I am using Geoserver to cache 4 band (RGB,Alpha) satellite imagery. The native projection of the imagery is NAD83 and I want them in WGS84, when I do the reprojection,using Arcmap, depending on the extent of the imagery, black borders or areas appear in the results. I can make the black areas transparent in Arcmap, but no matter what I do (Set Null, change no data value to zero, clip the black borders,etc) in Arcmap, when I cache the imagery using Geoserver, the black areas re appear in the results both in Open layer viewer of Geoserver and when I serve them as WMTS in Arcmap or QGIS.
If you have A LOT of imagery, you could utilize GDAL's virtual mosaic building tool and create a virtual mosaic, then add it to Geoserver as Ian suggested. Take a look at Geoserver's ImagePyramid format.
An example workflow could be:
- Create virtual mosaic
- Tile mosaic into an ImagePyramid using gdal_retile
- Add ImagePyramid to Geoserver (i.e.- add the imagepyramid store, point it to your tiles location, publish it etc.
Then on the 'Data' tab for the layer in Geoserver, you could set the "Output Transparent Color" to black #000000 as discussed here.