CODEWITHSUNDEEP
c++openglvbo
benjammin
benjammin

Reputation: 537

C++ Vertex Buffer Issue

I'm attempting to add a vertex buffer to the Mesh.cpp file of the pixel city procedural city-generating program. Part of the current code looks like this:

for (qsi = _quad_strip.begin(); qsi < _quad_strip.end(); ++qsi) {
  glBegin (GL_QUAD_STRIP);
  for (n = qsi->index_list.begin(); n < qsi->index_list.end(); ++n) {
    glTexCoord2fv (&_vertex[*n].uv.x);
    glVertex3fv (&_vertex[*n].position.x);
  }
    glEnd ();
}

This draws textures onto the rectangular sides of some of the buildings. Just going off the VBO tutorials I've found online, I attempt to convert this to use a vertex buffer like so (I store vboId in Mesh.h):

for (qsi = _quad_strip.begin(); qsi < _quad_strip.end(); ++qsi) {
    void * varray = (char *) malloc(sizeof(GLfloat)*5*qsi->index_list.size());
    GLfloat *p = (GLfloat*)varray;

    int counter = 0;
    for (n = qsi->index_list.begin(); n < qsi->index_list.end(); ++n) {
      memcpy(&p[counter+0],&_vertex[*n].uv.x, sizeof(GLfloat)*2);
      memcpy(&p[counter+2],&_vertex[*n].position.x, sizeof(GLfloat)*3);
      counter+=5;
    }
    glGenBuffersARB(1, &vboId);
    glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboId);
    glBufferDataARB(GL_ARRAY_BUFFER_ARB, sizeof(GLfloat)*5*qsi->index_list.size(), p, GL_STATIC_DRAW_ARB);

    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glEnableClientState(GL_VERTEX_ARRAY);

    glTexCoordPointer(2, GL_FLOAT, sizeof(GLfloat)*5, (GLfloat*)0); 
    glVertexPointer(3, GL_FLOAT, sizeof(GLfloat)*5,  (GLfloat*)2);

    glDrawArrays(GL_QUAD_STRIP, 0, qsi->index_list.size());

    glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0);

    glDisableClientState(GL_TEXTURE_COORD_ARRAY);
    glDisableClientState(GL_VERTEX_ARRAY);

    free(varray);
   }

However, this code simply doesn't work. Nothing is rendered. Sometimes, when I mess with the parameters into glTexCoordPointer or glVertexPointer, I really skewed/garbage data drawn on the screen (or the program crashes), but nothing that has even come close to remotely working.

Upvotes: 0

Views: 204

Answers (1)

derhass
derhass

Reputation: 45362

Your vertex pointer is wrong. When using VBOs, the pointer is interpreted as the byte offset relative to the currently bound buffer. So, you need sizeof(GLfloat)*2 bytes as offset.

glVertexPointer(3, GL_FLOAT, sizeof(GLfloat)*5,  (char*)0 +sizeof(GLfloat)*2);

As a side note: you could save the additional data copy if you would create the VBO with the correct size but NULL as the data pointer (which basically creates the data storage) and memory-map it with glMapBuffer(), completely avoivding the malloc()ed temporary buffer you use.

Upvotes: 1

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