Triangulating Concave and Convex Polygons — Ear Clipping

The Problem

OpenGL and other low-level rendering APIs are limited to rendering convex polygons. This causes a complications to arise when trying to render concave primitives. In my case, I wanted to be able to render arbitrary 2D "ground,"  and not succumb to manually breaking apart the polygon into OpenGL-compatible primitives. Thus the search for a triangulation algorithm arose. All I could find were algorithm names without accompanying implementations or explanations, and long mathematical abstracts on computational geometry. Any implementations I did find seemed to be extremely over-engineered and over-complicated for easy comprehension and adaptation. I finally settled on writing my own implementation of the ear-clipping algorithm to facilitate triangulation.

What We Will Accomplish

139-vertex polygon

Triangulated

Yet Another Quad Tree Tutorial

There was nothing to show for Screenshot Saturday this week, but that doesn't mean there hasn't been any progress! I spent the weekend writing a quad-tree implementation and integrating it into the existing engine. It was not a very hard process, and there are already dozens of tutorials available online for this, but this guide may still come in handy for someone.

How Do Quad Trees Work

You may be familiar with a binary tree. Essentially there's a "root" structure (known as a "node") that branches out into two nodes (binary = two). Then, each of these nodes splits into two of their own. This continues as long as necessary. The nodes that aren't split are called "leaf" nodes.
Quad trees work exactly the same way, except the nodes are split into four parts rather than two. This is useful for video games because the screen is a rectangle, and can thus easily be split into smaller subsequent rectangles. Here's a screenshot of a stress-test quad tree in progress:

The small, 8x8 quads represent particles with physics. Each quad you see can contain a maximum of 32 (an arbitrary value) particles before it automatically splits into 4 nodes. The tree cannot go more than 6 levels deep, though, because that would eventually cause a stack overflow due to the recursion inherent to quad trees.

You can see this live, in-action by downloading a demo here (pardon the dependencies, I used my old wrapper classes to write this). Hold keys to generate particles, or use the mouse to set them in certain locations. Right-click the mouse to test for a collision with the large blue quad. The result will show up in the console window as a 1 (true) or 0 (false). You can notice that the operation is performed incredibly quickly, even with an insane amount of particles on the screen. That is the power of a quad tree.

Note: If you get DLL errors, you may need to install the VC++ redistributable from here.

Making a Menu

I've been a bit quiet the past few weeks on here, but I've been actively participating in Screenshot Saturday on /r/gamedev. You can see last weeks here, and the one prior here.

This week, I've worked on creating a flexible, slick UI system for making menus. It proved to be pretty complex, and I used a seriously ass-backwards technique to get it accomplished.
You can think of a button as an physical entity, right? You need to be able to check collision, move it around for transitions, and swap textures easily. So I created a CButton class with a CEntity inside. At first, I just had a button texture rendered to the screen as a mesh, and the text rendered on top without being a part of the scene. This worked okay, but I wouldn't be able to add shader effects to the text portion of the buttons, which would look weird. So I had to have them both be a part of the scene.

The problem arose when I wanted to assign a texture to the entity. Since the button background is a texture, and the my text-rendering system renders directly to the frame buffer  there was no way to easily "blit" them together. I decided to create a frame buffer object for every instance of the button (main, hover, click) and render both parts of the button to it, then assign that texture to the button entity.
As you can see, this is serious overkill. It requires three render passes, two different shaders, and a ton of OpenGL state change. I'm not too worried about this, though, since it will only be called at the beginning of the game, prior to any performance-critical sections. It also works fast on my machine, no frame rate issues whatsoever.

After making the button class, I made another, higher-level class to wrap everything up for flexible menu creation. Now I can just call something like
     CMenu::AddButton("Button.tga", "Play Praecursor", math::rect_t(0, 0, 256, 64));
and the wrapper will automatically search for the hover and click versions of the button texture. I added these buttons to a pre-made background, added a lighting effect for a torch (it always looks good), and called it good!

Final result!

Animating 2D Sprites in OpenGL

In working on my latest game, Praecursor, it came time to develop a system for easily animating sprites on the screen. I found a very nice sprite for a hero on OpenGameArt, and wanted to integrate his various animations into player actions. In the process, I created a CAnimation class that would extend the CRigidBody class, making it eligible to act just like a physical entity and be rendered on the screen with relative ease. The entire process was fairly challenging, requiring a custom file format parser and a new fragment shader. The following is a tutorial-like thought process that went into the development of animate-able sprites.

Layout

The CAnimation class is supposed to act exactly like a regular physical entity, but should support loading of custom .icanim files and have various functions related to animation. These include toggling animation, setting an automatic animation rate, quick-swapping sprite sheets, and manually switching sprites.

Quick-swapping seems like a pointless functionality of an animation class; after all, why not just create a separate instance and load it with the new sprite sheet? Well, I didn't think about this until I started trying to swap-out animations for running, jumping, and standing with the main player instance. When I would just have a list of animations and do m_Player = m_allAnimations[JUMP], the player would lose his physics properties, such as gravity or jump force. I tried a few workarounds, but none of them turned out like expected, so I decided to add a SwapSpriteSheet() method to the CAnimation class. This will attach a new texture to the material and give the shader new parameters based on width and heights.

In the future, I think I will change the class to incorporate animation boundaries, so I don't need to load a separate image for each animate-able action. I would be able to do something like SetAnimationIndex(0, JUMP), and the animation would only loop through sprites [0:JUMP], then if I wanted to just play the standing animation, I could do SetAnimationIndex(JUMP + 1, STAND), assuming the standing animation comes after the jumping one in the sprite sheet. This would likely all but eliminate the need for the swapping method.

Rendering Text in OpenGL Using FreeType Fonts

EDIT (04.14.2013): Tutorial updated to handle new-lines (\n character)

I finally decided to tackle the rendering of TrueType fonts in my engine. It proved to be a much bigger challenge than I had originally anticipated; I perused over dozens of outdated guides, tutorials, code snippets, and documentation files to finally achieve something legible on screen. I decided to provide a complete guide to this process so that if anyone should decide to follow in my footsteps, they will know where to go.

FreeType 2 Installation

OpenGL works with pixels, and TrueType fonts are not stored like image files, so we need the FreeType 2 library to create font bitmaps. If you are well-versed in 3rd party API installation, feel free to skip this section. I have only performed the compilation using Visual Studio 2010 and 2012.

• Download the latest headers for FreeType 2 from SourceForge. As of this write-up, the latest version is 2.4.11.
• Unzip the archive and go to the builds folder to locate your platform. I will be demonstrating this with the win32/vc2010 build.
• Open freetype.sln in Visual Studio. If you are running VS2012, you will need to update the solution to use the latest compiler and libraries.
• Select the Release candidate and go to Build->Build Solution.
• The compiled .lib file will be located in objs/win32/vc2010/freetype[version].lib, where [version] is "2411" as of this writing.
• Copy this file to somewhere in your current project directory's library path.
• Copy the contents of the include/ folder to somewhere in your current project directory's include path.