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#110 grid cracking: Black spots / sampling errors

non_fatal
open
nobody
9
2012-09-20
2004-05-01
Timothy M. Shead
No

Enclosed is a simple test case for the "black spots"
sampling errors that have been reported off-and-on.
The world.rib file contains a single NuPatch arranged
to create a disk, while sample_buster.sl is an imager
shader that does a straight color assignment to any
pixel with an alpha < 1. By rendering geometry with
the constant shader and a contrasting color, any
sampling errors or cracks in the geometry show-up as
isolated pixels - normally, these types of errors would
tend to be masked by filtering and oversampling.

Cheers,
Tim

Discussion

  • Logged In: YES
    user_id=73804

    Additional observations:

    Higher pixel sample rates produce more discontinuities,
    presumably more samples means more opportunity for error,
    although these errors would tend to be hidden better without
    the sample_buster shader.

    Now that I know what I'm looking for, I've verified that
    these discontinuities exist in all primitive types including
    conics, polygons, SDS, and NURB patches, although polygons
    have noticeably fewer problems than other types.

    The distribution of discontinuities is strongly
    view-dependent. In a minority of cases I stumbled upon
    distributions that appeared to be UV-aligned, although I
    haven't been able to reproduce them.

    Amusingly, 3delight seems to have this same problem, but
    BMRT does not.

    Cheers,
    Tim

     
  • Chris Foster
    Chris Foster
    2008-02-18

    Logged In: YES
    user_id=1086736
    Originator: NO

    Bug #1525403: "cracks in SubdivisionMesh" is a duplicate of this bug - see there for extra test cases.

     


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