updated readme, shots

README.md

@@ -1,5 +1,80 @@
 ## A high-level object-oriented Game Boy Advance bitmap 3D software engine
 
-Engine layout: stripped-down version of [https://github.com/wgroeneveld/gba-sprite-engine/](https://github.com/wgroeneveld/gba-sprite-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://github.com/wgroeneveld/gba-sprite-engine/](https://github.com/wgroeneveld/gba-sprite-engine/) combined with more _tonc_ library functions. 
+
+#### Show me the money
+
+Sure thing. **Demo 1**: without wires
+
+![design](https://github.com/wgroeneveld/gba-bitmap-engine/blob/master/img/wireless.gif?raw=true)
+
+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
+
+![design](https://github.com/wgroeneveld/gba-bitmap-engine/blob/master/img/wired.gif?raw=true)
+
+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:
+
+![design](https://github.com/wgroeneveld/gba-bitmap-engine/blob/master/img/wired2.gif?raw=true)
+
+30 FPS, winning 10 frames using `bmp8_line()`.
+
+**Demo 2**: load actual vertex content from Blender/Babylon based on [this](https://david.blob.core.windows.net/softengine3d/part3/index.html):
+
+![design](https://github.com/wgroeneveld/gba-bitmap-engine/blob/master/img/monkey.gif?raw=true)
+
+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!
+
+#### 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:
+
+```C
+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]`:
+
+```C
+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. 
 
-**Work in progress**, using mode4 to create a software 3D engine from scratch.

demos/demo2-blender-import/converter.js

@@ -26,6 +26,7 @@ function done() {
         "}\n"
 }
 
+// partially reverse-engineered https://david.blob.core.windows.net/softengine3d/part3/index.html
 for(var meshIndex = 0; meshIndex < jsonObject.meshes.length; meshIndex++) {
     var verticesArray = jsonObject.meshes[meshIndex].vertices;
     var indicesArray = jsonObject.meshes[meshIndex].indices;

demos/demo2-blender-import/src/monkey.cpp

@@ -35,5 +35,5 @@ void MonkeyScene::tick(u16 keys) {
 
     TextStream::instance().setText(std::to_string(fps < 0 ? 0 : fps) + std::string(" FPS"), 1, 1);
 
-    monkey->rotate(2, 2);
+    monkey->rotate(10, 10);
 }