[Gdal-dev] Re: MrSID dataset

[Andy Tompkins]

> 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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Kirk,

Thank you for replying with such good information.  You are correct,
just 
changing the second fudge-factor (0.99 - for the rounding in the pixel 
translation) fixes my problem.  I look forward you your rewrite of the 
MrSIDDataset!

Thanks again,
  Andy.