Lighting Particles Without Pixel Shaders

In Prototype 2, the world is split up into three zones: green, yellow, and red. Each zone has a distinct style and color palette, as well as a few different times of day. Without lighting and shadowing, particles look wrong in many situations -- too flat, too light or dark, and sometimes just the wrong color (see Figure 3 for example). We realized they needed lighting but didn't want to add the expensive pixel shader code in order to do per-pixel shadowing and image-based lighting, as this would have vastly reduced the number of particles that we could render.

Figure 3A (top) has improved colors over Figure 3B (bottom).

Our solution was to do lighting per-vertex, but as a pre-pass into an intermediate "particle lighting" buffer. For each particle vertex, we render the lighting contribution to a pixel in the lighting buffer. This way we can use the pixel shader to do lookups into the shadow buffer and image-based lighting textures, using the same lighting code as the rest of the game and avoiding the performance pitfalls of vertex texture lookups on some platforms.

This lighting buffer is then read in the particle's vertex shader and combined with the vertex color, resulting in no extra instructions in the pixel shader. The only concern here was the performance of the vertex shader texture lookup on some platforms, particularly the PS3 and some earlier DX9 GPUs.

In these cases we actually rebind the particle lighting buffer as a vertex buffer and just read from it as we would any other vertex stream. This is trivial on the PS3 as we have full control over how memory is viewed and accessed, and for the DX9 GPUs that support it, we use the ATI R2VB extension (as detailed in this link [pdf]).

Putting It All Together 

Particles are a significant part of bringing the world of Prototype 2 to life. Various performance management systems work together to deliver effects without exceeding available resources. Lighting and shadowing add a huge amount of visual quality, and by doing it per-vertex, we are able to light every particle in the world at considerably less cost than we otherwise could have. And finally, one of the most important aspects of effects tech development is giving the artists the tools they need to do their job-and to help us do ours. After all, they're the ones that make us all look good!

The author would like to acknowledge Kevin Loose and Harold Westlund who authored many parts of the original Radical particle effects systems.

Listing 1: Add-Alpha Shader Code

// Add-alpha pixel shader. To be used in conjunction
// with the blend factors {One, InverseSourceAlpha}

float4 addalphaPS(
float4 vertexColour : COLOR0,
float2 uvFrame0 : TEXCOORD0,
float2 uvFrame1 : TEXCOORD1,
float subFrameStep : TEXCOORD2 ) : COLOR

{

// Fetch both texture frames and interpolate

float4 frame0 = tex2D( FXAtlasSampler, uvFrame0 );
float4 frame1 = tex2D( FXAtlasSampler, uvFrame1 );
float4 tex = lerp(frame0, frame1, subFrameStep);

// Pre-multiply the texture alpha. For alpha-blended particles,
// this achieves the same effect as a SourceAlpha blend factor

float3 preMultipliedColour = tex.rgb * tex.a;
float3 colourOut = vertexColour.rgb * preMultipliedColour;

// The vertex alpha controls whether the particle is alpha
// blended or additive; 0 = additive, 1 = alpha blended,
// or an intermediate value for a mix of both

float alphaOut = vertexColour.a * tex.a;
return float4( colourOut, alphaOut );

}