Q/A From Today's ECW/JP2 SDK Webinar

Below are the questions and answers that we covered in today's ERDAS ECW/JP2 SDK Webinar.

Q1: Compression is always followed by the loss of detail information, right? How does ECW handle that? Which compression rates are useful?

A1: ECW does not support numerically lossless compression at this time. However, if you use a target compression of 4 to 6:1 on RGB data, you should have a visually lossless image at a 2x zoom. See my blog for more information: http://field-guide.blogspot.com/2010/07/understanding-target-compression-ratio.html


Q2: Would you recommend ECW over JP2?

A2: I would recommend ECW over JP2 for all applications where an 8-bit lossy compression works. In the GIS community, a visually lossless setting is usually fine. We did a test when I was at Georgia Tech to see what compression was needed for the majority of users using a statewide dataset. The results are documented on my blog, linked above.

But remember, the ERDAS ECW/JP2 SDK supports JP2 very solidly! Any product using the SDK will have fast JP2 support and even faster ECW support.


Q3: It seems ECW cannot be used on ArcGIS Server and image server, any comment?

A3: ERDAS will release a new product, ECW for ArcGIS Server, spring 2011 to address this very issue. Look for it!


Q4: For aerial photos and orthophotos, which is better for delivering products without losing resolution and data, ECW or JP2?

A4: Pixel resolution is never lost in compression. The first thing you’ll notice in compression is very minor loss of contrast. As you compress further, a little more contrast is lost, and in heavier compression artifacts appear around road lines, edges and other linear features on aerial imagery. The question to ask is, “Do I need visually lossless or numerical lossless compression?” In most GIS applications, visually lossless compression works very well. In some (but not all) remote sensing applications, numerically lossless compression is needed.

The reason data vendors deliver numerically lossless data is that the data provider does not know if you are a GIS user doing imagery analysis or an image expert doing detailed remote sensing. They deliver one version to all, numerically lossless JP2. No problem, the ECW/JP2 SDK supports this!


Q5: Does a standard/report exist for the compression rates for several products (vector map production, base ortho maps, etc.)?

A5: I wish I knew of one, but I do not. This is an area for cartographers and remote sensing researchers to look for research grants. We need a standard.


Q6: How much faster can ERDAS ECW/JP2 SDK make image compression?

A6: I tested and documented this on my blog. Some SDK customers say their applications using v4.2 ECW encode 7x faster than it did in v3.3. Our research indicates most applications will experience greater than 2 x improvements over v3.3. But, JP2 is faster in v4.2 as well. Please see the following link for some tests I ran: http://field-guide.blogspot.com/2010/04/single-core-vs-multi-core-image.html


Q7: What can I provide a customer if he cannot read the ECW format - can I offer him a simple/free ECW reader/decoder?

A7: Many GIS applications can read ECW. ERDAS has a free viewer, ER Viewer that can be used as well. See: http://www.erdas.com/products/ERDASERMapper/ERDASERViewer/Details.aspx


Q8: At times, ECW cannot be opened directly in ArcGIS 10. Any suggestions about how to do that?

A8: There are two possibilities. One, ArcGIS 10 uses a version of GDAL that uses an older version of the ERDAS ECW/JP2 SDK . GDAL has now addressed that and supports v4.2; ESRI will pick up the ball and do the rest.

Two, some organizations who have licenses for the ERDAS ECW/JP2 SDK have added things to the ECW file that are not in compliance with the file format. This will cause problems. You might want to contact the organization that created the data to make sure they do not add any ‘goodies’ to the file format.


Q9: Are there any plans to use GP-GPU for compression?

A9: Yes. I cannot give specifics at this time, but I can say ‘yes.’


Q10: Can I lose accurate pixel value when I compress ECW data?

A10: At the present time, ECW has a visually lossless capability, not a numerically lossless capability.


Q11: Do you have any plans to support the updating of parts of an existing ECW file?

A11: Yes, this is under discussion. I will assume your question indicates an interest and will put your name in the request column. Thanks!


Q12: What would you use for storing floating point data, e.g. DEM data?

A12: Up until now, geospatial people have been using some sort of DR-RLE compression, such as that found in ERDAS’s IMG file format. Going forward, as the floating point JPEG2000 standard takes hold we will see more people use JP2 for lossless floating point data.

The question to ask is whether the maximum 2.5x reduction in file size on encoding worth the slowdown in decoding?


Q13: Which format is useful for signed16-bit data, ECW or JP2?

A13: ECW does not support signed data right now. Therefore, JP2 is the solution at this time.


Q14: Spanish users are having issues with plug-ins when using the Spanish version of the ERDAS ECW/JP2 SDK. Are you going to solve this soon?

A14: The ECW/JP2 SDK should not have a problem with this. If there is a problem with the free plug-in, it is in the plug-in code, not the ECW/JP2 SDK. We are working with Autodesk Germany on the AutoCAD 2011/2011 Plug-in to solve language support issues right now.


Q15: Paul, can you say anything publicly about when the AutoCAD 2010 plug-in will appear?

A15: We are waiting on Autodesk to help with a few installer questions. After that, we are ready to go.


Q16: Does ECW support stereo imagery data?

A16: Not inside the ECW file format. There are some companies that have modified the ECW file format (breaking the file for others), to support stereo parameters inside the file format. We have asked them to work with us to get it formally supported in the formal standard.

Until that time, the stereo parameters can be stored in the external .aux file.


Q17: Why is it necessary to have a plug-in to work with ECW in ERDAS software?

A17: When older versions of ERDAS software were made, ERDAS had not purchased ER Mapper and ECW support was incomplete. ERDAS purchased ER Mapper in May 2007, so that is when native ECW support became standard.


Q18: For unsigned 8-bit, unsigned 16 bit mss data, which format is more useable, ECW or JP2?

A18: ECW is an unsigned 8-bit data format. It can store panchromatic, RGB and multispectral data. 16-bit data should be stored in JP2 at this time.


Q19: Can the ECW format support hyperspectral data?

A19: ECW is a lossy, unsigned 8-bit data format that supports 65,535 bands. Give ECW a try!


Q20: We need to be able to make the background pixels of imagery transparent in ArcMap. ECW doesn't support this, but JP2 does. Is there any way to get the background pixels in ECW files to "disappear" in ArcMap (i.e. set the bg values to 255 or 0)?

A20: ECW added support for transparency in ECW/JP2 SDK v4.1, released in the fall of 2010. We had beta versions available in the summer, but this was too late for ESRI to get it supported in ArcGIS Desktop 10. GDAL is now updated, and the ball is in ESRI’s court. Let them know you want it!

In the ERDAS IMAGINE 2011 Exporter and MosaicPro, we enable you to define whatever value you want to be transparent; you can even define a region using a polygon.

Using ERDAS IMAGINE 2011’s “Create ECW Transparency” feature, you can also add a transparency layer to an existing ECW file via DNs or polygons without recompressing.


Q21: ECW or JP2 - where can I use each format and I how do I know?

A21: Both file formats are broadly accepted in the geospatial industry. If you wish to buy a product that does not support ECW/JP2, tell that product’s manager to give me a call. Remember, the SDK to provide desktop/client ECW, JP2, ECWP reading is free to the software developer.


Q22: Will you post the webinar.

A22: As usual, I forgot to record the webinar! I will re-record and post it later this week.

ERDAS ECW/JP2 SDK Product Page Link: http://www.erdas.com/products/ERDASECWJPEG2000SDK/Details.aspx

YouTube Video: http://www.youtube.com/watch?v=y6rjEgOkhoc&feature=plcp