Mailing Lists: Apple Mailing Lists
Image of Mac OS face in stamp
 
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: spatial quality

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

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

Interesting, so in theory one *could* look at the q tables and subsampling factors and come up with some kind of quality measure. However, there's no universal definition of this, as different encoders work very differently, so QuickTime provides no measure at all unless lossless encoding was used? Fair enough then.

As to why it would be useful, lots of reasons. It's all part of the metadata and you can never have too much of that :-). For example, right now I'm trying to compare the subjective quality of digital camera pictures recorded in 'normal' and 'fine' modes. Other than guessing based on the relative file size there doesn't seem to be any way of telling which is which.

Regards,

-Rolf
--
Rolf Howarth, Square Box Systems Ltd, Stratford-upon-Avon UK.
_______________________________________________
quicktime-api mailing list | email@hidden
Help/Unsubscribe/Archives: http://www.lists.apple.com/mailman/listinfo/quicktime-api
Do not post admin requests to the list. They will be ignored.
References: 
 >spatial quality (From: Rolf Howarth <email@hidden>)
 >Re: spatial quality (From: Daniel Muller <email@hidden>)



Visit the Apple Store online or at retail locations.
1-800-MY-APPLE

Contact Apple | Terms of Use | Privacy Policy

Copyright © 2007 Apple Inc. All rights reserved.