Making it robust
Now we have this far implemented, the shadows will be looking quite nice – when static – however problems will become apparent when moving the light sources around. The most glaring is that of self-intersection.
If the light source is too large in relation to the object, or too near, the inner penumbra edge will intersect the hull. First we need to detect this case. We find the vector from the boundary point to the next edge point in the shadow (moving anti clockwise here since we're on the shadow start boundary point). Then we compare the angle between the outer penumbra edge and our newly found hull edge, and the angle between the outer and inner penumbra edges. If the angle to the boundary edge is smaller, the we've got an intersection case we need to fix.
First we snap the current fin to the boundary edge, and calculate the new intensity for the inner edge via the ratio of the angles. Then we create a new shadow fin at the next point on the hull. This has an outer edge set to the same vector and intensity as the existing fins inner edge, while the new fins inner edge is calculated as before. By gluing these two fins together we create a single smooth shadow edge. Technically we should repeat the self-intersection test and keep adding more fins as needed, however I've not found that this is needed in practice.
You will also notice one other problem with this as it stands, the shadow fins will 'pop' along the edges of the hull as a light rotates. This is because we're still using an infinitely small point light to find the boundary points. To solve this we should take the physical radius into account when finding them. A robust way of doing this is to shift the light source position towards the hull by the radius distance before we find our boundary points. With these two fixes in place the fins will be visually stable as either the light or the hull moves (or both!).
Depending on the style of game and the view used (such as a side scrolling platformer as opposed to a top down shooter) the way light and shadow interacts with the various level objects will be different. What seems sensible for one may appear wrong in another. Most obviously is with objects casting shadows onto objects at the same depth.
The image above shows the same scene with different shadow offsets. Imagine that the scene is a top down viewpoint – the grey areas are impassable walls while the green-grey is the floor showing a T junction (imagine hard!). Now the image on the right seems slightly out of place – the shadows are being projected on top of the walls, yet these are much higher than the light source – realistically they shouldn't be lit at all but solid black walls aren't very visually appealing. The second shows the shadows being offset and only obscuring the floor and any objects on it.
Now if you were to imagine the same scene as a 2D platformer, you might prefer the left image. Here it seems to make more sense that the objects should shadow those on the same level. This decision is usually very dependant on the geometry and art direction of the level itself, so no hard and fast rules seem to apply. The best option seems to be to experiment and see which looks best.
Adding control over this is a small modification. At the moment the scene on the left is the common case, and by generating shadow volumes that are a close fit to the edge of the shadow caster we've already done all the hard work, all we need to do is store a shadow depth offset in our ConvexHull and apply it to the depth of the shadow geometry. The existing depth testing will reject fragments drawn behind other objects and leave them at the original intensity.
Emmissive / Self Illumination pass
This is a simple addition that can produce some neat effects – and can be seen as a generalisation of the wireframe 'full-bright' pass. After the lights have been drawn, we clear the alpha buffer again as before, but instead of writing light intensities into it we render our scene geometry with their associated emissive surface. This is an alpha texture used to control light intensities as before, and can be used for glowing objects, such as a computer display or a piece of hardware with a bank of LEDs - everything that has its own light source but is too small to require an individual light of its own. Because these are so small, we skip the shadow generation and can do them all in one go. Then we modulate the scene geometry by this alpha intensity as before. Unusual effects are possible with this, such as particles which light up their immediate surroundings, or the bright local illumination provided by a neon sign (with one or two associated lights to provide the lighting at medium and long range).
After a lot of work, much maths and a few sneaky tricks and we've finally got a shadow system that's both physically accurate and robust. The hardware requirements are modest indeed – a framebuffer with an alpha component is about all that's required, we don't even need to stray into extensions to get the correct effect. There is a whole heap of possible optimisations and areas for improvement, notably caching the calculated shadow geometry when neither the light nor the caster has changed, and including some sort of spatial tree so that only the shadow casters that are actually needed are used for each light.