edubuntu-bugs team mailing list archive

[Mark Harrison <648379@xxxxxxxxxxxxxxxxxx>]

Given the symptoms:

1) Occurs during resizing using factors that are not powers of 2 (i.e., 2, 4, 8, etc.).  Comment #2
2) The thatched pattern is the color (254, 254, 254), where pure white is (255, 255, 255).  Attachment to Comment #1
3) Only occurs for pure white (255, 255, 255) starting image. Comment #3

I would strongly suspect that this is not a bug, but a rounding error
inherent in filter.  It just happens that only images starting from pure
white end up with rounding errors large enough to change a pixel value
by one.  The Lanczos filter actually takes on negative values at places,
so that might be enough to put a resampled pixel one value below where
it should be when the scaling isn't a nice power of 2.

In other words, this is a limitation of the filter, not a programming
bug.

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https://bugs.launchpad.net/bugs/648379

Title:
  Artifacts when scaling down with Sinc Lanczos3

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