<br><br><div class="gmail_quote">On Fri, Oct 12, 2012 at 3:20 PM, Trevor Wekel <span dir="ltr"><<a href="mailto:trevor_wekel@otxsystems.com" target="_blank">trevor_wekel@otxsystems.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Hi Frank,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I was not aware of VRTWarpedDataset so did not consider it initially.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">So I took a quick look at vrtwarped.cpp. I have a question. In vrtwarped.cpp, the ProcessBlock() seems to cache the results of the warp operation (GDALCopyWords
around line 1174) into each raster band. Does this mean that all blocks will end up being cached?<u></u><u></u></span></p>
<p class="MsoNormal"><br></p></div></div></blockquote><div>Trevor,</div><div><br></div><div>Yes, VRTWarpedDataset will always push produced tiles into the GDAL block cache. Note that most drivers go through the block cache too.</div>
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-CA" link="blue" vlink="purple"><div><p class="MsoNormal"></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">For FDO, we basically call a ReadNext() to suck the contents of the tile into a buffer supplied by the caller. The ReadNext() drives the calls to _getTile().
As far as I can tell, our access pattern is linear and we only access each tile once. If we switch to using VRTWarpedDataset, will we have to call ProcessBlock() for each tile? Will this end up caching all the blocks as we do the linear read?</span></p>
</div></div></blockquote><div><br></div><div> In a web context, the ideal would be a long lived server which keeps around the VRTWarpedDataset for subsequent use. I not sure how practical that is in the FDO/MapGuide use case. In this situation caching the blocks can help subsequent views but there might be better other layers at which to do that.</div>
<div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-CA" link="blue" vlink="purple"><div><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">As an alternative to using VRTWarpedDataset, would it be ok to instantiate a GDALWarpOperation as a member of the FdoRfpStreamReaderByTileResample? Pseudo
code would be something like this:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas">FdoRfpStreamReaderGdalByTileResample::FdoRfpStreamReaderGdalByTileResample()<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas">{<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas">m_warpOp = new GDALWarpOperation();<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">FdoRfpStreamReaderGdalByTileResample::_getTile()<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">{<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">warpOptions = GDALCreateWarpOptions();<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">// populate warp options<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">m_warpOp->Initialize(warpOptions);<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">m_warpOp->WarpRegion(0,0,origXSize,origYSize);<u></u><u></u></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">This may improve the speed of the _getTile() call if a) Initialize() can be called more than once and b) the new GDALWarpOperation() does a lot of setup work</span></p>
</div></div></blockquote><div><br></div><div>It is more likely to be the Iniitalize() that is expensive and reinitialzing an existing WarpOperation is not a well trod path and likely to lead to pain. I advise against it. In retrospect, sticking with your existing approach is likely ok but you might want to get a sense of how much time is spent in the setup code vs. the actual warping. Larger tiles will help.</div>
<div><br></div><div>Best regards,</div><div><br></div><div> </div></div>-- <br>---------------------------------------+--------------------------------------<br>I set the clouds in motion - turn up | Frank Warmerdam, <a href="mailto:warmerdam@pobox.com" target="_blank">warmerdam@pobox.com</a><br>
light and sound - activate the windows | <a href="http://pobox.com/~warmerdam" target="_blank">http://pobox.com/~warmerdam</a><br>and watch the world go round - Rush | Geospatial Software Developer<br><br>