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August 2008

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2008.08.08 05:15 "Writing custom fields with big counts.", by Tom Harris
2008.08.08 08:53 "Re: Writing custom fields with big counts.", by Andy Cave
2008.08.08 13:29 "Re: Writing custom fields with big counts.", by Edward Lam
2008.08.11 03:16 "Re: Writing custom fields with big counts.", by Tom Harris
2008.08.11 13:18 "Custom fields and BIGTIFF (was: Writing custom fields with big counts)", by Edward Lam
2008.08.11 02:19 "Re: Writing custom fields with big counts.", by Tom Harris

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:

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.


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" <>
> To: <>
> 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>
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