Used to control whether multiple background rasters are merged together into a single composite background raster or written independently into the print output file. Background rasters are raster attachments that are printed below all non-raster graphics. It is more efficient to merge rasters, especially if the attachments overlap. However, Adobe Reader's Layers control cannot individually turn on or off rasters that have been merged inside PDF files. It may also be desirable to prevent merging of background rasters when producing Windows metafiles with transparent backgrounds. The default behavior is to merge background rasters. This property only applies when printing in non-rasterized mode.