If you tried to set an image transparent with the normal transparent tools you found out it only works with one color. If fact all it does is turn off that color you select and is not really making anything transparent. Transparent as defined by Merriam-Webster is “fine or sheer enough to be seen through”. There is no see though when you just turn off a color. Using the data connect with FDO we can make polygon features transparent should we be able to do the same for images? Well we can with a little trick.
First we need to use the data connect to add the image to our map. Then save the image layer to “LAYER” file. When you save the layer it creates a file with an extension of layer, If you are not aware of what a layer file is, it’s an xml file that contains information on the data file as to how it is connected to the map, the data it contains (spatial & data), and how it is displayed or theme. Once you saved the image file to a Layer remove that image layer from the display manager and disconnect from the file(s) in the data connect palette.
Now browse to the image.layer file you created and open it in “Notepad”. Scroll down until you see the tags;
“<FeatureName>rasters:Name</FeatureName>” “<Geometry>Image</Geometry> “
Now insert between the two tags
Save the file after you edited it. The number 0.5 is the amount of transparency with the larger the number the more transparent the image will be. 0.9 is almost translucent and 0.1 is of little transparency. Now use the Load Layer tool from the Data Icon in the Display Manager to add the image to the map.
So why would anyone need an image to be transparent to start with when we can place it at the bottom of the draw order and make all the other layers transparent? First off, how often have you reopened the map to find out the draw order was not as you had it set when you closed the map drawing? Another reason you may want to set an image to be transparent would be if you have overlapping images in the map. Maybe you have an current aerial image and an aerial of the same are that is 10 yeas ago and you need to do a time lapse study of the area. Toggling the images on and off can get frustrating after a while, where as having one transparent will allow you to see though it to do the analysis a lot easier.
Here are a few before and after screen captures of the results.
How we normally do it with the image layer at the bottom of the display draw order and the polygon with transparency.
Here is with the image with transparency at 0.5 and the image layer at the top of the display draw order.
A closer look of the above. Notice how the street centerlines show though the image file.
If you look close enough you will see a grid pattern in the image after we make it transparent, I only guess that is the way Map3D renders the image but after a test print of the map the grid is not getting plotted so I see no harm in it being there. Also do not forget about the MAPPLOTTRANSPARENCY command if you want to plot with transparency.