Question

I am new to OpenGL as learning exercise I decided to draw a set of horizontal lines from a grid of m x n matrix containing the vertices locations

This is what I have

and If I use LINE_STRIP

A code snippet using vertex arrays and indices will be great, I cant seem to be able to get the concept just from a text book I need to see and play with a code example
Any help will be much appreciated!

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@Thomas
Got it working with the following code

totalPoints = GRID_ROWS * 2 * (GRID_COLUMNS - 1);
indices = new int[totalPoints];

points = new GLModel(this, totalPoints, LINES, GLModel.DYNAMIC);
int n = 0;
points.beginUpdateVertices();
for ( int row = 0; row < GRID_ROWS; row++ ) {
  for ( int col = 0; col < GRID_COLUMNS - 1; col++ ) {
    int rowoffset = row * GRID_COLUMNS;
    int n0 = rowoffset + col;
    int n1 = rowoffset + col + 1;

    points.updateVertex( n, pointsPos[n0].x, pointsPos[n0].y, pointsPos[n0].z );
    indices[n] = n0;
    n++;

    points.updateVertex( n, pointsPos[n1].x, pointsPos[n1].y, pointsPos[n1].z );
    indices[n] = n1;
    n++;
  }
}
points.endUpdateVertices();

Then I update and draw by doing

points.beginUpdateVertices();
for ( int n = 0; n < totalPoints; n++ ) {
   points.updateVertex( n, pointsPos[indices[n]].x, pointsPos[indices[n]].y, pointsPos[indices[n]].z );
}
points.endUpdateVertices();

This is the result

---

Fix it by changing the nested for loop

for ( int col = 0; col < GRID_COLUMNS; col++ ) {
for ( int row = 0; row < GRID_ROWS - 1; row++ ) {
    int offset = col * GRID_ROWS;
    int n0 = offset + row;
    int n1 = offset + row + 1;
    indices[n++] = n0;
    indices[n++] = n1;
  }
}

Now I can have any number of rows and columns

Thanks agin!

Answer 1

You need to draw a line for each segment and resuse an index, i.e. for the first part you’d draw a line for (0,1), (1,2), (2,3) and so on.

Edit:

Suppose you have a 4×5 array (4 lines, 5 vertices per line). You could then calculate the indices like this (pseudo code):

Vertex[] v = new Vertex[20]; // 20 vertices in the grid
for(int row = 0; row < numrows; row++) // numrows = 4
{
  int rowoffset = row * numcols ; //0, 4, 8, 12
  for(int col = 0; col < (numcols - 1); col++) //numcols = 5
  {
     addLineIndices(rowoffset + col, rowoffset + col +1); //adds (0,1), (1,2), (2,3) and (3, 4) for the first row
  }
}

Then issue the draw call for numrows * (numcols - 1) linesegments (GL_LINES), i.e. 16 in the example. Note that addLineIndices would be a function that adds the index pair for one line segment to an index array which is then supplied to the draw call.