Question

ECW writer compression

  • 5 March 2020
  • 1 reply
  • 23 views
nicholas
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Badge +10

I am transitioning from using ERDAS Imagine to FME to generate ECW files

I need to understand the difference between the terminology used by both systems with regards to compression.

FME ECW writer refers to "Compress by Percentage (0-99)" with a default of 75

this means that the output image will be 75% compressed, or one-quarter the size

ERDAS Imagine refers to "Target ECW Compression" with a default of 20

this means that the output image will be compressed by a ratio of 20:1, or one-twentieth the size.

Two questions;

1) Why does FME use a compression percentage and not a compression ratio (the convention established by ERDAS)

2) Why is FME's default compression different to ERDAS's default compression?

1 reply

jovitaatsafe
Rank
Userlevel 1
Badge +11

Hi @nicholas,

We use compression percentage due to the programming libraries that we're using for ECW, in particular this parameter phrasing comes from GDAL. I suspect that the differences in default compression would also be attributed to the different libraries that we're using to the ones that ERDAS uses.

It might be worth posting a new Idea here if you wish to see changes in the compression method from the user side of things so that it's a more consistent experience when working with raster compression.

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