For many years ERDAS IMAGINE has not provided correct transparency creation with the IMAGINE MrSID Encoders. This was addressed in 9.3 and continues on through the 9.3 Service Packs and into ERDAS IMAGINE 2010. Yet, it is not obvious how to do this.
The user must set the NoData (NullData) in ImageInfo (LayerInfo) or in Image Command Tool (Image Metadata Editor in 2010), then use the IMAGINE MrSID Encoders. Voilà you have a true transparent background data.
ERDAS will make the correct transparency creation ability more prominent and flexible in a future release.
Here is one big thing to remember; ERDAS uses 0 as the transparent value while ESRI uses 255. ERDAS as well as other remote sensing and photogrammetric packages have a black background page while ESRI has a white background page. The more careful image providers now create 8-bit data with a range from 1 to 254 rather than 0 to 255. All 0 and 255 values will be outside of the actual image footprint. This approach allows their customers to be flexible with whatever package they they are using at the moment.
2 comments:
Yes, delivering data with values from 1-254 is the "only" option. And the background completely 0-0-0 or 255-255-255. This will make it easier to change the background color later as you can just convert 0-0-0 to 255-255-255 in all files or the other way around if you need. In uncompressed files that is - for JPEGs and other compressed formats its not that easy.
For MrSID, the user defines it as DN values to be background/null, and those DN values may change, but MrSID keeps track of the change and the new values are the background transparent DNs.
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