Programmatically Generate OpenGL Vertex Data in iOS

Question

Basically what I want to do is generate my vertex coordinates programmatically instead of storing it in a statically predefined array. Unfortunately I am not able to convert a very simple example to a dynamic array.

Everything works fine if I stick to static arrays:

typedef struct {
    GLfloat Position[3];
    GLfloat Color[4];
    GLfloat TexCoord[2];
    float Normal[3];
} Vertex;


Vertex sphereVertices[] = {
    {{1, -1, 1}, {1, 0, 0, 1}, {1, 0}, {0, 0, 1}},
    {{1, 1, 1}, {0, 1, 0, 1}, {1, 1}, {0, 0, 1}},
    {{-1, 1, 1}, {0, 0, 1, 1}, {0, 1}, {0, 0, 1}},
    {{-1, -1, 1}, {0, 0, 0, 1}, {0, 0}, {0, 0, 1}}
};

GLubyte sphereIndices [] = {
    0, 1, 2,
    2, 3, 0
};

...

glGenBuffers(1, &sphereIndexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, sphereIndexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(sphereIndices), sphereIndices, GL_STATIC_DRAW);

glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, Position));

...

glDrawElements(GL_TRIANGLES, 6 * sizeof(GLubyte), GL_UNSIGNED_BYTE, 0);

As soon as I switch my indices to a dynamic array only the first triangle shows up.

GLubyte *sphereIndices;

+(void)initialize {
    sphereIndices = malloc(6 * sizeof(GLubyte));
    sphereIndices[0] = 0;
    sphereIndices[1] = 1;
    sphereIndices[2] = 2;
    sphereIndices[3] = 2;
    sphereIndices[4] = 3;
    sphereIndices[5] = 0;
}

Probably this has something to do with pointers. Does anybody know what I am doing wrong?

Thanks!

zlajo · Accepted Answer · 2012-08-15 10:30:00Z

4

I was able to solve the problem myself. As I already expected the problem had to do with my missing understanding of pointers.

As far as I found out, there is no way to get the size of a dynamic array. Because of that sizeof(sphereIndices) always returns 4 which is the size of the pointer not the size of the data. That's why glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(sphereIndices), sphereIndices, GL_STATIC_DRAW) only sends 4 indices to openGL instead of 6.

I fixed the problem by introducing an additional variable to keep track of the number of indices.

answered Aug 15, 2012 at 10:30

zlajo

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