A Guide to Instanced Geometry

As stated in the title, this is more of a guide than a tutorial. It isn't for the complete beginner, some experience is necessary. Throughout this I assume you have a general knowledge of what a mesh is, basic understanding of how matrices work, basic knowledge of GLSL, and experience with C++ or another object-oriented language. I am just writing about my own personal implementation of instanced geometry, which is definitely open for comments and suggestions. The code snippets are stripped down versions directly from my basic 2D OpenGL rendering engine I have dubbed IronClad.

What Is Instanced Geometry?

Primitive rendering techniques have many copies of a single object's data. Say you wanted to draw a tiled map, with 2 unique tiles. Say, for instance (no pun intended), a floor tile and a wall tile. Now, each of these objects contains, at the very least, 8 floats for vertex positions, and 8 floats for texture coordinates. That's 8 * 4 + 8 * 4 = 64 bytes. If each instance of a tile contains this information for rendering, and you have 1000 wall tiles, that's 64 kilobytes of memory! And that's not even considering the other tile types. Obviously, this is an example of a very simple mesh with only 4 vertices. Most games have models with hundreds if not thousands of vertices, so you can see why it'd be a serious problem to have multiple copies of that data.

Of course, there's a simple solution to this problem; you keep around one copy of the data in the first object you create, and the other objects simply refer to the original, just in their own position. Well, that's exactly where instancing comes in!

Working With Shadows

The logical course of action after finishing lighting is to move on to shadows. I opted for a relatively simple, software-based approach. It involves casting rays from the light source to individual edges of the tiles in the collision map, and drawing black quads based on the rays.

In my current implementation, the shadows are independent of the other lights. Thus, shadows cast by one light will be left untouched if there is another light in the way. This obviously causes problems, and a better method that will calculate cumulative shadows is currently in progress. But for a single light, the following algorithm suffices. Hopefully some OpenGL beginners trying to create shadows can gain some knowledge from my own trial-and-error!

The steps are as follows:

• Determine the light position.

• Cast a ray from the light to the first vertex (top-left point)
• Calculate slope and extend the ray, so instead of it being from the light to the vertex, it goes from the vertex to some really distant value.
• Keep this ray for later (I'll call it TopLeft later).

• Repeat this process for each vertex, so you get 4 rays, which I'll call TopLeft, TopRight, BottomRight, and BottomLeft in the example code below.

• Now that we have determined the sectors for shading, we test to see which quads are necessary to fill. This is not shown in the diagrams, but in practice, if the light is above the tile, the sector created by the TopLeft and TopRight rays will cut across the tile itself, which is obviously undesirable. The following must be true:
• TopLeft.Start.y < LightPos.y     : Draw quad
• TopRight.Start.x > LightPos.x    : Draw quad
• BottomRight.Start.y > LightPos.y : Draw quad
• BottomLeft.Start.x < LightPos.x  : Draw quad

• Draw the quads with whatever rendering method you wish, be it using glBegin()/glEnd(), glDrawArrays(), FBOs, or what have you.

Here is some C++ example source code using immediate mode rendering. CRay2 is just a class containing two CVector2's, which basically are an (x, y) coordinate pair. There is a minor bug with this method, when the LightPos.x is equal to the Ray.Start.x value, the shadow is not created as intended. When I find a fix, I'll update this code. If anyone has a fix, feel free to comment below.

Let There Be Light!

"And George said unto Collapse, "let there be light," and there was, and George saw that it was good."

Edit (2.1.2012): My lighting shader has changed quite a bit since this post; I opted for a multi-pass shader, rather than maxing out the shader variables.

After several weeks of reading about shaders, learning GLSL, learning about lighting, observing other projects, finally writing my own, and spending hours debugging, tweaking, and improving, it's finally done. My shader supports multiple lights, and gets re-written on the fly to support larger and larger amounts, due to GLSL loop limitations. Whenever I want to increase the amount of lights I use, I merely say:

    LightingShader.SetMacro("NUM_LIGHTS", ++lights);

Which will then re-write, re-compile, and re-link the shader with a new light count. Though I was considering not releasing the shader source code, here it is anyway:

Here are some screen-shots of lighting:

Original shader - one light (in-game)

Final shader - multiple lights (test zone)

AI Experimentation

Collapse is a game set in a post-apocalyptic world in which a human-created AI faction known as the Mechs have taken over. Therefore, AI must be an essential part of the programming.
After extensive research in path-finding algorithms, I decided upon the ubiquitous A*. Then, after days of trying to implement it, using many different techniques from just simple std::vector<Node*> lists to a much more complex and efficient std::priority_queue<Node> binary heap, I gave up. None of them worked properly, obviously due to faults in my programming, but I simple couldn't figure it out.
So I settled on a different approach: pre-defined movement paths. Obviously, this would significantly dumb-down AI decision-making, but I had to at least get something working. The map worked fine, but, again, after days of tweaking and planning and modifying, I couldn't even get the enemy tank to follow the path. I used a rather strange approach with lots of vector math and collision detection, which probably wasn't necessary. The algorithm (in pseudo-code) looked something like this:

Spawn:

    Target = FindNearestPath

    End

Update:

    If reached Target

        Target = FindNextTile

        If no Target

            Turn 3 degrees

    Else

        Adjust angle and drive.

    End

FindNearestPath:

    Find tile with minimum distance to entity.

    Return tile

FindNextTile:

    For each tile in AI map

        If tile collides with line-of-sight

          and tile not collides with entity

            Return tile

    Return no tile

Levels in Collapse

Collapse is the first game I've ever made that actually required levels, so it was quite interesting figuring out how to approach level design, loading, and storage. I definitely had to create some sort of level editor in order to easily design levels. Eventually, after much consideration, I decided that the easiest way to approach level handling would be to split every level into 4 basic components: Terrain, Collision, AI, and Game-Logic. 

Terrain

This map is pretty self-explanatory. It's stored with a .ctm extension (Collapse Terrain Map) and contains information about all the tiles on the map. Currently, since my artwork is fairly limited in the tile department, there are only two tiles available for placement: a floor and a wall. I knew that I'd be expanding later, and I couldn't bear to have a massive switch statement for every single type of file. So, I wrote a quick Python script to gather information about which tile images were available to use in the level, and then stored it in a file call ValidNames.dat. The script is really simple, since I have a special folder dedicated to files, it's pretty easy to find them all:

Collision

Having a separate map for collision makes it so I don't need to store which map tiles are passable and which aren't, and it also allows for more flexibility in terms of destroying tiles for any given reason. For example, when the player fires the tank weapons, if they make contact with the collision map, the collision tile contacted is removed and the terrain tile is changed to a "broken" version. The map is stored as a .ccm file (Collapse Collision Map) and just contains x, y coordinates of 32x32 tiles representing impassable areas.

AI

Now you may wonder, why would you need a map for AI? Well, after trying some basic path-finding techniques, A* in particular, I found that I could not do it efficiently enough. Plus, it would mean that all enemies were informed of the player's location, thus always swarming towards him.  So then I decided to opt for a "line-of-sight" approach, but there was way too much collision detection going on due to diagonal lines creating insanely large amounts of rectangles to check collisions with. Even after optimization, it was still ridiculously slow. So, now I've decided that enemies will patrol certain areas, following a predetermined path, and upon making line-of-sight contact with the player (using a much more efficient vector math approach rather than raw line-rectangle collision), turn to a more aggressive path-finding approach to destroy the player. 

Game-Logic

This map, which is still unimplemented, will contain information such as light sources, player spawn-locations, enemy spawn-locations, start/end level markers, maybe some sort of scripted event triggers, and other things that don't fit into the other categories.

My First Game - ShapeWars

When you're only a beginner in game development, it's hard to think of a fun, interesting, challenging, yet not overwhelming idea. More often than not, you're going to make a clone of some other game, be it Mario, Tetris, etc. I saw a few YouTube videos of the infamous game Geometry Wars on the Xbox Live! Arcade and thought it'd be a very perfect fit for a first game.

I spent several months developing my clone, Shape Wars, in C++ using SDL. I had a fairly decent game done, but for a first attempt, as most developers would agree, the code-base was a complete mess. It was impossible to extend or maintain the game in any way. So I started anew. I decided to approach it in a much more professional manner, maintaining a Github, a changelog, a readme, etc.

The release of version 1.0, after a complete re-write was (according to the changelog) on January 13th, 2012. Everything was re-done: the artwork, the music, the level system, the UI, everything. I switched from a level system that read from a file, to one that gradually generated more difficult enemies in increased numbers. I added a (in my opinion) VERY clean menu. From then on, it became a ton easier to maintain everything, but many flaws remained. Even today, there's a really nasty bug in the game that causes a segfault at the most inopportune moments. It has something to do with how I handle bullet and enemy deletion. Pesky std::list and iterators.

Here are a few screenshots:

Main Menu:

In-Game:

Download Link - Github (.zip)

Unwise

Why Start A Blog?

I asked myself this question many, many times before actually beginning. I originally thought it would be extremely time consuming and boring to maintain a dev-blog, but as the change-logs to my games got longer and longer, I felt that starting a blog would make my life easier. This thing should keep me pretty motivated to brainstorm my ideas, release previews, share screenshots, and express my thoughts. Generally, this blog is going to be full of stuff like tutorials, algorithms, code-snippets, screenshots / releases of my projects, and brainstorming ideas about problems I'm having.

My Background

After that, I decided that it was time to leave the turtle of SDL rendering behind and make the jump to 3D. After about a week of experimentation with OpenGL, I realized the amount of math I would need to know for even the simplest game was just too much. Plus, I have absolutely no idea how to use modeling programs, and have no desire to learn. So I retreated back to 2D, except using a mix of OpenGL and SDL. SDL for window and event handling, and OpenGL for rendering. So, with many new-found features at my disposal, I began developing my first full-fledged game: Collapse.
I only got into game-dev fairly recently, maybe a year or so ago. I started out experimenting with Python+PyGame, making simple Snake and Tic-Tac-Toe clones. I felt the curly-brackets calling back to me, so I made the switch to C++ and SDL. The last game I made in purely SDL, ShapeWars, was already pushing the limits of SDL when large quantities of bullets and enemies swarmed. I had to keep all of the game artwork fixed, rotation was simply too slow and not an options.