Re: flickering with zbump

["adrian" <adrian.wyer(at)fluid-pictures.com>]

errr yeah, what Halfdan said.....

a

Adrian Wyer
Fluid Pictures
33 Glasshouse Street
London
W1B 5DG
----- Original Message ----- 
From: "Bernard Lebel" <3dbernard(at)gmail.com>
To: <XSI(at)Softimage.COM>
Sent: Wednesday, August 09, 2006 2:58 PM
Subject: Re: flickering with zbump


> Halfdan actually answered that question on the mr list:
>
>
> "It's a combination of mental ray, the shader libraries and floating
> point inaccuracy.
>
> A shader returns NaN (not a number) value which overwhelms any other
> value in the array of samples which are filtered down to a pixel. If
> you increase your filter size, for example, the black spots get
> bigger.
>
> When a NaN is converted to 8- or 16-bit value, the conversion function
> returns zero, which is why you see black spots. If you save the image
> out as float, the NaN values will be saved out with it.
>
> This is usually caused by a shader passing a negative value to sqrt().
> A simple example:
>
>  nd = mi_vector_dot( &state->dir, &state->normal );
>  val = sqrt( 1.0f - nd );
>
> Under some circumstances, even if both state->normal and state->dir
> are a supposedly unit vectors, the dot product can give a value
> slightly larger than 1.0, due to floating point inaccuracies. This
> then gets inverted and passed as a very small negative number to
> sqrt() which then returns a NaN. This NaN value then proceeds to
> contaminate any and all calculations it is involved with downstream.
>
> http://en.wikipedia.org/wiki/NaN
>
> These kinds of errors are very often highly non-trivial to uncover. My
> recommendation is to talk to your vendor. The more examples they get,
> the easier it is to track these things down."
>
>
>
> Cheers
> Bernard
>
>
>
> On 8/9/06, Kris Rivel <kris(at)krisrivel.com> wrote:
> >  What's with those damn black artifacts?  Can anyone at Soft give a clear
> > explanation of the problem and maybe put as at ease and tell us it will 
> > be
> > fixed in the next release.  It happens way too often and there doesn't 
> > seem
> > to be a clear repro for it.  I know I've submitted some nasty scene files 
> > to
> > illustrate it.  The problem is that its somewhat random but seems to 
> > happen
> > often when using OpenEXR, HDRI or shaders like zbump or dark tree.  I had 
> > it
> > pop up again on a job where I was using an OpenEXR image as an 
> > environment
> > for reflections.  I got lots of the little buggers so I converted it to 
> > HDRI
> > and it worked fine.  It just happens a bit too often I think.  If its an 
> > MR
> > thing, we need to bash their heads in a bit to address the problem.
> >
> >  Kris
> >
> >  kim aldis wrote:
> >  Didn't the new version fix that?
> >
> >
> >
> >  -----Original Message-----
> > From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On
> > Behalf Of adrian
> > Sent: 09 August 2006 12:20
> > To: XSI(at)Softimage.COM
> > Subject: Re: flickering with zbump
> >
> > we had issues with this, but it manifested itself as 2x2 or 4x4 pixel
> > black squares.....
> >
> > resolved by lowering the bump factor, or some convoluted rendertree
> > chicanery to stop zbump returning NAN values
> >
> > not much help i know
> >
> > a
> >
> > Adrian Wyer
> > Fluid Pictures
> > 33 Glasshouse Street
> > London
> > W1B 5DG
> > ----- Original Message -----
> > From: "Chris Marshall" <chris(at)eclipsecreative.co.uk>
> > To: <XSI(at)Softimage.COM>
> > Sent: Wednesday, August 09, 2006 12:22 PM
> > Subject: flickering with zbump
> >
> >
> >
> >
> >  Hi All,
> > Does anyone get flickering problems when using zbump?
> > I've seen it on two jobs now, and it's so bad we've had to go back to
> > regular bumpmapping, which isn't ideal.
> > It happens when you have an object breaking through the surface of
> >
> >  the one
> >
> >
> >  using zbump. The flickering occurs around the area where the one
> >
> >  object
> >
> >
> >  penetrates the other.
> > Anyone seen this or can offer some advice on fixing it?
> >
> > Thanks
> >
> > Chris
> >
> >
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