CODEWITHSUNDEEP
objectopenglbuffervbovertex
sgtHale
sgtHale

Reputation: 1547

A simple Vertex Buffer Object (C++) that doesnt render

Im trying to use VBOs to render just a normal 2d textured square onto an FBO. Immediate mode functions work flawlessly but not this VBO. GL_TEXTURE_2D is already enabled for the code. What is wrong with it?

unsigned int VBOid = 0;
unsigned int Iid = 0;

float *geometry;
unsigned int *indices;

int num_geometry = 1;
int num_vertices = 4;
int num_indices = num_geometry*num_vertices;
geometry = new float[num_geometry*num_vertices*4];
indices = new unsigned int[num_indices];

indices[0] = 0;
indices[1] = 1;
indices[2] = 2;
indices[3] = 3;

/* Fill geometry: 0, 1, = vertex_xy
*                2, 3 = tex_coord_uv
*/
geometry[0] = 0.0f;
geometry[1] = 0.0f;
geometry[2] = 0.0f;
geometry[3] = 0.0f;

geometry[4] = 50.0f;
geometry[5] = 0.0f;
geometry[6] = 1.0f;
geometry[7] = 0.0f;

geometry[8] = 50.0f;
geometry[9] = 50.0f;
geometry[10] = 1.0f;
geometry[11] = 1.0f;

geometry[12] = 0.0f;
geometry[13] = 50.0f;
geometry[14] = 0.0f;
geometry[15] = 1.0f;

glGenBuffers(1, &VBOid);
glBindBuffer(GL_ARRAY_BUFFER, VBOid);
glBufferData(GL_ARRAY_BUFFER, sizeof(geometry), geometry, GL_STATIC_DRAW);

glGenBuffers(1, &Iid);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, Iid);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

//GL_TEXTURE_2D is already enabled here
//Buffers are already bound from above

glBindTexture( GL_TEXTURE_2D, 2); //I used 2 just to test to see if it is rendering a texture correctly. Yes, 2 does exist in my program thats why i arbitrarily used it
//glClientActiveTexture(GL_TEXTURE0); I dont know what this is for and where to put it

glEnableClientState(GL_TEXTURE_COORD_ARRAY);
//glActiveTexture(GL_TEXTURE0); same here I dont know what this is for or where to put it
glVertexPointer(2, GL_FLOAT, sizeof(GLfloat)*4, 0);
glTexCoordPointer(2, GL_FLOAT, sizeof(GLfloat)*4, (float*)(sizeof(GLfloat)*2));

glDrawElements(GL_QUADS, num_indices, GL_UNSIGNED_INT, indices);

glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);

glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

Upvotes: 3

Views: 488

Answers (1)

Christian Rau
Christian Rau

Reputation: 45948

The problem is your usage of sizeof(geometry) (and the same for indices) inside the glBufferData calls. Those variables are actually just pointers, no matter if they point to dynamically allocated arrays (which the compiler doesn't know). So you will always get the size of a pointer (4 or 8 bytes, depending on platform).

Replace sizeof(geometry) with num_geometry*num_vertices*4*sizeof(float) and sizeof(indices) with num_indices*sizeof(unsigned int). Well, in fact you don't need any indices here at all and can just draw the whole thing with a simple

glDrawArrays(GL_QUADS, 0, 4);

Always be aware of the differences between an actual (compile-time sized) array and a mere pointer pointing to a dynamicallly allocated array, with the result of the sizeof operator being one of those differences (and the requirement to free the memory of the latter using delete[] at some later point in time being another, but not less important, difference).

Upvotes: 3

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