openslide.level-count: '2'

Benjamin Gilbert bgilbert at
Sat Sep 26 18:44:46 EDT 2015

On 09/22/2015 02:56 AM, Mathieu Malaterre wrote:
> On Tue, Sep 22, 2015 at 8:36 AM, Benjamin Gilbert <bgilbert at> wrote:
>> In general, OpenSlide does not synthesize intermediate levels; we try to
>> deliver the information actually present in the slide file, and leave
>> the post-processing to the application. (There is one driver, hamamatsu,
>> which does synthesize levels due to some quirks of the format.)
> Tiles are Progressive JPEG ?

Hamamatsu VMS has two levels, consisting of one JPEG each.  (If the 
slide hits the 16-bit limit in width or height, level 0 is divided into 
several JPEGs.)  A "tile" is a run of MCUs between two restart markers, 
so tiles are e.g. 4096x8 pixels.  There's an index file giving the byte 
offsets of a subset of the restart markers.

Since substantial downsampling is necessary for this format, and because 
OpenSlide has access to a format-specific performance optimization that 
is unavailable to the application -- scaled decoding through libjpeg -- 
OpenSlide performs the downsampling in this case.

--Benjamin Gilbert

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