how does openslide do image downsampling

[Benjamin Gilbert]

On 01/26/2015 10:02 PM, 石川　拓海 wrote:
> I'm using a NDPI format (Hamamatsu format) image. Does OpenSlide creates
> extra slide levels for this format?

For each real level in a Hamamatsu VMS or NDPI slide, OpenSlide tries to 
create synthetic levels which are downsampled by factors of 2, 4, and 8 
from the real level.  If a synthetic level would have the same 
downsample factor as a real level in the slide file, the synthetic level 
is skipped.

VMS slides have only two real levels, so synthetic levels will always be 
created.  Some NDPI slides (older ones?) space the downsamples of their 
real levels by a factor of 4, so every other level (plus the smallest 
ones) will be synthetic.  Other NDPI slides space their real levels by a 
factor of 2, so the only synthetic levels will be the last three.

> If so, do you know how does libjpeg do image downsampling? Gaussian
> pyramid?

As I understand it, libjpeg only partially decodes each JPEG MCU.  I 
don't know the details.

--Benjamin Gilbert