Question

I have an Nx4 array of vertices that I’d like to plot using glVertexArray and glDrawArray. as a 2D scatter plot. I currently initialize the array like so:

GLint data[4][MAX_N_POINT];

for (int j=0; j<MAX_N_POINT; j++)
{
    data[j][0] = 200 + rand()%20 - 10; // X Dimension
    data[j][1] = 400 + rand()%20 - 10; // Y Dimension
    data[j][2] = 600 + rand()%20 - 10; // Z Dimension
    data[j][3] = 1; // W Dimension
}

Then I draw the 2d scatter with:

glVertexPointer(4, GL_INT, 0, data);
glDrawArrays(GL_POINTS, 0, nPlot);

Which shows up correctly as this:

note: the colored squares in the background are so I can easily judge if the points are falling in the correct location

However as I’m only want a 2d plot I’d like to skip over the 2 unused dimensions. I tried telling glVertexPointer that my vertices only have 2 points and then setting the stride between vertices as 2 as well but that doesn’t seem to be working as I thought it should.

glVertexPointer(2, GL_INT, 2, data2); 
glDrawArrays(GL_POINTS, 0, nPlot);

However when I plot this I get a very different result with points showing up at XxY, YxZ, ZxW:

Have I misunderstood the proper function of the stride variable? If not how can I properly set the stride so the bottom plot resembles the top plot?

Answer 1

The stride is not the “gap” between vectors, the stride is the distance between vector start locations. So in your case your vectors are “sizeof(int)*4” apart, which is the stride distance to use:

glVertexPointer(2, GL_INT, sizeof(GLint)*4, data2);