I have had this problem before, such as placing shadow inside lakes and other bodies of water; but once again for the old style map of London I recently created I needed a technique to place shadow inside of building polygons.
The old maps had a solid black edge that was clearly inside of the building footprint, in fact smaller buildings were just solid black.
Method 1 – Photoshop
Although you can create inner shadow quite simply in Photoshop, to get the appearance I was after was not possible and this is why.
Open your building footprints into Photoshop, I had them as polygons saved as a PDF.
Right-click on the layer in the Layers panel and select Blending Options… to bring up the Layer Style dialogue.
Tick to choose Inner Glow and adjust the settings. I wanted an inner drop shadow coming in from the bottom right, so my settings were as seen below:
At first glance this method looks fine but on closer inspection there is a problem. Because it is raster the effect is being applied as if it were one object or one shape.
Zoom in on a more intricate building and you can see the problem. The shadow is displaced from the edge of the feature in places. If I exaggerate the settings you can see this more clearly across all the buildings.
Method 2 – Illustrator
As you can see, keeping the footprints as individual vector polygons doesn’t have this problem.
So to recreate this effect I used drop shadow and masks in Illustrator. This is how.
Firstly if creating from a PDF export rather than AI file, then you may wish to disable or preferably delete the clipping masks of the frame and of the group. I have done so here because it makes it easier to understand what is going on. Delete the top clipping mask. Then select the clip group, choose Object > Clipping Mask > Release. Then delete this clipping mask also from the Layers panel.
Contract the group with the arrow, again to make it easier to follow.
Now select the buildings group and go to Object > Path > Outline Stroke.
Then copy the group (Edit > Copy, or Ctrl + C) and paste twice using Edit > Paste in Front (or Ctrl + F). I have also zoomed in to 600% just to make it easier to see as I have very detailed building outlines.
Next, use the Rectangle tool to drag a new white rectangle to complete cover your page and buildings.
Reorder so the rectangle Path is between the first (bottom) and second (middle) groups of buildings.
For clarity, turn off the visibility of the first and third building groups in the Layers panel.
Select the second group and the white rectangle. Open the Pathfinder panel (Window > Pathfinder) and select Minus Front. If your buildings are as complex as mine then it may take a while and say (Not Responding) for half a minute or so.
What we end up with is our white rectangle with the building shapes cut out of it. If I add a background you can see this more clearly:
Keeping the rectangle selected, go to Effect > Stylize > Drop Shadow…
Here we can control the settings of the shadow. As we want the shadow to go into the building from the bottom right we need to set the X and Y Offsets to negative (minus).
If you have a large dataset like mine, then I suggest turning preview off until you have modified your settings. Do this every time you tweak your parameters.
This all looks fine, but on a map you’re likely to have layers or features beneath that are currently being hidden by the white mask.
So, turn on the visibility of the upper group of buildings. Select that group and choose Object > Compound Path > Make.
Select the compound path and the white mask (the top Group), then go to Object > Clipping Path > Make. If you have complex data then a warning message will pop up. Just accept it. It worked for all of London so it should work on your map too.
There may be small white artefacts left on the edges of the building shapes. To fix this, find and select the white mask once more – now a group within a clip group. Open the transparency panel (Windows > Transparency) and change the mode to Multiply. This will make any white fragments effectively transparent.
If I remove that background layer and zoom back out a little, you can see we have achieved the desired building shadows to add to our map. This can then be saved either as a PDF or any raster format for loading into our final map.
Copies of the special limited edition map of London can be purchased here. Or visit os.uk/maps for our full range of paper mapping including the 225th anniversary map of the Western Highlands of Scotland in a 1960s style and my previous map of Western Arabia Terra, Mars.
Finally, I promise to try to avoid using white buildings on a white page in future tutorials!