Le 08/12/2023 à 20:23, Laurențiu Nicola via gdal-dev a écrit :
On Fri, Dec 8, 2023, at 21:11, Even Rouault wrote:
- GeoTIFF DEFLATE 280 MB
Is it with PREDICTOR=3 ? (If not, then the ZARR datasets should also
be compressed with FILTER=DELTA to have a fair comparison)
I would expect a fair comparison to be between PREDICTOR=3 and
FILTER=DELTA and not the other way around,
yes, poor wording of mine. I meant that if using PREDICTOR=3, one should
compare with FILTER=DELTA. But looking more closely, they are not
strictly equivalent. PREDICTOR=3 applies the delta as b[0]-a[0],
b[1]-a[1], b[2]-a[2], b[3]-a[3] where a[0...3] and b[0...3] are seen as
the 4 byte representation of the float32, whereas FILTER=DELTA does the
difference b_float - a_float as floating point. This isn't the same...
but it does seem to level the field significantly:
- GeoTIFF DEFLATE PREDICTOR=3 255 MB
- Zarr DELTA BLOSC zlib NONE 290 MB
- Zarr DELTA BLOSC zlib BIT 266 MB
- Zarr DELTA BLOSC zlib BYTE 259 MB
- Zarr BLOSC zlib NONE 281 MB
- Zarr BLOSC zlib BIT 253 MB
- Zarr BLOSC zlib BYTE 249 MB
So FILTER=DELTA yields worse compression, while GeoTIFF PREDICTOR=3
does very well. No need for Zarr! I don't remember PREDICTOR=3 doing
so well in the past, so.. no idea :-).
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