Thanks, Julien.  This is helpful.

On Jan 15, 2011, at 2:28 PM, Julien Jalon wrote:
> When requesting a thumbnail, it's up to the underlying plug-in to honor the 
> requested size.
> 
> If icon == NO, Quick Look will return the plug-in's image result as is.
> 
> If icon == YES, Quick Look will force the requested size (because it adds 
> icon decoration which should match the requested size) - so if you request a 
> 512x512 icon, you should always get a 512x512 image.

When icon == YES, I do get back an image at the specific size I request.  With 
some file types, like PDF and HTML, the larger image still looks reasonably 
good, but with OmniGraffle files I get a scaled-up version of the smaller 
image, with horrible pixelation.

> Many plug-in just use a low-resolution image stored in the file as a Quick 
> Look thumbnail. It's totally acceptable but nowadays, "low-resolution" should 
> be more toward 512x512 than 128x128.

Peeking into OmniGraffle's document format (optionally either a plist or a 
package), I see it does exactly that for both the thumbnail *and* the preview, 
and its low-resolution image is small.  Also there is an option on a 
per-document basis to store no Quick Look info at all, in which case all you 
see is the document icon.

> Previews are more complex (generally slower to generate) and are meant to be 
> displayed mainly in Quick Look panel but they also appear in Cover Flow, 
> Column view, Info window

I haven't found a way to display Quick Look previews except using 
QLPreviewPanel.  I assume there is currently no public API for displaying a 
preview in an arbitrary view?

I want to display nice thumbnails of OmniGraffle files (actually all file 
types, but these in particular), so I guess I can treat .graffle as a special 
case and extract the preview myself from the .graffle file.  The preview is 
stored as a PDF; if it's a multi-page PDF, I'll grab the first page.  If I find 
no preview, I'll fall back on my regular thumbnail logic, which will give me 
the document icon.

> and, starting with SnowLeopard, in live previews in Finder's icons.

I see the preview being used in Finder's Column mode, but in Icon mode it seems 
to be the thumbnail.  I'll report this to Omni; the thumbnail really should be 
bigger in case I go into Icon mode and zoom the icons to full size.  This is 
with the latest OmniGraffle Pro.

--Andy

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