Introduction

Originally, dynamic shadowing techniques were possible only in a limited way. But with the advent of powerful programmable graphics hardware, dynamic shadow techniques have nearly completely replaced static techniques like light mapping and semi-dynamic techniques like projected shadows. Two popular dynamic shadowing techniques are shadow volumes and shadow mapping.

A closer look

The shadow volumes technique is a geometry based technique that requires the extrusion of the geometry in the direction of the light to generate a closed volume. Then, via ray casting, the shadowed portions of the scene can be determined (usually the stencil buffer is used to simulate ray-casting). This technique is pixel-accurate and doesn't suffer from any aliasing problems, but as with any technique, it suffers from its share of disadvantages. Two major problems with this technique are that it is heavily geometry dependent and fill-rate intensive. Because of this, shadow mapping is slowly becoming more popular.

Shadow mapping on the other hand is an image space technique that involves rendering the scene depth from the light's point of view and using this depth information to determine which portions of the scene in shadow. Though this technique has several advantages, it suffers from aliasing artifacts and z-fighting. But there are solutions to this and since the advantages outweigh the disadvantages, this will be the technique of my choice in this article.

Soft shadows

Hard shadows destroy the realism of a scene. Hence, we need to fake soft shadows in order to improve the visual quality of the scene. A lot of over-zealous PHD students have come up with papers describing soft shadowing techniques, but in reality, most of these techniques are not viable in real-time, at least when considering complex scenes. Until we have hardware that can overcome some of the limitations of these techniques, we will need to stick to more down-to-earth methods.

In this article, I present an image space method to generate soft-edged shadows using shadow maps. This method doesn't generate perfectly soft shadows (no umbra-penumbra). But it not only solves the aliasing problems of shadow mapping, it improves the visual quality by achieving aesthetically pleasing soft edged shadows.

So how does it work?

First, we generate the shadow map as usual by rendering the scene depth from the light's point of view into a floating point buffer. Then, instead of rendering the scene with shadows, we render the shadowed regions into a screen-sized buffer. Now, we can blur this using a bloom filter and project it back onto the scene in screen space. Sounds simple right?

In this article, we only deal with spot lights, but this technique can easily be extended to handle point lights as well.

Here are the steps:

• Generate the shadow map as usual by writing the scene depth into a floating point buffer.
• Render the shadowed portions of the scene after depth comparison into fixed point texture, without any lighting.
• Blur the above buffer using a bloom filter (though we use a separable Gaussian filter in this article, any filter can be used).
• Project the blurred buffer onto the scene in screen space to get cool soft-edged shadows, along with full lighting.

Step 1