Even Rouault wrote: > > Le jeudi 02 octobre 2014 01:06:57, David Strip a écrit : > > On 10/1/2014 12:02 PM, Jukka Rahkonen wrote: > > > > For comparison: > > Tiff as zipped 347 MB > > Tiff into png 263 MB > > If I have understood right both zip and png are using deflate > > algorithm so there might be some place for improving deflate compression in > GDAL. > > > > I was curious how png could achieve such better compression if it is > > using the same deflate algorithm. I wouldn't think different > > implementations would account for so much improvement. It turns out > > the png compression uses a "filtering" step ahead of compression. This > > is explained here. The filter is similar to a differential pulse code > > modulation, in which the pixel is represented as the difference from > > the pixels to the left, left upper diagonal, and above. This typically > > reduces the magnitude of the value to something close to zero, making > > the encoding more efficient. > > True, a way to improve things might be to specify -co PREDICTOR=2. Should > apply to both LZW and DEFLATE. > This is one of the filter that might be used by PNG, except that PNG has > different > filters, so it will eventually beat TIFF deflate.
Not a bad suggestion. Original 424 MB DEFLATE without predictor 380 MB DEFLATE with -co predictor=2 280 MB -Jukka Rahkonen- _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev