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October 2004
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2004.10.02 16:07 "Re: quad-tile", by Andrey Kiselev
On Sat, Oct 02, 2004 at 10:41:39AM -0500, Bob Friesenhahn wrote: > >- Closed-circuit testing is a tiny bit problematic when lossy storage > >is involved. I'm not just thinking jpeg, even mere YCbCr subsampled > >images, whatever compression scheme, are lossy. This tiny problem can > >perhaps be solved by having the data include tolerance levels for the > >closed-circuit testing? > > For many of GraphicsMagick's tests we use an image comparison function > which computes statistical error values. For algorithms which should > not produce error, the allowed error is zero, but for algorithms which > produce error (e.g. JPEG) we manually inspect the result to ensure > that it seems ok, and then set the maximum allowed error value to > slighly higher than the actual result. The error values include the > maximum pixel error (in case a single pixel is wrong), and the total > mean error. > > Provided that the libtiff test suite provides an image compare > function, the allowed degree of error may be included with the test > specification. Good idea. I think we can go this way too. > For operations which must always produce a certain result (the case > for most libtiff operations), we use a checksum (SHA-128) and compare > the resulting checksum with a checksum included with the test. This > is a very solid way to prepare a test, but is a bit tedious since each > result must be validated by the person who writes the test. This approach is used by Frank for the GDAL test suite and it will be used in libtiff when we will connect the external image sets to the tests. Andrey -- Andrey V. Kiselev Home phone: +7 812 5274898 ICQ# 26871517