The key difference is that when you open a raster in v21 it keeps the file open and uses a Windows file handle. Each process only gets about 2000 file handles so if you try to open more than 2000 rasters you run out of handles. When this happens, the engine will probably crash. Also, each raster tile is actually two files - the base level TIFF and the overview PPRC. So you can only open about 1000 tiles like this before the engine runs out of puff.
The old system in v15 (which is still present in v21) opens the raster when it needs some data and then immediately closes it again, so each time you require some raster data there is some overhead involved in opening the raster, verifying the format, loading metadata etc. It is not very efficient but it works for large seamless tables.
GDAL (the open-source raster library used in v21) has a hybrid system where it maintains a pool of file handles and allocates them to rasters on demand. So it can also open a large number of rasters simultaneously - with similar performance implications.
For the best performance, you can use the raster tools to merge all the tiles into a single large raster. The best format to use for this purpose is MRR.
Another alternative is to build a GDAL virtual raster (VRT) that merges all the TIFF tiles. This will be very much like a seamless table.
The only other thing you can do is prevent both the native TIFF driver and the GDAL driver from opening the TIFF files. The TIFF files will then fall through to the old (v15) system. The PPRC files will be ignored. You do this by turning off the TIFF drivers in the backstage settings for the raster engine. The problem with that solution is that it applies to any and all TIFF files you try to display.
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Sam Roberts
Founder, Roberts Geospatial Engineering
Australia
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Original Message:
Sent: 10-27-2022 10:51
From: Nick Hall
Subject: Seamless table v21 Vs v15
Hi,
I have a large seamless table of just over 10,000 tiles. All the ghx and pprc files have been created for the tiffs.
When I open it in v15 I can zoom to the entire layer and, eventually, it will draw the entire map.
When I try and do the same in v21, no matter how long I wait, MapInfo crashes.
I'm not really looking for solutions (as zoom layering works fine) I'm more interested in why? Is it a graphics thing or a memory allocation thing? Am I missing a setting in v21? why would 32 bit be better than 64 bit?
Thanks
Nick
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Nick Hall
Mapchester LTD
Manchester
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