One confused image monkey

Melanie Harlow’s blog @ ESRI has left me like one of these …

Hmmm? Conversion from 89MB of MrSID’s to a JPEG2000 balooned to 922MB. What exactly does 75% compression mean in that context? Sounds pretty incorrect to me unless 75% means lossless which isnt exactly a fair test. Recompress to ~1:15 lossy jp2 and I would expect the total dataset size to be largely the same as the original MrSID.

Hmmm? I do believe you are missing a format in your comparison. ECW will save you space, use a complex algorithm, and improve the performance of reading the data. This statement is completely wrong. Sticking with a more verbose format like TIFF will require you to read more data, more files and offer less effective caching mechanisms

Before you go off and convert all your raster data to a highly compressed format to save space, please note that the more complex the compression the slower it is to read the data. Therefore, we generally recommend when speed is your #1 concern, that you use a TIFF raster dataset with JPEG compression

Hmmm? If the output mosaic is only 30,000 x 20,000 pixels then why not do a proper mosaic into a single MrSID, JP2 or ECW file? Keeping small tiled datasets removes the benefits these wavelet formats provide, gives worse performance and requires you to store redundant overview data thats simply not required if kept as a single file.

A good example on a larger dataset was the FOSS4G 2010 Benchmarking raster over Barcelona. 3 band, 8 bit 220,000 x 150,000 pixel dataset. Mosaiced to 1 ECW = 4.8 GB. The equivalent JPEG Compressed TIFF tiles totalled 112 GB. Thats a significant difference without even looking at the performance gains.

I will wait for Part Duex that I’m sure will clarify this =)

3 thoughts on “One confused image monkey”

  1. Those numbers look to be way off, don’t they? No way should a “subjectively equivalent” lossy compression to JP2 larger than the equivalent JPEG.

    The article looks to essentially be repeating the technical advice offered by ESRI on the underlying tech of ArcGIS Server Image Extension, which does perform “well” with JPEG-compressed TIFF data and ad hoc overviews.

    But also performs rather well with ECW data and JPEG-compressed TIFF overviews – certainly no worse than with the TIFF at the base.

    The main argument in favour of the technology is the retention of the survey product which is usually delivered as orthorectified TIFF. 30k x 20k px isn’t really a realistic quantity of data for a typical e.g. aerial survey project, you are more likely to be looking at a few thousand raw TIFF orthos each of which is a few hundred MB in size.

    If new data is acquired you can add it to the catalog without re-deriving all the second-order overviews etc. This has some benefits over the “just mosaic it all to ECW” approach.

    Under those sorts of conditions Image Extension and the new Mosaic Datasets are a quick way to get new or updated imagery into your dissemination / visualisation platform, but hardly the best fit for all purposes.

  2. You now can’t serve your data thought the web because it’s in ecw format!!!..Gives us $50k and we will let you…….Ha ha ha luv ERDA$ supporting OGC on our terms just like E$RI…

  3. Spot on Tom; there will never be a single format good for all scenarios. In my experience most people leave data in small tiles because it’s “easier” rather than because it permits updating pieces of larger mosaics. *shrug*

    Simon, simon simon. $50k? You must be smoking something … and i think you missed the point entirely. I’m not saying jpeg compressed tiffs arent a useful format, but if your looking at file storage and trying to say it gives you worse performance then you’re way off the money.

    I’ve clearly missed anonymous cowards .. at least i know your local :-)

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