OpenGL VBO drawing problems

Question

I'm having a problem currently with a particle engine I'm making. With the engine you can add more than one emitter in to the engine, the idea being that each particle system can emit its own particles.

The problem I'm getting however is that when I add a second particle system, the drawing of the first seems to be affected, by which I mean it's not drawn at all. The draw call of each particle system is being called correctly.

What I am thinking the issue is however is that although multiple VBOs are created, only one is actually used.

I'll show the important parts of my functions that affect the VBOs. My shader uses a uniform location to store WVP matrices. I should also mention each particle system should be using its own shader program.

This below is my initializeBuffers function called when the particle system is created:

void ParticleSystem::InitializeBuffers()
{
    glGenVertexArrays(1, &VaoId);   
    glBindVertexArray(VaoId);

    //glGenBuffers(1, &VboId);
    glGenBuffers(1, &PositionBufferId);
    glGenBuffers(1, &IndexBufferId);
    glGenBuffers(1, &WVPId);

    std::list<Particle>::iterator iterator = particles.begin();
    //positions.reserve(5);

    for (std::list<Particle>::iterator iterator = particles.begin(), end = particles.end(); iterator != end; ++iterator) 
    {
        positions.push_back(iterator->GetPosition());
        //verticesToDraw.insert(verticesToDraw.end(), iterator->GetVertices()->begin(), iterator->GetVertices()->end());
        indicesToDraw.insert(indicesToDraw.end(), iterator->GetIndices()->begin(), iterator->GetIndices()->end());
    }


    //glBindBuffer(GL_ARRAY_BUFFER, VboId);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IndexBufferId);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indicesToDraw[0]) * indicesToDraw.size(), &indicesToDraw[0], GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER, WVPId);

    for (unsigned int i = 0; i < 4 ; i++) {
        glEnableVertexAttribArray(WVP_LOCATION + i);
        glVertexAttribPointer(WVP_LOCATION + i, 4, GL_FLOAT, GL_FALSE, sizeof(Matrix4f), (const GLvoid*)(sizeof(GLfloat) * i * 4));
        glVertexAttribDivisor(WVP_LOCATION + i, 1);
    }

    for(std::list<BaseBuildingBlock*>::iterator iterator =  buildingBlocks.begin(), end = buildingBlocks.end(); iterator != end; ++iterator)
    {
        (*iterator)->InitializeBuffer(programId);
    }

    /*
    glBindBuffer(GL_ARRAY_BUFFER, WorldId);

    for (unsigned int i = 0; i < 4 ; i++) {
    glEnableVertexAttribArray(WORLD_LOCATION + i);
    glVertexAttribPointer(WORLD_LOCATION + i, 4, GL_FLOAT, GL_FALSE, sizeof(Matrix4f), (const GLvoid*)(sizeof(GLfloat) * i * 4));
    glVertexAttribDivisor(WORLD_LOCATION + i, 1);
    }
    */
    //return GLCheckError();
}

This is the draw function and the code that actually draws the instanced elements, the wvp matrices are formed by the particle system earlier in the function.

void ParticleSystem::Draw(Matrix4f perspectiveCameraMatrix)
{
    // scale TEST
    //GLint gScaleLocation = glGetUniformLocation(program, "gScale");
    //assert(gScaleLocation != 0xFFFFFFFF);
    //glUniform1f(gScaleLocation, scale);

    //Pipeline p;
    //Matrix4f* WVPMatrices = new Matrix4f[particles.size()];
    //Matrix4f* WorldMatrices = new Matrix4f[particles.size()];

    WVPMatrices.clear(); 
    WorldMatrices.clear(); 

    glUseProgram(0);

    glUseProgram(programId);

    //Matrix4f perspectiveMatrix;
    //perspectiveMatrix.BuildPerspProjMat(90,1, 0.01, 200, 100 - 0 /*getWidth() / 32*/, 100 - 0  /*getHeight() / 32*/);


    //********************************************************************************************************
    // Method 1

    // Think I need to next define a camera position.

    if(particles.size() == 0)
    {
        return;
    }

    verticesToDraw.clear();


    Matrix4f scaleMatrix;
    Matrix4f worldMatrix;
    Matrix4f rotateMatrix;
    Matrix4f finalMatrix;

    //ColourId = glGetUniformLocation(programId, "UniformColour");


    int i = 0;



    for (std::list<Particle>::iterator iterator = particles.begin(), end = particles.end(); iterator != end; ++iterator) 
    {

        verticesToDraw = *iterator->GetVertices();
        indicesToDraw = *iterator->GetIndices();
        //positions.push_back(iterator->GetPosition());
        worldMatrix.InitTranslationTransform(iterator->GetPosition().x, iterator->GetPosition().y, iterator->GetPosition().z);
        rotateMatrix.InitRotateTransform(iterator->GetRotation().x, iterator->GetRotation().y, iterator->GetRotation().z);
        scaleMatrix.InitScaleTransform(iterator->GetScale().x, iterator->GetScale().y, iterator->GetScale().z);
        finalMatrix = perspectiveCameraMatrix * worldMatrix * rotateMatrix * scaleMatrix;
        //p.WorldPos(iterator->GetPosition());
        //p.Rotate(iterator->GetRotation());
        WVPMatrices.push_back(finalMatrix.Transpose());

        /*glUniform4f(ColourId, iterator->GetColour().r, iterator->GetColour().g, iterator->GetColour().b,
        iterator->GetColour().a);*/
        //WorldMatrices[i] = p.GetWorldTrans();
        i++;

        //iterator->Draw();
    }


    //glEnableVertexAttribArray(0);

    if(colourOverLifeBuildingBlock != NULL)
    {
        colourOverLifeBuildingBlock->Test();
    }

    glBindBuffer(GL_ARRAY_BUFFER, VboId);
    glBufferData(GL_ARRAY_BUFFER, verticesToDraw.size() * sizeof(verticesToDraw[0]), &verticesToDraw.front(), GL_STATIC_DRAW);
    glEnableVertexAttribArray(POSITION_LOCATION);
    glVertexAttribPointer(POSITION_LOCATION, 3, GL_FLOAT, GL_FALSE, 0, 0);

    int size = particles.size();

    glBindBuffer(GL_ARRAY_BUFFER, WVPId);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Matrix4f) * size, &WVPMatrices.front(), GL_DYNAMIC_DRAW);

    glDrawElementsInstanced(GL_TRIANGLES, indicesToDraw.size(), GL_UNSIGNED_BYTE, 0, particles.size());

    glBindBuffer(GL_ARRAY_BUFFER, 0);

    //glDisableVertexAttribArray(0);
    //glFlush();

}

The particle system entire header is below:

#include <gl\glew.h>
#include <array>
#include <vector>

class ParticleSystem
{

public:
    ParticleSystem(Vector3 pos, Quaternion rot, float spawnRate, int particlesToSpawn); // Constructs a particle system.
    ~ParticleSystem(); // Destructor.
    void Update(float elapsedTime); // Updates the particle system.
    void Draw(Matrix4f perspectiveMatrix); // Draw the particle system
    void CreateShaders();
    void InitializeBuffers();

    // Long amount of get sets.
    /*float* GetMinLifeTime();
    void SetMinLifeTime(float lt);
    float* GetMaxLifeTime();
    void SetMaxLifeTime(float lt);*/
    int* GetParticlesToSpawnAtATime();
    void SetParticlesToSpawnAtATime(int particlesToSpawn);
    float* GetSpawnRate();
    void SetSpawnRate(float spawnRate);
    Vector3* GetPosition();
    void SetPosition(Vector3 newPosition);
    Quaternion* GetRotation();
    void SetRotation(Quaternion rotation);
    std::list<BaseBuildingBlock*> GetBuildingBlocks();

    VelocityBuildingBlock* GetVelocityBuilding();
    ColourOverLifeBuildingBlock* GetColourOverLifeBuildingBlock();
    LifeTimeBuildingBlock* GetLifeTimeBuildingBlock();
    UniformColourBuildingBlock* GetUniformColourBuildingBlock();
    ScaleBuildingBlock* GetScaleBuildingBlock();
    /*Vector3* GetMinVelocity();
    void SetMinVelocity(Vector3 min);
    Vector3* GetMaxVelocity();
    void SetMaxVelocity(Vector3 maxVelocity);*/
    Vector3 GetMinParticleStartPoint();
    void SetMinParticleStartPoint(Vector3 minParticleStartPoint);
    Vector3 GetMaxParticleStartPoint();
    void SetMaxParticleStartPoint(Vector3 maxParticleStartPoint);
    bool CreateColourOverLifeBuildingBlock();
    bool DeleteColourOverLifeBuildingBlock();
    bool CreateUniformColourBuildingBlock();
    bool DeleteUniformColourBuildingBlock();
    bool CreateScaleBuildingBlock();
    bool DeleteScaleBuildingBlock();


    /*Colour GetStartColour();
    void SetStartColour(Colour startColour);
    Colour GetEndColour();
    void SetEndColour(Colour endColour);*/
    Vector3* GetMinParticleRotationAmountPerFrame();
    void SetMinParticleRotationAmountPerFrame(Vector3 minParticleRotationAmount);
    Vector3* GetMaxParticleRotationAmountPerFrame();
    void SetMaxParticleRotationAmountPerFrame(Vector3 maxParticleRotationAmount);
    void Save(TiXmlElement* element);

private:
    // Spawns a particle.
    void SpawnParticle();

    GLuint VaoId;
    GLuint VboId;
    GLuint IndexBufferId;
    GLuint PositionBufferId;
    GLuint WVPId;
    GLenum programId;

    std::vector<GLfloat> verticesToDraw;
    std::vector<GLubyte> indicesToDraw; 
    std::vector<Vector3> positions;
    std::vector<Matrix4f> WVPMatrices; 
    std::vector<Matrix4f> WorldMatrices; 

    std::list<Particle> particles; // List of particles
    Vector3 position; // position of the emitter
    Quaternion rotation; // rotation of the emitter.
    float spawnRate; // spawnrate of the emitter.
    int particlesToSpawnAtATime; // The amount of particles to spawn at a time.
    float minLifeTime; // The minimum time a particle can live for.
    float maxLifeTime; // The maximum time a particle can live for.
    float timer; // Timer
    ShaderCreator* shaderCreator;

    //Vector3 minVelocity; // The minimum velocity a particle can have.
    //Vector3 maxVelocity; // The maximum velocity a particle can have/



    //std::list<BaseBuildingBlock> buildingBlocks;

    // I'm thinking of eventually making a list of baseBuildingBlocks. 
    std::list<BaseBuildingBlock*> buildingBlocks; 
    VelocityBuildingBlock* velocityBuildingBlock;
    ColourOverLifeBuildingBlock* colourOverLifeBuildingBlock;
    LifeTimeBuildingBlock* lifeTimeBuildingBlock;
    UniformColourBuildingBlock* uniformColourBuildingBlock;
    ScaleBuildingBlock* scaleBuildingBlock;

    Vector3 minParticleStartPoint; // The minimum position a particle can start at.
    Vector3 maxParticleStartPoint; // The maximum position a particle can start at.
    Vector3 minParticleRotationAmountPerFrame; // The minimum amount of rotation that a particle can rotate every frame.
    Vector3 maxParticleRotationAmountPerFrame; // The maximum amount of rotation that a particle can rotate every frame.
    Colour startColour; // StartColour is the colour that a particle will start with.
    Colour endColour; // EndColour is the colour that a particle will end with.

    //TEST
    float scale;

};

#endif

Now I'm wondering, is there some way I have to switch the active VBO? or am I totally on the wrong track. I used a shader debugger and both VBOs defiantely exist.

Answer 1

you'll need to correctly set up your vertex attribs before each draw call - i.e., you have to call glBindBuffer followed by glEnableVertexArray & glVertexAttribPointer for each of your attributes before each draw call. in the code you posted, this happens only for the particle position, but not for the 'WVP_LOCATION' attribute which apparently contains your transformation matrices ( you do upload the data to the GPU via glBufferData, but don't set up the attribute ) - meaning that once you have more than one particle system, only the transformation matrices of your second particle system are ever going to be accessed for rendering.

one a side not, what you're trying to do here seems to be quite inefficient - you're essentially pushing one transformation matrix to the GPU for each of your particles, per frame. Depending on how many particles you want, this is going to kill your performance - you should consider updating the particle's position etc. with a transform feedback.

edit: just realized that the opengl wiki link doen't really explain a lot. a transform feedback is a way to record vertex shader outputs ( or, if a geometry / tessellation shader were present, that output would be recorded instead ). The output variables are written into a VBO - afterwards, they can be used for rendering like any other vertex attribute. The whole concept is extremely similar to using a framebuffer object for recording fragment shader outputs; It allows for particle systems that exist entirely on the GPU, with a vertex shader computing the updated position, life time & other attributes in each frame. A very nice tutorial, which shows the basic setup of such a transform feedback, can be found here