For CityEngine areas with no NoData should be transparent.
So you need to add a transparency layer. ArcMap has the ability to save with transparent color for those image formats that support a transparent color: GIF and PNG, of these two the PNG (Portable Network Graphic) is the better format as it supports 24bit color and transparency where GIF only has 8bit palette color and transparency.
To make the export transparent select in the export dialog (File::Export)
On the color dialog:
Pick the large box at the top that says 'no color' and the PNG will be exported with a transparent background.
I was also looking for a method to make NoData area transparent, and found this posting. However, when I tried this method, it still showed some color (see the light grey for the original raster extension). I want to apply transparency to several raster layers' NoData for view analysis. Although there is no value, the view shed result picked up that area as visible part. I am not sure is it because of the color applied?
I hope to use raster for every view category rather than polygon so I can also apply the elevation at the same time.
In the image below, the black spots represent parks, and other areas are NoData. But it still picked up that part as part of layer.