Clarify someones work please

[Halsafar]

//$Author:        DigiBen            [email][email protected][/email]         //
    // We want to calculate the texture coordinate for the current vertex.
    // To do this, we just take the current x and y and divide them by the
    // MAP_SIZE of our terrain.  This is of course assuming the height map
    // has the same width and height.  Notice that we negate the v value (IE, (u, v)).
    // This is because we need to flip the texture upside down so it lines up correctly.
    // This simple math we use to find the texture coordinate can be explained easily.
    // We know that our terrain is made up of a grid, and that if we want to stretch
    // the entire texture over top of that grid, we just divide the current section
    // of the grid by the total section of the grid, which gives us a ratio from 0
    // to 1.  This works great because the (u, v) coordinates for a texture map go
    // from (0, 0) in the top left corner to (1, 1) in the bottom right corner.

    // Give OpenGL the current texture coordinate for our height map
    glTexCoord2f(   (float)x / (float)MAP_SIZE,    
                  - (float)z / (float)MAP_SIZE    ); 

That is correct, even I figured that out myself..

I do not get why the v has to be negetive....What is he talking about???
What does a -v do?

[NoteMe]

It is for flipping it up side down. If not the textures will not line up corectly. That is the right way to do it if you are rendering a terrain, from left to right, and then the next line of traingles/quads goes from right to left. If you draw it down on a paper then you see why. It's pretty logical when you can see it in front of you.