Converting OpenGL Primitives to OpenGLES - c++

Question

I'm trying to convert the following code from OpenGL 1.5 spec. to the OpenGLES 1.1 spec
(num_x and num_y are passed into the function by arguments)
::glBegin(GL_LINES);
for (int i=-num_x; i<=num_x; i++)
{
glVertex3i(i, 0, -num_y);
glVertex3i(i, 0, num_y);
}
for (int i=-num_y; i<=num_y; i++)
{
glVertex3i(-num_x, 0, i);
glVertex3i( num_x, 0, i);
}
::glEnd();
Here is my corresponding converted code: (ignore the inefficiency of my loops, I am trying to get the conversion to work properly first)
I'm trying to build two things here:
A float array of all the x,y,z coordinates needed to draw the grid
An index array of all the verticies needed.
Both arrays are then passed to OpenGL which renders them.
An example of what the arrays should look like:
GLshort indices[] = {3, 0, 1, 2,
3, 3, 4, 5,
3, 6, 7, 8, };
GLfloat vertexs[] = {3.0f, 0.0f, 0.0f,
6.0f, 0.0f, -0.5f ,
0, 0, 0,
6.0f, 0.0f, 0.5f,
3.0f, 0.0f, 0.0f,
0, 0, 0,
3, 0, 0,
0, 0, 0,
0, 6, 0};
int iNumOfVerticies = (num_x + num_y)*4*3;
int iNumOfIndicies = (iNumOfVerticies/3)*4;
GLshort* verticies = new short[iNumOfVerticies];
GLshort* indicies = new short[iNumOfIndicies];
int j = 0;
for(int i=-num_x; j < iNumOfVerticies && i<=num_x; i++,j+=6)
{
verticies[j] = i;
verticies[j+1] = 0;
verticies[j+2] = -num_y;
verticies[j+3] = i;
verticies[j+4] = 0;
verticies[j+5] = num_y;
}
for(int i=-num_y; j < iNumOfVerticies && i<=num_y;i++,j+=6)
{
verticies[j] = i;
verticies[j+1] = 0;
verticies[j+2] = -num_x;
verticies[j+3] = i;
verticies[j+4] = 0;
verticies[j+5] = num_x;
}
I need to also build an array if indicies to pass on. I 'borrowed' the array structure from the iphone 'teapot' example.
In each row, we have the number of indicies followed by the indicies referenced.
int k = 0;
for(j = 0; j < iNumOfIndicies; j++)
{
if (j%4==0)
{
indicies[j] = 3;
}
else
{
indicies[j] = k++;
}
}
::glEnableClientState(GL_VERTEX_ARRAY);
::glVertexPointer(3 ,GL_FLOAT, 0, verticies);
for(int i = 0; i < iNumOfIndicies;i += indicies[i] + 1)
{
::glDrawElements( GL_LINES, indicies[i], GL_UNSIGNED_SHORT, &indicies[i+1] );
}
delete [] verticies;
delete [] indicies;
Please add code questions as comments, not answers

Answer 1

I can see several things wrong with your converted code:
1.- Using the wrong type for the variable verticies, it should be:
GLfloat* verticies = new float[iNumOfVerticies];
2.- Incorrectly filling verticies, second loop should be:
for(int i=-num_y; j < iNumOfVerticies && i<=num_y;i++,j+=6)
{
verticies[j] = -num_x;
verticies[j+1] = 0;
verticies[j+2] = i;
verticies[j+3] = num_x;
verticies[j+4] = 0;
verticies[j+5] = i;
}
3.- Incorrect filling of indicies, I think you should delete the lines:
if (j%4==0)
{
indicies[j] = 3;
}
else
4.- Incorrect use of glDrawElements, replace the loop by this single line:
::glDrawElements( GL_LINES, iNumOfIndicies, GL_UNSIGNED_SHORT, indicies);