Re: [Algorithms] negative UV coords

[Emil P. <hu...@co...>]

With wrap mode it indeed wraps, rather than flip. The fractional part is
defined x - floor(x), which is always in [0, 1), so frac(-0.3) = 0.7.

I think the confusion in this thread is because some are talking about
negating texture coordinates (which flips) and some are talking about what
happens when the texture coordinate is interpolated from the positive to
negative side of zero (where it's just wrapping, like it does across any
other integer coordinate).

-Emil


-----Original Message-----
From: gda...@li...
[mailto:gda...@li...] On Behalf Of Tom
Plunket
Sent: 29 July 2008 00:45
To: Game Development Algorithms
Subject: Re: [Algorithms] negative UV coords


> 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.

Again, it would seem (to me) to depend on the implementation, since
it depends how you interpret the fractional portion of the values. 
E.g. I seem to remember hearing that different hardware implemented
modulus differently, so -3.5 % 2.0 could yield 1.5 or .5 (or
possibly a negative version of one of those?), depending on your
implementation.  So then the question is one of whether you
interpret -.3 U as .3 or .7.  Can one say with certainty that one
way is wrong and the other right?  I mean, to me if I have the mode
"wrap" set, I sure wouldn't expect it'd implicitly mean "flip."  For
that, I'd choose a "mirror" mode instead.

-tom!

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