[Gdal-dev] MrSID dataset

[Kirk McKelvey]

Andy,

The MrSID formats decode natively only at "octave" (integral
power-of-two) zoom-levels, in other words 1:1, 4:1, 16:1, et cetera.  It
looks like the logic in MrSIDDataset::IRasterIO() is doing two things:
  1. deciding which octave to decode (since it may be given a non-octave
zoom-level)
  2. translating the given pixel coords into that octave
Then the octave decode is resampled to fulfill the original zoom-level
requested.

So my question is whether you can take just one or the other of your
changes to fix the problem?  The first fudge-factor (0.1) is
manipulating the octave decision and the second fudge-factor (0.99)
looks like it is manipulating the rounding in the pixel translation.
I'm guessing you can fix your problem with just the second change.

As for whether it is the "right" change, well, someone probably put that
fudge-factor in there because they were seeing black lines of their own,
and if you take it out, theirs will come back.  The right change
involves using the LizardTech classes and methods to do zoom and
coordinate calculations.

I recently began rewriting MrSIDDataset for speed and code correctness,
and I'll make sure to address this issue while I'm in there.  You may
have to wait a few weeks though since this work is competing with other
tasks.

Cheers,
Kirk


-----Original Message-----
From: gdal-dev-bounces at lists.maptools.org
[mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Andy Tompkins
Sent: Wednesday, September 21, 2005 5:17 AM
To: gdal-dev at lists.maptools.org
Subject: [Gdal-dev] MrSID dataset

I have just started using GDAL and I have a question about the MrSID
dataset.

I find that I get a black line drawn down the left and bottom sides of
the image after decoding the file with dataset->RasterIO().  I believe
that I have found a fix for this problem.

change MrSIDDataset::IRasterIO(), line 761 from:
    nZoomMag * 2 < (dfZoomMag + 0.1) && iOverview < nOverviewCount; to
    nZoomMag * 2 < dfZoomMag && iOverview < nOverviewCount;

and change MrSIDDataset::IRasterIO(), lines 772, and 773 from:
    nTmpXSize = (int) floor( nXSize / (double) nZoomMag + 0.99 );
    nTmpYSize = (int) floor( nYSize / (double) nZoomMag + 0.99 ); to
    nTmpXSize = (int) floor( nXSize / (double) nZoomMag );
    nTmpYSize = (int) floor( nYSize / (double) nZoomMag );

This seems to fix the problem.  I guess I am wondering if I have missed
something and/or if there is a better/correct way to fix my problem? 
Also, if
this is a good/correct fix, how does this change get into GDAL?

Thank you in advance,
  Andy Tompkins
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