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jpeg tile validity checker

wmiswmwmiswm Global Mapper UserPosts: 220Trusted User
in Technical Support
I encountered an unexpected issue during my mbtiles output. The tiles are in jpeg format. When I browse through the tiles, I noticed that some tiles are invalid jpeg files. I have checked and confirmed that the drive is without any error. The tiles are legitimate files, only that they cannot be read as a jpeg picture. The percentage is not high, around 0.1% - 0.01%. I can of course pick the files up and delete them, and run the output once more to fill the gaps, but I have 3 million tiles in one layer. It will take hours to finish the eyeball checking. Is there any automatic way to do this?

Best Answer

  • bmg_bobbmg_bob Global Mapper Programmer Posts: 2,026
    Answer ✓
    I suggest writing a Global Mapper script to iterate through the folders where the JPEG files are stored, and attempt to load each one. You should get error messages in the output log when the import fails. Be sure to log the name of each file before you attempt to load it, and use UNLOAD_ALL after each import command.

    If you are not familiar with scripting, we have a webinar you can look at to get an introduction.

    When you ran your MBTiles export, did you choose the "Skip Empty Tiles" option? I wonder if these invalid JPEGs are the result of exporting a tile with no data.

Answers

  • bmg_bobbmg_bob Global Mapper Programmer Posts: 2,026
    How did you determine that the JPEG files are invalid? Was a reason given?
  • wmiswmwmiswm Global Mapper User Posts: 220Trusted User
    the "jpg" file cannot be opened by any photo editor.
  • bmg_bobbmg_bob Global Mapper Programmer Posts: 2,026
    Can you open the JPEG file in Global Mapper? If not, what error message does Global Mapper display?
  • wmiswmwmiswm Global Mapper User Posts: 220Trusted User
    GM report the following:

    Not a JPEG file: Starts with 0x00 0x00
    JPG Overlay.cpp -67
    followed by version info etc.

    The percentage of such bad files is not high, in about 0.1-0.01%. But I have 3 million files to examine. Can you suggest any automatic way to pick these bad files?
  • wmiswmwmiswm Global Mapper User Posts: 220Trusted User
    Hi Bob, I did not choose the "Skip Empty Tiles" option. I know by selecting it will make the final output smaller, but do not quite sure how the mapping app will behave when it encounters a missing tile. There are plenty of blank tiles in the map, all of them are legitimate jpg files with background color only.

    Your suggested way will work, I am sure. But please keep this in mind for future GM updates, either improve the reliability of the output, or provide a build in checking mechanism. Thank you very much.
  • bmg_bobbmg_bob Global Mapper Programmer Posts: 2,026
    Please send a description of your workflow, a copy of your data (if possible), and a couple of the invalid JPEG files to Blue Marble Geographics Support (geohelp@bluemarblegeo.com). This will get the problem into the support stream, and help us to make sure that all exported JPEG tiles are valid.
  • wmiswmwmiswm Global Mapper User Posts: 220Trusted User
    I have already cleaned up the files and do not have backup of the defective files, but shall send next time when this happen again. Thank you very much.
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