Exporting raster GeoPackage should leverage a mix of PNG and JPEG tiles

[emxsys]

A mix of PNG and JPEG image tiles within a GeoPackage provides both transparency and good compression across a zoom level's map coverage. Partial map tiles, e.g., those containing a border of the map, should be generated as PNG images with transparency for the border region. Whereas full coverage map tiles should be generated as opaque JPEG images with compression.

[emxsys]

This issue was fixed via a combination of modifications in the WorldWind GeoPackage Module (worldwind-gt-geopkg) and the WorldWind GeoPackage Extension (worldwind-gs-geopkg).

• GeoPackageReader (worldwind-gt-geopkg): added support for transparent PNG images by changing the image type from BufferedImage.TYPE_3BYTE_BGR to BufferedImage.TYPE_INT_ARGB and set the background fill to transparent white.
• GeoPackageGetMapOutputFormat (worldwind-gs-geopkg): set the format option mime-type to "JPEG-PNG" (image/vnd.jpeg-png) so that the corresponding JpegPngMapResponse can test for the presence of transparency in an image and set the resulting map output to PNG if transparency is detected, otherwise JPEG with compression; set the tile's GetMapRequest transparent setting to true so that transparency is conditionally enabled in the super's RenderedImageMapOutputFormat.directRasterRender method.