorithms-list mailing list
>>> GDA...@li...
>>> https://lists.sourceforge.net/lists/listinfo/gdalgorithms-list
>>> Archives:
>>> http://sourceforge.net/mailarchive/forum.php?forum_name=gdalgorithms-list
>>>
>>>
>>>       
>> -------------------------------------------------------------------------
>> This SF.Net email is sponsored by the Moblin Your Move Developer's challenge
>> Build the coolest Linux based applications with Moblin SDK & win great prizes
>> Grand prize is a trip for two to an Open Source event anywhere in the world
>> http://moblin-contest.org/redirect.php?banner_id=100&url=/
>> _______________________________________________
>> GDAlgorithms-list mailing list
>> GDA...@li...
>> https://lists.sourceforge.net/lists/listinfo/gdalgorithms-list
>> Archives:
>> http://sourceforge.net/mailarchive/forum.php?forum_name=gdalgorithms-list
>>
>>
>>     
>
>
>
>
>
> This message and its attachments may contain legally privileged or confidential information. This message is intended for the use of the individual or entity to which it is addressed. If you are not the addressee indicated in this message, or the employee or agent responsible for delivering the message to the intended recipient, you may not copy or deliver this message or its attachments to anyone. Rather, you should permanently delete this message and its attachments and kindly notify the sender by reply e-mail. Any content of this message and its attachments, which does not relate to the official business of the sending company must be taken not to have been sent or endorsed by the sending company or any of its related entities. No warranty is made that the e-mail or attachment(s) are free from computer virus or other defect.
>
>
> -------------------------------------------------------------------------
> This SF.Net email is sponsored by the Moblin Your Move Developer's challenge
> Build the coolest Linux based applications with Moblin SDK & win great prizes
> Grand prize is a trip for two to an Open Source event anywhere in the world
> http://moblin-contest.org/redirect.php?banner_id=100&url=/
> _______________________________________________
> GDAlgorithms-list mailing list
> GDA...@li...
> https://lists.sourceforge.net/lists/listinfo/gdalgorithms-list
> Archives:
> http://sourceforge.net/mailarchive/forum.php?forum_name=gdalgorithms-list
>
>