HeadsUp NeHe Tutorials

Since time immemorial, I have learned from the NeHe OpenGL Tutorials.  These tutorials have been great, particularly up through the 2.1 version of OpenGL.  With the advent of more and more shader programming, WebGL, and other advancements, the old tutorials are now listed as “Legacy”.

These tutorials are still useful for a couple of reasons.  Not everyone programs with shaders as yet, and they are a great way to flush out all the challenges with a new OpenGL interface, such as what is in HeadsUp.  I have implemented tutorials 2-8, just for kicks.  The picture here shows lesson8.lua, which is about blending, some lighting, and using texture objects.  I’ve implemented an extremely simple brain dead Targa image viewer, just to get some images into texture objects.  The rest is pure OpenGL.

The way I’ve done it is to grab the GLUT or C++ version of the code if it exists, and then just do some massaging of the code until it compiles.  It’s typically a fairly simple straight forward process.  I’ve even added some of the most common glxxx functions to the global namespace for convenience.   One example is dealing with color.  Of course, you can be very explicit:

gl.glColor4f(1,o,o,1)

And if you want to do that without the ‘gl.’ prefix, you can simply do:

glColor4f(1,0,0,1)

That allows your code to look exactly like the typical ‘C’ version of the code. But wait a minute, this is Lua, so convenience is the name of the game. We can do some overloading and get an even better effect:

function glColor(...)
local arg={...};
    if #arg == 3 then
        gl.glColor3d(arg[1], arg[2], arg[3]);
    elseif #arg == 4 then
        gl.glColor4d(arg[1], arg[2], arg[3], arg[4]);
    elseif #arg == 2 then
        gl.glColor4d(arg[1], arg[1], arg[1], arg[2]);
    elseif #arg == 1 then
        if type(arg[1] == "number") then
            gl.glColor3d(arg[1], arg[1], arg[1]);
        elseif type(arg[1]) == "table" then
            if #arg[1] == 3 then
                gl.glColor3d(arg[1], arg[2], arg[3]);
            elseif #arg[1] == 4 then
                gl.glColor4d(arg[1], arg[2], arg[3], arg[4]);
            end
        end
    end
end

With this function, yoiu can use several calling conventions:

glColor(0.63)        -- Set a grayscale value
glColor(0.63, 0.5)   -- Set a grayscale value with alpha
glColor(0.25, 0.30, 0.30, 1)  -- Set a full color value
glColor({0.24, 0.30,0.30,1})  -- Set a full color, with alpha using a table
glColor({0.24, 0.30, 0.30})   -- Set a full color using a table

The only one that is missing is:

glColor(vec3(0.24, 0.3,0.3))  -- Set color using a vec (float[3])

If you are familiar with using the processing environment, this flexibility in setting color values might seem more familiar. There is something nasty about the difference between counting from ‘0’ as is typical in C, and counting from ‘1’ which is standard for Lua tables. By using this sort of construct, you can get it both ways. If you want the typical C version, including passing array structures, then use the standard C looking functions. If you want to pass your Lua based tables around, then use the more generic versions of the function, and pass tables around.

The same is true for Vertex objects.

I find this to be a useful construct. Although the flexibility can be a bit much when you try to think about the many ways you can do something, really it just feels natural because you just do whatever feels natural to your programming style, and it will probably work. You can stick with copy/paste of code you find from elsewhere, or you can taylor it to the Lua environment as suits your needs.

Now, on to those shaders!

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