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);