This is a value between 0 (no transparency, the default) and 100 (completely transparent, i.e., no visibility). Setting a value above 0 causes the image to be partially transparent, so that the map resource underneath the current resource is partially visible across the map. If only one resource occurs in the map, this property should not be set above the default value of zero (0).

Note that setting this property to a value above zero (0) may incur a performance penalty, as the image must be processed for partial transparency. If you want features to be visible in map resources below the current one, a better option is usually to use the ImageDescriptor's properties of TransparentColor and TransparentBackground. By setting the TransparentColor to a color not used by layers in the map resource, and setting TransparentBackground to true, the background of the current resource will be completely transparent, and in these areas the data source below will be visible. These settings may be set in the design-time resource items editor for the MapResourceManager.

Reference