In most API's that use handles, functions which effect the handle take the handle as their first argument. Take for example how we did our 2D framework:
// This is NOT OpenGL, it's from the 2D framewrok // First we intialize the texture manager TextureManager.Instance.Initialize(Window); // Next we generate a texture handle int texBird = TextureManager.Instance.LoadTexture("Assets/Bird.png"); // Finally we draw the texture by passing the handle // as the first paramater to the draw function TextureManager.Instance.Draw(texBird, new Point(0, 0));
OpenGL does not work like this.
OpenGL has the concept of a "currently bound texture". When you plan to use a texture for anything (rendering usually), you bind it to the OpenGL context (with the
GL.BindTexture function). After a texture is bound, all subsequent operations that use textures will use the currently bound texture.
This is what the function call for binding a texture looks like:
GL.BindTexture(TextureTarget proxy, int texId);
To bind no texture, set the second argument (texId) to 0. By default, no texture is bound. What is the first argument? That
TextureTarget enumeration.... It's a list of texture types that can be bound. We only care about the following entries:
There are a lot of other options, but seeing how those are the four texture types we can actually create, they are the only ones we care about.
What happens if you enable Texture2D, then try to bind a Texture1D object? Nothing. No errors, nothing. But also, nothing will be drawn. More often that not you will be using Texture2D for this argument. If you are not getting anything rendering on screen, make sure you have the right texture unit enabled.
A bound texture will remain bound until it is either unbound, or deleted. It's GOOD PRACTICE to unbind your texture (by binding 0), after you are done drawing with it.
Here is what we've learned so far:
// Enable Texturing GL.Enable(EnableCap.Texture2D); // Generate a texture handle int handle = GL.GenTexture(); // Bind the handle we generated as the active texture object GL.BindTexture(TextureTarget.Texture2D, handle); // ???
Now that we have generated a texture, and told OpenGL that we want to make changes to it by binding the texture handle, it's time to actually fill the texture with some data. We will discuss how to do this in the next section.