[gdal-dev] ECW codec compression question

[Frank Warmerdam]

chalda wrote:
> Hello,
> 
> I have question about setting of compression of the ECW codec. I have 
> read the article on the page http://www.gdal.org/frmt_ecw.html and there 
> is said that option TARGET=percent sets size reduction as a percentage 
> of the original image.
> 
> I'm using the ECW codec through gdal_translate by this comma
> gdal_translate -of JP2ECW -co "TARGET=10" input_image.tif out_image.jp2
> 
> But when I'm changing the number option of the TARGET then no change of 
> quality or the size of the image occurs. Or better the changes begins 
> around the number 50 but before all options 0-50 returs the same 
> "quality results".
> 
> By the mentioned page that shouldn't be correct. Could somebody explain 
> me please what how the quality option works here?

Ondrej,

The TARGET parameter sets a target reduction for the algorithm, but in
some cases low information images will be compressed more than requested.
If your image has a relatively low information density it may be that the
least compression the library is capable of providing is 50%.

The thing to keep in mind is that TARGET is a hint to the library - not
an exact definition of how much compression will be achieved.

Best regards,
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