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Performance issues loading JPEG tiles

Matt_ThalesMatt_Thales Global Mapper UserPosts: 4
edited December 2009 in SDK

I've been having performance problems when loading a collection of jpeg tiles. All I'm doing is building a list of filenames and then calling GM_LoadLayerList for each one in turn.

The test map I've been using contains 59 tiles and is taking a half hour to import during which the PC is unresponsive. However if I remove some tiles it only takes 5 mins to import 50 tiles. The problem only appears with jpegs I've had thousands of tiffs and ecws loaded in less time using the same techniques.

I'm using v1.32 of the SDK.

Any ideas?



  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited December 2009

    When a JPG file is loaded the entire thing has to be loaded into memory (or decompressed to a new file for access) as the JPG format does not allow for random access. Other formats, like TIFF and ECW, allow for random access so loading is very fast as just the header has to be loaded and not the entire data file. So loading JPG files will be much slower than the other format that allow for random access.


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  • STHSTH Global Mapper User Posts: 434Trusted User
    edited December 2009
    I also try to work with many JPEGs and I have to say this is the most inconvenient format to work with with regards to speed. I usually end up with creating a MapCatalogue of all the JPEGs and then select the "load" criteria so only a few JPEGs are loaded simultaneously.

    If space is not an issue I would recommend to batch convert to JPEG compressed TIFFs with Overviews. Even though this compresses the images again it is far better to work with.
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