mapbox/mbtiles-spec — pull request #33 by morabitowoolpert (2013-05-06 13:47:17+00:00)
Title: Mixed Image File Format

Propose adding a "mixed" image file format, like the MapServer or ArcGIS Server "MIXED" cache modes. This would indicate that the database contains both JPEG and PNG images, supporting a scenario--useful in overlay layers--where PNGs are used only where transparency is needed and JPEGs are used elsewhere. This minimizes cache size while guaranteeing that the layer will cleanly overlay on a basemap layer.