The init method
The first opportunity to execute any OpenGL calls is your canvas's init method (technical note - the method is actually an implementation of the GLEventListener interface, not the GLCanvas class). This is called when the underlying OpenGL context is created (and could conceivably be called again if the native context was lost and re-created, but it's generally OK to consider it a one-time deal). You can use this method to set up anything that will stay constant over the lifetime of the application. Tutorial 2 calls gl.setSwapInterval(1) which makes GL wait for the vertical blank interval before swapping the front and back buffers. This reduces shearing and allows for silky-smooth animation, and also prevents your game from trying to render frames more frequently than the monitor can physically display them.
init is also a good place to load textures, set up display lists and vertex arrays, and set global lighting parameters like the ambient light level.
The reshape method
The second opportunity to execute GL calls is in reshape. This is called whenever the viewport changes, and once before the first call to display. This is where you set up the viewport and the GL_PERSPECTIVE matrix, and it's as good a place as any to set the default GL_MODELVIEW matrix. Combined, these three settings are like defining the camera in your scene, but it can do things that no physical camera can.
The viewport is the 2d window that OpenGL will draw to. display is called with height and width parameters, and in most cases I can think of these are just passed straight through to glViewport. The only exception I can think of to that is if you are managing multiple views in the same context, and you wish to render to several different viewports. However I would look into multiple GLCanvas instances first.
On to the matrices. Despite the name, GL_PERSPECTIVE doesn't have to be a perspective transformation at all. Though a perspective view makes sense in a lot of games, if you're planning an RTS or other top-down view, or even a totally 2D game (OpenGL turns out to be great for 2d too -), then an orthogonal transform is what you want. There are two helper functions to create matrices, as otherwise the math can get tricky:
- glFrustum. A Frustum is a shape like an Incan pyramid, and defines the viewing volume for a perspective view, i.e. things further away are smaller. Suitable for first person shooters, many third person games, and anything where realism is key.
- glOrtho. Ortho is short for Orthogonal, meaning that vectors that were at right angles before the tranformation will still be at right angles afterwards. Take a moment to think why perspective transforms are not orthogonal. This function sets up the camera for a parallel projection, i.e. things further do not get smaller. This is handy for 2d games, or top down god games, RTSs and surprisingly more gameplay styles.
Both functions produce a matrix that defines the viewing volume. This is the volume of space that is visible to the camera. Note that it has a front and back - like a real camera, if something is too close to the lens you can't see it very well, and unlike a real camera there is a maximum distance beyond which OpenGL will ignore objects. There is a problem with setting the near plane to 0.00001 and the far plane to 9999999 though - depth buffer precision. Depth buffering is a fantastically useful technique we'll cover in detail later, but for now let's just note that we don't want the ratio of near:far to get too large, or you'll see rendering artifacts.
glFrustum takes six parameters. The first five define the position of the front face of the viewing volume - left, right, bottom, top and near. The sixth sets the far distance. The shape of the front face should be the same as the shape of your viewport - if the user drags out a tall, skinny window, then to prevent distorting your graphics, the view volume should be tall and skinny too, like in the following code:
float aspect = (1.0f) * height / width;
gl.glViewport(0, 0, width, height);
gl.glMatrixMode(GL.GL_PROJECTION);
gl.glLoadIdentity();
gl.glFrustum(.5f, -.5f, -.5f * aspect, .5f * aspect, 1.f, 500.f);
This sets up a pretty standard perspective view, where a square of side 1 would fill the screen if it was centered one unit in front of the camera - as long as the screen is square.
gl.glViewport(0, 0, width, height);
gl.glMatrixMode(GL.GL_PROJECTION);
gl.glLoadIdentity();
gl.glOrtho(-width / 64.0f, width / 64.0f, -height / 64.0f, height / 64.0f, 1, 100);
This view would show each unit square as a square 64 pixels across, no matter how large the screen is. Resizing the window would show more of the world, rather than scaling up the same view. It's the camera matrix that JavaPop uses.
Now the OpenGL context is set up and ready for you to draw your geometry in the display method. We'll get to that very soon.gl.glMatrixMode(GL.GL_MODELVIEW);
gl.glLoadIdentity();
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