Starting a GameDev Series
I tried Ground Control some time ago with my sons, it was working very nicely on Windows but was horribly slow on Linux with Wine. Visuals were flawless, just very slow. I started to investigate the Wine codebase to fix it. Little did I know what I embarked on.
Mission log in practice
I’m leading by example and therefore writing a mission log on my recent trip in graphics programming.
Wine as the starting point
Long story short, Ground Control (GC) uses a suboptimal way to draw its 3D
objects. It uses one
DrawPrimitive() call per polygon. It also leverages the
D3DPT_TRIANGLEFAN option, which makes it easy to draw any polygon.
The usual way is to use
D3DPT_TRIANGLESTRIP or at least
Unsurprisingly, it is written in the documentation of of Direct3D, hinting that batches of 100 triangles is optimal. More surprisingly, performance in Windows is actually pretty decent even without batching.
makes it impossible to batch per design. Which is why it was even removed in
more recent version of Direct3D1.
Yet in Wine, as each Direct3D call has some overhead, the slowdown is pretty horrible, specially for rendering the map, which is actually an endless loops of calls of triangles.
I simply patched a simple buffering of those calls. The result was that performance was even better than in Windows.
Problem solved some would say. Nonetheless I wasn’t satisfied as I wanted to understanding why there is so much overhead in Wine.
So I started to write a simple Direct3D application to have the simplest code that exhibits that performance issue, in order to profile it. I was very curious as what exactly the overhead was, and if solving it would be beneficial for other scenarios.
I suspected a synchronous wait somewhere in the call chain, finding which could lead to even more performance increase, and not only for that particular use case.
I didn’t know at first that writing that simple application would prove itfself much more complicated than anticipated. Yet it was a incredible learning experience.
I didn’t know anything about 3D programming before, so I spend some time reading online tutorials. I discovered that GFX programming evolved from “function calls” to “shaders”.
In a nutshell, it is the same as using plain SQL with pre-written helpers to write its own stored procedures.
As i am learning, and I usually go with history as it is much easier for me to understand gradually, I limited myself to the “fixed pipeline”. Which is also what GC used at the time.
Direct3D or OpenGL ?
At first I tried to write some D3D. It became soon apparent that the lack of a working environment to develop makes everything harder to learn.
Indeed, Microsoft evolved from d3d7 to d3d11 and d3d12 that dont have fixed pipelines anymore. Very much like OpenGL 3.1+ hasn’t support of it either.
Yet, while old D3D stuff mostly disappeared from the internet, old OpenGL one is still very easy to find.
Therefore after some initial struggle with d3d, I settled with OpenGL as my learning journey.
Departing from my initial goal
Now, with the journey destination revisited, I decided that rewriting a simple RTS game could be much more interesting than a synthetic testing application.
Simple, because I don’t have much skills in art, and I don’t want to spend all my time on eye candy. And it should still eventually serve as a benchmark for Wine if I port it to Direct3D.
Choosing the tech stack & the overall design
The language is C. No particular version, but it should be very easy to understand for beginners. Therefore avoiding new syntax, but still using obvious ones, such as C99 style comments. About “why not C++ ?”, it has lots of niceties, but is somehow overwhelming to enter a new C++ codebase. And I’m much more fluent in C than C++, so let’s not mix all the learning curves at once.
The codebase from Wine is very well written, which will serve as my reference for codestyle.
Also, modern compilers have gone a long way in optimizations, so that pointer juggling is not always needed. Even using a functional style and passing structs around by parameters & return value results in many cases in the same code than passing a pointer to a mutable structure.
So, the code will be always naive at first, and then optimized once it makes sense.
Also, some modding should be possible, but not via the usual lua extensibilty, but via the quake2 native DLL model. Some SDK should be provided so that building a mod without rebuilding the whole source code is possible. Very much like how one can build Linux Modules out of the source tree.
And finally, only OpenGL 1.2 features will be leveraged, as it is much simpler to understand, and port to other platforms later.
Using those limits narrow the scope nicely in order for the journey to have a chance to be successful.
It is deprecated in d3d9 and removed in d3d10. ↩