OpenGL vertices in shader storage buffer

Question

I'm working on a particle system using the compute shader right now. I put all my particles in a shader-storage-buffer. A particle contains two vertices, the current position and the previous position.

struct Particle{
    glm::vec4   _currPosition;
    glm::vec4   _prevPosition;
};

After I dispatch my compute shader I want to draw all the particles directly from the shader-storage-buffer. So this is what I do:

glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, shaderStorageBufferID);
_shaderManager->useProgram("computeProg");
glDispatchCompute((_numParticles/WORK_GROUP_SIZE)+1, 1, 1);
glMemoryBarrier(GL_SHADER_STORAGE_BARRIER_BIT);
_shaderManager->useProgram("shaderProg");
glBindBuffer(GL_ARRAY_BUFFER, shaderStorageBufferID);
glVertexPointer(4,GL_FLOAT, sizeof(glm::vec4), (GLvoid*)0);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_POINTS, 0, _numParticles);
glDisableClientState(GL_VERTEX_ARRAY);

The problem is that I see _numParticles on the screen but one half is rendered with the _prevPosition attribute of my particle structure. That means one particle is interpreted as two vertices which are drawn on the screen. But I want him to skip the _prevPosition attribute in every particle structure. Where's my mistake?

Maybe the way I initialize my shader-storage-buffer is important:

GLuint shaderStorageBufferID;

glGenBuffers(1, &shaderStorageBufferID);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, shaderStorageBufferID);
glBufferData(GL_SHADER_STORAGE_BUFFER, numParticles*sizeof(Particle), NULL ,GL_STATIC_DRAW);
struct Particle* particles = (struct Particle*) glMapBufferRange(GL_SHADER_STORAGE_BUFFER, 0, numParticles*sizeof(Particle), GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT);
for(int i = 0; i < numParticles; ++i){
    particles[i]._currPosition = glm::vec4(i, 0, 0.0, 1.0f);
    particles[i]._prevPosition = glm::vec4(i, 0, 1.0, 1.0f);
}

Answer 1

Your Particle struct contains two vec4's. Your compute shader writes two vec4s per array element.

Yet this line:

glVertexPointer(4,GL_FLOAT, sizeof(glm::vec4), (GLvoid*)0);

Tells OpenGL that you're passing an array of vec4s. You're not. You're passing an array, where every element is two vec4s. And you want to skip the second one.

So tell OpenGL that, by providing a proper stride:

glVertexPointer(4, GL_FLOAT, sizeof(Particle), (GLvoid*)0);

Oh, and BTW: you're still using the wrong barrier. Just because your code happens to function doesn't mean that it's guaranteed. Unsynchronized load/store operations can be tricky unless you're doing everything correctly.