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Re: spatial quality

Rolf,

I asked the same question 2 years ago and I got a reply from Sam Bushell <email@hidden>.

BTW, now the same question could be asked for JPEG 2000, it seems that for the lossless compression it's possible to find a specific marker but for lossly compression there are some tricks (not part of the JPEG 2000 standard).

What's the Apple's reply for JPEG 2000?


Daniel Muller
--
Opus Software
Mac Development

<http://www.opus.plugin.ch/>
<mailto:email@hidden>

Date: Wed, 20 Sep 2000 10:49:57 -0700
From: Sam Bushell <email@hidden>
Subject: Re: Graphic importer and image quality

Daniel Muller <email@hidden>:
When importing a JPEG file with GetGraphicsImporterForFile, the image
quality (spatialQuality field in ImageDescription record retrieved by
GraphicsImportGetImageDescription) is always set to
codecNormalQuality even when the JPEG file has been saved with the
Graphics Exporter and image quality specified in
GraphicsExportRequestSettings for codecHighQuality.

How can I get the image quality of lossly compression image like
JPEG, some ImageDescription have extensions but not the JPEG grip?

It's hard to see how this value would be particularly useful. The
properties of a JPEG file that really determine its quality are the
quantization tables and chroma subsampling factors (altogether over a
hundred numbers). QuickTime's JPEG compressor chooses values for these
based on a spatialQuality setting, but there are plenty of q-tables and
subsampling factors that don't correspond to a spatialQuality setting, and
third-party JPEG encoders may choose wildly different q-tables.

So any spatialQuality value we came up with would be a lie anyway.

The only accurate piece of information that is conveyed by the image
description's spatialQuality is that a lossy compression was used -- if the
compression was lossless, the graphics importer should report that by
setting spatialQuality to codecLosslessQuality.

Sam Bushell
QuickTime Engineering


Is there any way to get the original JPEG image compression quality for an arbitrary image opened by QuickTime? In the case of an image created by QuickTime the image description gives back the quality used during compression, but when opening other images it's always 512 (50%).

-Rolf
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References: 
 >spatial quality (From: Rolf Howarth <email@hidden>)



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