2021.12.16 09:01 "[Tiff] Ensuring buffer alignment when creating tiff files", by Milian Wolff
we would like to directly mmap large image buffers from tiff files. Our software checks whether:
- image is not tiled
- just a single strip, or all strips are contiguous
- strip buffer is aligned to BitsPerSample (e.g. 2byte for 16bit greyscale images)
- not compressed (TIFFTAG_COMPRESSION == COMPRESSION_NONE)
- follows system byte order (TIFFIsBigEndian == system byte order)
- not swapped (TIFFIsByteSwapped)
- contiguous planar config (TIFFTAG_PLANARCONFIG == PLANARCONFIG_CONTIG)
So far so good, this seems to work well for the main buffers. But now we received a test file with an embedded mipmap via SubIFD. Sadly, these buffers are all misaligned, i.e. we have these SubIFD:
BitsPerSample (258) SHORT (3) 1<16>
SubIFD (330) IFD (13) 4<0xa006f6 0xa80ad4 0xaa0d32 0xaa8ed0>
But `TIFFGetStrileOffsetWithErr` returns the following values for the respective IFDs:
8006f5 a00ad3 a80d31 aa0ecf
As you can see, these offsets are misaligned for a 2byte/16bit greyscale image.
Looking at the libtiff API, we cannot find anything that would allow us to ensure that the SubIFDs are aligned correctly. Are we missing something or is this simply not possible currently?
We think that it would only require a small change in the code base, namely ensuring that the seek at  ends at an aligned address based on the BitsPerSample for the current IFD.
Milian Wolff | firstname.lastname@example.org | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
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