Export to RASTER/BMP: wrong file header

Parasite
Parasite Global Mapper User
edited January 2012 in Bug Report
Hi guys,

I am experiencing a difficulties while exporting to the raster/BMP file. I bet I found a bug or something. :)
Global Mapper 13.00 trial.

Steps to reproduce:
1. Open an image in GM (ASTER GDEM tile in my case, square GeoTIFF 3601x3601 px)
2. Opened OK, shows a nice DEM visualisation.
3. Export->RASTER->BMP (all options by default)->OK.
4. Exported to the single BMP file OK, 38 905 258 bytes.
This file won't be opened nor accepted by the major GISes :(

Let's dig a little deeper now. On this bmp file we got a header: 
00000000   42 4D AA A5 51 02 00 00  00 00 36 00 00 00 28 00   BMª¥Q.....6...(.
00000016   00 00 11 0E 00 00 EF F1  FF FF 01 00 18 00 00 00   ......ïñÿÿ......
00000032   00 00 74 A5 51 02 00 00  00 00 00 00 00 00 00 00   ..t¥Q...........
00000048   00 00 00 00 00 00 00 96  99 00 AC B1 00 E8 EC 00   .......–™.¬±.èì.
The byte offset 18...21 is the image WIDTH. We got "00 00 0E 11" there, which is clearly an "3601".
The byte offset 22...25 is the image HEIGHT, and we got "FF FF F1 EF" there, which is the value "4294963694", OR "MINUS 3601".

The image HEIGHT cannot be negative nor THAT great in BMP headers.
Changing the bytes "FF FF F1 EF" to the correct ones "00 00 0E 11" is fixing the problem, and the file will be welcomed by all viewers\raster anupulation software.

Is this a bug, or how could I make PROPER BMP headers while exporting via GM?

Thanks.

Comments

  • global_mapper
    global_mapper Administrator
    edited January 2012
    The BMP format stores the height in the header as negative if the BMP is a top-down image rather than a bottom-up image, but if other applications can't handle it I will go ahead and change the export to be bottom up to make everybody happy.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • global_mapper
    global_mapper Administrator
    edited January 2012
    I have completed updating the BMP export to generate bottom-up rather than top-down BMP files so that they will work everywhere. I have placed a new build at http://www.globalmapper.com/global_mapper13.zip with the change for you to try. Simply download that file and extract the contents into your existing v13.xx installation folder to give it a try. If you are using the 64-bit v13 version there is a new build at http://www.globalmapper.com/global_mapper13_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • Parasite
    Parasite Global Mapper User
    edited January 2012
    The BMP format stores the height in the header as negative if the BMP is a top-down image rather than a bottom-up image
    Mmm...not pretty sure what you meant by that. I just open a GeoTIFF (a single file) - how could I check, is it a top-down or a bottom-up? It is not a mosaic nor any multi-file scene...

    But anyway the IMAGE_SIZE can't be negative in both WIDTH and HEIGHT. It is just a violaton of the file format (a very old and obsolete one BTW, so many classic tools won't recognize any newest "tricks"), and also has no logical means anyway. How would you imagine the image with negative HEIGHT? An image behind the black hole's event horizon? :)

    See attached the properties of the image with the negative value - and the memory needed to simply OPEN such of file.
    I have completed updating the BMP export to generate bottom-up rather than top-down BMP files so that they will work everywhere.
    Great! Now it is working as a charm.
    See attached the same file properties report from the same software.

    Thanks.
  • global_mapper
    global_mapper Administrator
    edited January 2012
    The TIFF format has a different way of storing orientation, but the fast majority are row-major and top down.

    The negative height doesn't physically mean anything, the actual height in pixels is just the absolute value of the specified value. If it is positive then the first row of pixels in the file is at the bottom of the image and each successive row is further up the image. If the height value is negative then the first row is at the top of the image (which is much more common in image formats) and the subsequent rows are further down the image.

    In any case Global Mapper can handle them either way and it was easy enough to switch.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • Parasite
    Parasite Global Mapper User
    edited January 2012
    The TIFF format has a different way of storing orientation, but the fast majority are row-major and top down.
    But we do exporting to BMP - and there are no such things as negative values anywhere in da header (at least for backward-compatibility). As far as I know...

    Anyway, the troubles were detected while opening all of previously-generated BMPs in ERDAS Imagine, ArcGIS and GeoExpress all together (where all are up to date). The above screenshots were made on GeoExpress as a sample. I bet those software must be strictly following all the file\protocol standarts since long ago, and if they all reporting the issue - means something is really wrong with the file... :)

    After the fix, everything is back to normal. Thanks again. You may now close the thread as [SOLVED].
  • global_mapper
    global_mapper Administrator
    edited January 2012
    The original BMP export was just tested by loading them with the built-in BMP handling applications in Windows, and they handle them just fine regardless of whether they use a negative or positive height (they also use that value to control the orientation so you can get a top-down one). No big deal though we are used to updating exports to make other applications happy :)

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com

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