A company called Logik State has announced their 3D engine for GBA, with screenshots.
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UK developer Logik State is showing off its new 3D polygonal engine for GBA, with screenshots at the link above. The screens show a car racing demo and a snowboarding demo. These could eventually become actual GBA games, but so far there is no publisher for them.
Here are some of the technical features, courtesy of Games Press:
15->20 fps 3D on a primarily 2d platform in a real game situationOver 2500 polygons per second when texture mapping the whole screenFull UV affine texture mapped polygonsFull 6 Degree's of freedomFull 3D collision systemHighly optimized ARM assemblyObject cullingBack-face cullingParticle systemN sided polygonsSupporting full 3D environments e.g. racing tracks,3D worldsObject LOD support3D sprites supportFull parent child object hierarchyProcedural texture supportAnimated texture supportAnimated camera(s) allow easy implementation of cut scenes and/or replaysRuns alongside a custom sound driver mixing 8bit digital PCM sound data with 6 channels for music and 2 channels for sound effectsCoupled with our custom sound driver this provides a complete solution for 3D graphics.
The company says they'll have movies of the engine up at their website soon.