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August 2008
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2008.08.08 13:29 "Re: Writing custom fields with big counts.", by Edward Lam
Hi Tom, I agree with Andy. Having said that, the TIFF6 spec says that tags have 32-bit value count. Looking at this page and the code, I think it's possible to do that with application tags in libtiff as well. First, make sure that you've defined your tags appropriately as per: http://www.remotesensing.org/libtiff/addingtags.html Now, what that page doesn't seem to explain is how to set your tag values. :) First note how to set up your tag in the Default Tag Auto-registration in the link above. In particular, field_passcount should be set to true. Looking at the code in tif_dir.c, if field_passcount is set to true, then you must include the value count as the first argument after your tag type, then followed by a pointer to your data. eg. TIFFSetField(my_fp, my_tag, my_int_count, my_buffer_ptr); Finally, note that "my_buffer_ptr" should point to a chunk of memory that contains my_int_count * sizeof(my_tiff_value_type) number of bytes. Cheers, -Edward Andy Cave wrote: > Hi Tom, > > Are you doing anything wrong. Yes: > > Why invent something non-standard, when TIFF provides the facility to do > exactly what you want. You have two options: > > 1. Put multiple images in a single file. Your main first image would be > the RGB data that most applications will view and your second image > would be your scientific data. This is all spec'd in the TIFF > documentation -see page 16 to start with (Multiple Images per TIFF File) > and then page 26 and later. > > 2. Make your image an RGB image with extra samples (which are your > scientific data), where these are marked as "unspecified". See page 31 > for details on this. > > Personally, I'd go for the second option (assuming most readers work > with it), as this keeps (preserves) your scientific data with the > equivalent RGB data. > > Alternatively and even simpler, just create two TIFF files with similar > names but one being slightly different to indicates it's the scientific > data. > > Regards, > > Andy. > > ----- Original Message ----- From: "Tom Harris" <celephicus@gmail.com> > To: <tiff@lists.maptools.org> > Sent: Friday, August 08, 2008 6:15 AM > Subject: [Tiff] Writing custom fields with big counts. > > >> Greetings, >> >> I am playing with an idea to add arbitrary extra fields to a vanilla >> RGB TIFF, these extra tags will hold the source image that was used to >> create the RGB image. I want to do this so tht I have data files from >> a scientific intrument that contain all sorts of strange data, but to >> the users look like normal multi-image 24 bit RGB files. >> >> I can add the extra field OK, but I find that although the spec allows >> 4 bytes for the count, libtiff only seems to use 2 bytes, so the >> maximum count is 65535. The TIFFFieldInfo struct only uses shorts for >> field_readcount & field_writecount, so I suppose this is why. However, >> even if I have -1 for these values (indicating that the number of >> values is set in the call to TIFFSetField), the maximum count that I >> seem to be able to write is 65535. Tiffdump will only report the field >> count as correct if it is less than 32768, any more and it gives me a >> count of zero, even though inspecting a hex dump of the tiff file >> shows the correct count. >> >> Am I doing something wrong? >> >> I was hoping to save my private data as an array of 1024*1024*4 >> unsigned shorts, but it looks like this will not work with libtiff. >> Perhaps I should be looking at writing a table of offsets to data in >> strips (exactly how images are saved at present). >> >> -- >> >> Tom Harris <celephicus(AT)gmail(DOT)com> >> _______________________________________________ >> Tiff mailing list: Tiff@lists.maptools.org >> http://lists.maptools.org/mailman/listinfo/tiff >> http://www.remotesensing.org/libtiff/ > > > _______________________________________________ > Tiff mailing list: Tiff@lists.maptools.org > http://lists.maptools.org/mailman/listinfo/tiff > http://www.remotesensing.org/libtiff/