Re: RLE & RGB to YBR_FULL
- From: Marco Eichelberg <eichelberg_nospam@xxxxxxxx>
- Date: Wed, 26 Mar 2008 11:26:29 +0100
Does this mean if I RLE encode an image and do color space conversion from RGB to YBR_FULL that I must mark the composite as "Lossy" (0028,2110) with "01" and make a new SOP Instance UID for the newly RLE compressed composite?
I would recommend this. DICOM requires a new SOP instance UID when a
modification takes place that might have impact on the diagnostic quality
of the image. While the error introduced by RGB to YCrBr color space conversion
is very small, it is probably difficult to estimate the effect on the image,
so marking the image as lossy compressed is the safer approach.
Does color space conversion make for lossy compression? I understand that JPEG 2000 has both YBR_ICT for lossy color space conversion and YBR_RCT for lossless color space conversion. Unforunately, the same does not apply to RLE does it?
In theory, color space conversion is lossless - the conversion from RGB
to YCbCr is an orthogonal coordinate transform and as such reversible.
The problem is the finite precision of the pixel representation,
which introduces rounding errors - these are responsible for the "loss",
i.e. the irreversible nature of the transformation when used with
integer numbers.
YBR_RCT is a transformation that has been designed specifically as
a color space to which a lossless integer transformation from and to
RGB is possible, at the expense of sacrificing a little bit of compression
efficiency. However, YBR_RCT requires 1 bit/sample more storage space
than the original RGB data if I remember correctly, and as such is
not useable in an uncompressed domain. The transformation is tightly
integrated into JPEG 2000.
Regards,
Marco Eichelberg
.
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