HOW TO CREATE SOFT GROUND PLANE SHADOWS USING SKETCHUP
Shadows are one of the most important visual cues that we have for understanding the spatial relationships between objects. Unfortunately, even modern computer graphics technology has a difficult time drawing realistic shadows at an interactive frame rate. One trick that you can use is to pre-render the shadows and then apply them to the scene as a textured polygon. This allows the creation of soft shadows and also allows the computer to maintain a high frame rate while drawing shadows.
Step 1: Activate and position the shadows
First, activate the shadows and position them using SketchUp's Shadows toolbar.
Step 2: Draw the Shadows Only
Next, we need to render the shadows without the geometry. To do this, create two pages in SketchUp. Put the objects in the scene in a different layer than Layer0, so that you can toggle the visibility of the layer containing the objects. Have the first page draw the shadows and show the layer containing the objects in the scene. Have the second page hide the layer containing the objects in the scene. When you move from the first page to the second page, the objects will disappear, leaving the shadows only.
Step 3: Draw the Shadows from Above
Next, position the camera to view the shadows from directly above so that we can use the resulting image to draw the shadows onto a ground plane polygon.
Step 4: Soften the Shadows
The shadows that are rendered by SketchUp always have hard edges. In order to make the shadows look more realistic, we can soften the shadows using software such as Photoshop or Gimp that includes an image blur tool. When you create the shadow image, you can use the "alpha" channel of the image to make portions of the image transparent. If you want fuzzy, semi-transparent shadows, then make sure to save the image out as a ".png" file including an (8 bit) alpha channel.
Step 5: Create a new Shadow Material
Next, we need to create a new material that uses the soft shadow image from the previous step as a texture. If the image that we created in the previous step has an alpha channel, then the alpha channel will be used to carve out transparent areas in the shadow material.
Apply the material to a ground polygon
Last, create a ground polygon that underlies the objects in the scene and apply the shadow material to it. This will create a semi transparent polygon where dark patches are the shadow areas. Since the shadows are pre-computed, you should turn off the Shadow option in SketchUp.
