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## A high-level object-oriented Game Boy Advance bitmap 3D software engine

**What's this?** A GBA MODE4 3D Software Engine - built from scratch.

This is (very) loosely based on David's 3D soft engine in C#/JS: https://www.davrous.com/2013/06/13/tutorial-series-learning-how-to-write-a-3d-soft-engine-from-scratch-in-c-typescript-or-javascript/

Engine blueprint: a stripped-down version of https://git.brainbaking.com/wgroeneveld/gba-sprite-engine combined with more *tonc* library functions.

### Show me the money!

Sure thing.

**Demo 1**: without wires

Camera rotates `2/256`

units on x and y each render cycle.
You 'should' see a cube forming based on 8 vertices. It's a simple example to showcase what the `Mesh`

class is about, and how `GBAEngine`

handles projections.

60 FPS. Yay.

**Demo 1b**: with wires

A lot less smooth. 20 FPS. I implemented Bresenham somewhat similar to David's approach, without any good results.
MODE4's weird byte write problems are causing trouble. Time to consult *tonc* and fix this:

30 FPS, winning 10 frames using `bmp8_line()`

.

**Demo 2**: load actual vertex content

From Blender/Babylon based on this.

A JS script has been provided that generates the needed C++ code. It's not great, and not very performant.

2 FPS. Ouch!! This thing has 507 meshes and 968 faces. GLHF! The GBA CPU does not seem to be very happy with that. Like everyone indeed says:

GBA Bitmap mode is not for gaming!

Use the `gba-sprite-engine`

instead.

I am aware of countless optimization opportunities but even thinking about that makes me sleepy. This is a high-level C++ engine, meaning redundant stack objects could also cause problems. And I'm okay with that - it's a proof-of-concept!

**Demo 3**: rasterization

The `RasterizerRenderer`

class draws triangles as 'fast' as possible, using horizontal scanlines.
There is a fast way to lines into VRAM. I tried implementing Z-buffering, but the buffer was too big and too slow as z-coords also had to be interpolated...

At this point, I do not think it's that interesting to go on to texture mapping other than the fun of it. Even with a lot of haxx and tricks, the colored monkey won't ever spin at 30FPS...

**Demo 3b**: with back-face culling

It did improve performance. I exported a few Babylon meshes, and the octahedron with 8 faces does run at 20FPS compared to 11FPS when back-face culling was implemented (that omits rendering certain faces if z < 0)

Changing colors indicate certain triangles were not drawn (into the background). It does not help a lot with our monkey, alas. Too many vertices...

More examples of meshes (box, cylinder, octahedron, sphere, torus) included.

### GBA-Specific problems

**Fixed-point math** sums up things nicely.

Numbers should be `.8f`

, so lots of shifting is needed. Problem with `Sqrt()`

BIOS functions is that they output `.8f`

but require `.16f`

?
So, calculating the length of a vector:

```
inline FIXED length() const {
FIXED toRoot = fxmul(v.x, v.x) + fxmul(v.y, v.y) + fxmul(v.z, v.z);
return Sqrt(toRoot << 8);
}
```

Second problem, sin/cos are expensive so we use *tonc*'s lookup tables - but they also come with weird requirements.

For instance, input is normally in radians. But we have fixed-point radians. And the lookup table is filled in `[1-512]`

slices but actually requires input in `[1-FFFFh]`

:

```
INLINE FIXED fxrad2lut(FIXED rad) {
int scale = fx2float(rad) / (2*M_PI / 512);
return (scale << 6) * 2;
}
INLINE FIXED fxsin(FIXED fxrad) {
if(fxrad == 0) return 0;
FIXED theta = fxrad2lut(fxrad);
FIXED sin = lu_sin(theta);
return fx12Tofx8(sin);
}
```

Divisions are a bit of a mess, but converting them to fixed-point and going back is not going to help much.
More details in `math.h`

.

In any case, lots of rounding errors occur. It is luckily not a problem due to GBA's limited screen dimensions.

**Limited iWRAM** size is another problem. Use `const`

arrays as much as possible!