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Extract a whole scale image from a SVS format file in C++

I am trying to extract a whole scale image from a SVS file in C++.

I saw an explanation from the OpenSlide homepage. It says the SVS format is "single-file pyramidal tiled TIFF".

So I tried to extract a whole-scale image like I did for TIFF image: I read all IFDs from the SVS file, but there is no 273 tag which contains an address pointing to a whole scale image.

That's why I am little confused now, the SVS format doesn't have a whole scale image inside the file?

I found an undefined private tag from a SVS file which number is 34675. Is this tag is for a whole scale image? Or is there a proper way to extract it?

Solution

  • Aperio SVS is a tiled format. All levels of the pyramid are tiled images. The base layer is the first TIFF directory. This page of the LibTiff documentation shows how to read tiled images.

    In short, you need to look for tag 324 (TIFFTAG_TILEOFFSETS), as well as tags 322 and 323 (TIFFTAG_TILEWIDTH, TIFFTAG_TILELENGTH). I highly recommend you use LibTiff for this, and don’t try to roll your own.

    The custom tag in the SVS file contains metadata, including the physical size of a pixel in micron (SVS doesn’t set the resolution TIFF tags).