Image updated. New support for smooth border and soft shadow rendering.
New property 'Smooth Border' to for border smoothing. This is mostly used in directional light.
'Spread Size' is to control the shadow spread size, i.e. the penumbra size. This makes more sense in spot light. The value does not directly determine the spread size. It also depending on the model size, so you may need to tune it for each model. 'Spread Sample Size' specifies the internal sample size used for filtering for blurring. 0 is the cheapest which kind of cheated using dithering. So you may find some noise in the penumbra. size 1 usually performs worth than 0, but gets better with 2 and above. However, increasing sample size will have an obvious impact on rendering performance.
One way to improve performance is to decrease 'Precision'. When 'Precision' is 1, Coin3D uses a texture size of 2048x2048 to store the shadow map. If you want soft shadow, then you should reduce the the shadow map size, because smaller image is easy to blur obviously. the texture size used is Precision * 2048 rounding to power of 2. So, 0.1 corresponds to 256x256, or 0.2 for 512x512, or 0.5 for 1024x1024.
For more technical references, you can consult the commit message here
. Note that the 'smoothBorder' attribute mentioned there is for Coin3d internal use. I have expanded it into three FreeCAD properties for easy use.