# Lab 13: Particle Systems

## 0. Getting Started

Double-click on CS470_Lab13.zip and extract the contents of the archive into a subdirectory called CS470_Lab13

Navigate into the CS470_Lab13 directory and double-click on CS470_Lab13.sln (the file with the little Visual Studio icon with the 10 on it) which should immediately open Visual Studio with the project.

## 1. Particle Representation

Similar to the way we use points to render the trees in Lab 11, particle systems makes each particle that it renders into a point. These points are slightly different from those of the previous lab in that they are given an initial velocity, age, and type.

struct ParticleVertex {
D3DXVECTOR3 initialPos;
D3DXVECTOR3 initialVel;
D3DXVECTOR2 size;
float age;
unsigned int type;
};

## 2. Particle Motion

Particle motion allows for each point to move in its own unique way, while retaining a sense of uniformity throughout the system being created. A fairly straightforward and popular approach based on simple Newtonian physics for particle motion would be the equation:

p(t) = 1/2t^2a + tv0 + p0

For this equation, a always represents the acceleration of each particle. Acceleration due to gravity (-9.8 m/s^2) is the most common value of a.

## 3. Randomness

In order to allow the particles to start at different positions, travel at different angles and speeds, we add randomness to the system. Luna uses the functions RandF() and RandUnitVec3() to randomly initialize particles at the application level. We also need to have a form of randomness within the shader, which is a bit trickier in that there are no function calls for random numbers. The following code shows how values are randomly generated at the shader level.

// Returns random float in [0, 1).
D3DX10INLINE float RandF()
{
return (float)(rand()) / (float)RAND_MAX;
}

// Returns random float in [a, b).
D3DX10INLINE float RandF(float a, float b)
{
return a + RandF()*(b-a);
}

// Returns random vector on the unit sphere.
D3DX10INLINE D3DXVECTOR3 RandUnitVec3()
{
D3DXVECTOR3 v(RandF(-1.0f, 1.0f), RandF(-1.0f, 1.0f), RandF(-1.0f, 1.0f));
D3DXVec3Normalize(&v, &v);
return v;
}

What this code does is it creates a texture based on time. First we load in a fixed number of textures, and then based on a time offset, we can sample one of these textures in a 'random' fashion.

## 3. Stream Output

Stream Output allows us to send geometry that was created in the geometry shader to a vertex buffer. This is useful for particle systems because now, rather than updating our system on the CPU, we can use a seperate geometry shader, whose sole purpose is to update the particle system, to render our geometry at a later time.

The first thing we need to do is make sure that our stream output doesn't get rendered, because we only want it to update the particle system, and not render it. This is accomplished by disabling rasterization by turning off the pixel shader, and the depth stencil buffer.

Add the following code to fire.fx to disable rasterization.

// Disable Rasterization
DepthStencilState DisableDepth
{
DepthEnable = FALSE;
};

Next we need to define our stream output.

// Define Stream Output
"POSITION.xyz; VELOCITY.xyz; SIZE.xy; AGE.x; TYPE.x" );

We also need to define the stream output technique.

// Define the Stream Output Technique
technique10 StreamOutTech
{
pass P0
{

// disable pixel shader for stream-out only

// we must also disable the depth buffer for stream-out only
SetDepthStencilState( DisableDepth, 0 );
}
}

Creating a Vertex Buffer for Stream Output

In order to bind a vertex buffer to a stream output, the vertex buffer must be created with the D3D10_BIND_STREAM_OUTPUT flag. The D3D10_BIND_VERTEX_BUFFER needs to be set as well, due to the fact that the vertex buffer will need to be used later in the pipeline.

The following code initializes the vertex buffer in the application.

D3D10_BUFFER_DESC vbd;
vbd.Usage = D3D10_USAGE_DEFAULT;
vbd.ByteWidth = sizeof(ParticleVertex) * 1;
vbd.BindFlags = D3D10_BIND_VERTEX_BUFFER;
vbd.CPUAccessFlags = 0;
vbd.MiscFlags = 0;

Binding/Unbinding to the Stream Output Stage

In order to bind a vertex buffer to the stream output stage, the following code is used.

// Method for Binding to the SO stage
void ID3D10Device::SOSetTargets(
UINT NumBuffers,
ID3D10Buffer *const *ppSOTargets,
const UINT *pOffsets);

Unbinding from the stream output stage is done by binding a new buffer(normally NULL) to the stream output stage. That code looks like this

// Unbinding from the SO stage
ID3D10Buffer* bufferArray[1] = {0};
md3dDevice->SOSetTargets(1, bufferArray, &offset);

## 3. GPU Based Particle Systems

The particle system class handles related code for creating, updating, and drawing a particle system.

Emitter Particles Emitter particles are different from normal particles in that they allow the geometry shader to create and destroy particles. Depending upon the type of particle system, generally once a specific criteria is met, an emitter particle will spawn other particles.

Our particle system will be initialized by a single emitter particle, which will in turn create a full particle system.

The Update/Draw Method

In order to render our particle system, we'll need two techniques. A stream output technique, that updates the particle system, and a second technique, used to render the particles.

Add the following code to PSystem.cpp in order to use the first technique to update the particle system.

//
// Draw the current particle list using stream-out only to update them.
// The updated vertices are streamed-out to the target VB.
//
md3dDevice->SOSetTargets(1, &mStreamOutVB, &offset);

D3D10_TECHNIQUE_DESC techDesc;
mStreamOutTech->GetDesc( &techDesc );
for(UINT p = 0; p < techDesc.Passes; ++p)
{
mStreamOutTech->GetPassByIndex( p )->Apply(0);

if( mFirstRun )
{
md3dDevice->Draw(1, 0);
mFirstRun = false;
}
else
{
md3dDevice->DrawAuto();
}
}

// done streaming-out--unbind the vertex buffer
ID3D10Buffer* bufferArray[1] = {0};
md3dDevice->SOSetTargets(1, bufferArray, &offset);

// ping-pong the vertex buffers
std::swap(mDrawVB, mStreamOutVB);

Add the following code in order to use the second technique to draw the particles.

//
// Draw the updated particle system we just streamed-out.
//
md3dDevice->IASetVertexBuffers(0, 1, &mDrawVB, &stride, &offset);

mDrawTech->GetDesc( &techDesc );
for(UINT p = 0; p < techDesc.Passes; ++p)
{
mDrawTech->GetPassByIndex( p )->Apply(0);
md3dDevice->DrawAuto();
}

## 3. Fire

Now that our application is set up, it's time to begin writing our particle system techniques. First, we'll write our technique to update the particle system. The first technique is responible for emitting new particles, as well as destroying old ones.

// The stream-out GS is just responsible for emitting
// new particles and destroying old particles.  The logic
// programed here will generally vary from particle system
// to particle system, as the destroy/spawn rules will be
// different.
[maxvertexcount(2)]
void StreamOutGS(point Particle gIn[1],
inout PointStream<Particle> ptStream)
{

Next, we need to update the age, and then see if the current particle is an emitter. If the current particle is an emitter, check and see if it needs to create a new particle. NOTE We never destroy an emitter, so the particle system won't die.

// Update Age and check to see if we need to emit a
// new particle.
gIn[0].age += gTimeStep;

if( gIn[0].type == PT_EMITTER )
{
// time to emit a new particle?
if( gIn[0].age > 0.005f )
{
float3 vRandom = RandUnitVec3(0.0f);
vRandom.x *= 0.5f;
vRandom.z *= 0.5f;

Particle p;
p.initialPosW = gEmitPosW.xyz;
p.initialVelW = 4.0f*vRandom;
p.sizeW       = float2(3.0f, 3.0f);
p.age         = 0.0f;
p.type        = PT_FLARE;

ptStream.Append(p);

// reset the time to emit
gIn[0].age = 0.0f;
}

// always keep emitters
ptStream.Append(gIn[0]);
}

If the current particle is not an emitter, we check to see whether or not it should be destroyed.

//      If not an emitter, possible destruction
else
{
// Specify conditions to keep particle
if( gIn[0].age <= 1.0f )
ptStream.Append(gIn[0]);
}
}

Next we need to write the second(Draw) technique. First we need to write our vertex shader as follows.

// Prepare particle to be drawn
VS_OUT vOut;

float t = vIn.age;

// constant acceleration equation
vOut.posW = 0.5f*t*t*gAccelW + t*vIn.initialVelW + vIn.initialPosW;

float opacity = 1.0f - smoothstep(0.0f, 1.0f, t/1.0f);
vOut.color = float4(1.0f, 1.0f, 1.0f, opacity);

vOut.sizeW = vIn.sizeW;
vOut.type  = vIn.type;

return vOut;

As you can see, the vertex shader simple takes particle, moves the particle based on the position equation, fades the color, and then it sends its information to the geometry shader to be rendered.

Now the geometry shader for the drawing technique. In order to do so, the shader takes the point passed in from the vertex shader, and, similar to the tree billboarding lab, expands it into a quad. This quad is then textured, blended, and faces the quad towards the camera, giving a 3D illusion.

Add the following code to the geometry shader, it should look familiar.

// Draw Particle
// do not draw emitter particles.
if( gIn[0].type != PT_EMITTER )
{
//
// Compute world matrix so that billboard faces the camera.
//
float3 look  = normalize(gEyePosW.xyz - gIn[0].posW);
float3 right = normalize(cross(float3(0,1,0), look));
float3 up    = cross(look, right);

float4x4 W;
W[0] = float4(right,       0.0f);
W[1] = float4(up,          0.0f);
W[2] = float4(look,        0.0f);
W[3] = float4(gIn[0].posW, 1.0f);

float4x4 WVP = mul(W, gViewProj);

//
// Compute 4 triangle strip vertices (quad) in local space.
// The quad faces down the +z axis in local space.
//
float halfWidth  = 0.5f*gIn[0].sizeW.x;
float halfHeight = 0.5f*gIn[0].sizeW.y;

float4 v[4];
v[0] = float4(-halfWidth, -halfHeight, 0.0f, 1.0f);
v[1] = float4(+halfWidth, -halfHeight, 0.0f, 1.0f);
v[2] = float4(-halfWidth, +halfHeight, 0.0f, 1.0f);
v[3] = float4(+halfWidth, +halfHeight, 0.0f, 1.0f);

//
// Transform quad vertices to world space and output
// them as a triangle strip.
//
GS_OUT gOut;
[unroll]
for(int i = 0; i < 4; ++i)
{
gOut.posH  = mul(v[i], WVP);
gOut.color = gIn[0].color;
triStream.Append(gOut);
}
}

## 4. Compiling and running the program

Once you have completed typing in the code, you can build and run the program in one of two ways:

• Click the small green arrow in the middle of the top toolbar
• Hit F5 (or Ctrl-F5)

The output should look similar to below

To quit the program simply close the window.

Most Particle Systems follow this general pattern of creating and rendering, the only difference would be the criteria for the two techniques in their respective shaders.