Archive for March, 2008

2008.03.28 - One Pass, Two Pass, Red Pass, Blue Pass

Here it is: another late-week journal update that pretty much chronicles my weekend accomplishments, only later.

But First, Beams!

First up, here’s a preview of the powered-up version of the final main weapon for the project:

Click to enlarge

The beam itself actually locks on to targets and continually damages them. Implementation-wise, it’s a quadratic bezier. Initially, I tried to calculate the intersection of a quadratic-bezier-swept sphere (i.e. a thick bezier curve) and a bounding sphere exactly. That’s all great, only it becomes a quartic equation (ax^4 + bx^3 + cx^2 + dx + e == 0), which is ridiculously difficult to compute programmatically (just check the javascript source on this page to see what I mean). So I opted for another solution:

I divided the curve up into a bunch of line segments, treated those segments as sphere-capped cylinders (capsules), and did much simpler intersection tests. PROBLEM SOLVED!

When Is Deferred Shading Not Deferred Shading?

I also implemented Light Pre-Pass Rendering, which is sort of a “Low-Calorie Deferred Shading” that Wolfgang Engel devised recently. Considering my original plan for lighting was to only allow right around 3 lights on-screen at a time, this gives me a much greater range of functionality. It’s a three-step process, as illustrated below.

Render Object Normals (Cut a hole in a box)

Step 1: render all of the objects’ normals and depth to a texture. Due to limitations that are either related to XNA or the fact that I want to allow a multisampled buffer, I’m not sure which, I can’t read from the depth buffer, so I have to render the object depth into the same texture that the normal map is rendering into.

Given two normal components, you can reconstruct the third (because the normal’s length is one). Generally, Z is used on the assumption that the Z component of the normal is always pointing towards the screen. However, with bump mapping (and even vertex normals), this is not a valid assumption. So just having the X and Y normal components is not enough. I decided to steal a bit from the blue channel to store the SIGN of the Z component. This leaves me with 15 bits of Z data, which, given the very limited (near-2D) range of important objects in the scene, is more than plenty for the lighting (as tested by the ugly-yet-useful “Learning to Love Your Z-Buffer” page).

Consequently, the HLSL code to pack and unpack the normals and depth looks like this:

float4 PackDepthNormal(float Z, float3 normal)
  float4 output;

  // High depth (currently in the 0..127 range
  Z = saturate(Z);
  output.z = floor(Z*127);

  // Low depth 0..1
  output.w = frac(Z*127);

  // Normal (xy)
  output.xy = normal.xy*.5+.5;

  // Encode sign of 0 in upper portion of high Z
  if(normal.z < 0)
    output.z += 128;

  // Convert to 0..1
  output.z /= 255;

  return output;

void UnpackDepthNormal(float4 input, out float Z, out float3 normal)
  // Read in the normal xy
  normal.xy = input.xy*2-1;

  // Compute the (unsigned) z normal
  normal.z = 1.0 - sqrt(dot(normal.xy, normal.xy));
  float hiDepth = input.z*255;

  // Check the sign of the z normal component
  if(hiDepth >= 128)
    normal.z = -normal.z;
    hiDepth -= 128;

  Z = (hiDepth + input.w)/127.0;;

And, it generates the following data:

Click to enlarge

That’s the normal/depth texture (alpha not visualized) for the scene. The normals are in world-space (converting from the stored non-linear Z to world position using the texcoords is basically a multiply by the inverse of the viewProjection matrix, a very simple operation).

Render Pure Lighting (Put your junk in that box)

Next step, using that texture (the object normals and positions), you can render the lights as a screen-space pass very inexpensively (the cost of a light no longer has anything to do with the number of objects it’s shining on, it’s now simply a function of number of pixels it draws on). Bonus points: you can use a variation on instancing to render a bunch of lights of a similar type (i.e. a group of point lights) in a single pass, decreasing the cost-per-light even further.

The lighting data (pure diffuse lighting, in my case, though this operation can be modified in a number of ways to do more material lighting types and/or specular lighting if necessary, especially if you have a separate depth texture) is rendered into another texture, and it ends up looking as follows:

Click to enlarge

That’s a render with three small point lights (red, green and blue in different areas of the screen) as well as a white directional light.

Render Objects With Materials (Make her open the box)

Finally, you render the objects again, only this time you render them with materials (diffuse texture, etc). However, instead of doing any lighting calculations at this time, you load them from the texture rendered in step 2.

This ends up looking like this (pretty much what you expect from a final render):

Click to enlarge

And that’s how light pre-pass rendering works, in a nutshell. At least, it’s how it works in my case, which is very simplistic, but it’s all I need for the art style in my game. It’s a lot easier on the resources than deferred shading, while still separating the lighting from the objects.


Hopefully, in my next update, I’ll have an actual set of background objects (as that’s the next item on the list, but it does require the dreadful tooth-pulling that is “artwork,” so we’ll see how quickly I can pull this together.

Until next time: Never give up, never surrender!

2008.03.16 - Progress: Like Regress, Only Forward!

It’s been tricky to make much progress these last couple weeks – having a (non-gaming) coding job and being able to come home and work gets tricky, so a large majority of my game coding time is weekend time. Also, couple some deadlines at work, and you’ve got a large case of “I don’t want to code when I hit home.”

However: I did make a good deal of progress these last few weeks.

If you look at the screenshot in my last entry, it should be plain exactly HOW MUCH. Suddenly, my little experiment looks considerably like a GAME.

Click to enlarge

High Quality Xvid, 32MB
Low Quality Xvid, 3.5MB

Particles Make Me Sneeze

The biggest hurdle for this section of the project was the general-purpose particle system. Even though I’ve done a bunch of crazy graphics-related stuff, a particle system has NEVER been on that list. But no longer!

For my particles, I wanted the following data:

  • Position (3D)
  • Rotation (around Z)
  • Image Index (which image in the particle map to use)
  • Scale (how big the particle is scaled, relative to the particle map’s specified world size)

The particle map mentioned in that list is a simple list of texture coordinates into the particle texture (Which contains images for all of the particles), as well as the size of a given particle in world space.

The particles in this system are actually rendered using shader constants (2 float4 shader constants per particle), which gave me right around 100 particles per draw call. On my PC, I can push the system up to 24,000 particles before it starts to slow down. On the Xbox 360, it’s closer to 6000. Both of those are well within my game’s target maximum of 2000 particles, and I could probably get that number higher if I had to.

The State Machine Tells Way Fewer Lies Than the Political Machine

One thing I learned when working on Mop of Destiny was how to set up a totally sweet state machine in C++. I got to port those concepts over to C#, which made it even EASIER, given all of the reflection support. Note that I do the majority of the reflection calls at application startup, so the expensive calls are already done when it’s time to do the actual game running.

Each state can have three functions associated with it: Begin, Tick, End.

Begin is called on the transition to that state from some other state.
Tick is called every time the state gets run (once per frame)
End is called on the transition from that state to some other state.

Also, each state can have a number of transitions associated. They take the form of: “BooleanFunction = TargetState”

Every frame, before calling tick, the state machine core will run each of the specified functions. When one of them evaluates to true, it switches states to the new TargetState, which will then be run (unless one of ITS transitions triggers). A state can also call the SetState function direction, but having the transitions in the function attribute makes it really easy to see where a state can transition to.

See You Later, Allocater!

One of the most important things that I have been doing with my code is ensuring that, during the run of a game level, no memory is allocated. At all. The reason is the .Net garbage collector (GC).

The GC, on Windows, is triggered every 2MB of allocations (among other scenarios, including low-memory and lost-focus cases). On the Xbox 360, the GC runs ever 1MB of allocations. Since the GC pauses all threads while it does its thing, it’s better that it’s never triggered during runtime…ESPECIALLY if the heap is complicated and garbage collection is going to take a while.

To handle this, I’ve created a few of my own data structures, including the OrderlessList<T>. I’ve used OrderlessLists alot throughout my code. Simply stated, it’s an array (allocated at the time of the object with some maximum number of elements) in which the order of the objects is unimportant (i.e. it can be reordered and it doesn’t matter). Given the property of being able to reorder at any time, removal from the list is a simple matter of copying the last list over the top of the element being removed, then decreasing the reported size of the list.

For the bullets (both the player and the enemy bullets), there’s an OrderlessList of live bullets, and an OrderlessList of dead bullets. Whenever a bullet is needed, an object is retrieved from the dead bullet list, given its properties, and added to the live bullet list. Whenever a bullet dies (goes off-screen or hits an enemy), it is deactivated and returned from the live bullet list to the dead bullet list. No allocations necessary.

That’s right, it’s the ol’ “pool of objects so you don’t have to allocate” trick. But hey, it works!

Rambling Is For Cowboys, Not Coders

Alright, enough talk! Tomorrow is another day at work, so it’s likely I won’t make any more progress until next weekend.

In the meantime, death is but a door, time is but a window; I’ll be back.

2008.03.05 - Mork Calling Drilian, Come in, Drilian

I haven’t had nearly as much time to get stuff done at home as I’d like, as work has been a bit of a scramble recently. Working ridiculously hard at a code-related day job and then coming home and trying to code is…difficult. And recently, highly unsuccessful.

However, this last weekend I was able to get a few things done.

Silos Are Not Just For Grain and Missiles

First up, I decided to check out Nevercenter Silo, a 3D modeling program that I swear has to be the easiest-to-use modeling software I have ever SEEN. For some reason, this software just completely clicks with me.

Maybe it’s that it allows me to start with something as simple as a box and push/pull/extrude/warp/etc it slowly into the shape that I want, or maybe it’s that it has built-in support for symmetrical modeling (you basically model HALF of a model and the other half changes shape along with it). It’s hard to say. However, it feels more like sitting down with clay and slowly morphing it into the shape that I want vs. the usually-cumbersome task of modeling a 3D mesh.

Consequently, not terribly long after I got it, I modeled what will likely be the first ship for my new game!

Click to enlarge

I used subdivision surfaces to do the modeling, so the source geometry (pictured to the left above) is actually rather simple. But when subdivided, it forms (what I think is) a pretty cool-looking spacecraft.

S.S. Procedural Is Leaving Port

I also managed to get my procedural content (textures, mostly) generation ported over to run on XNA (from C++, so it was a bit of work). Since I’m aiming to release this game as part of the XNA dealy on the 360 (with an option for XBLA if I’m really, really lucky), I’ve been taking great care in ensuring that it is going to run well on the Xbox 360. So far, so good. I have background asset generation working, etc etc.

But more importantly, I have my ship model loading into the game.

AND it’s procedurally textured using my super-spiffo texture generation framework, so that’s awesome, too.

Check it: (Yes, that is a wireframe bezier patch in the background and yes, that means I’m also going to have procedural geometry generation)

Click to enlarge

So, to sum: not a lot of coding done (the overwhelming majority of that has been done at work), but I do finally have some pretty new screenshots!