When comparing GameApi vs S2 engine HD, the Slant community recommends S2 engine HD for most people. In the question“What are the best 3D game engines?” S2 engine HD is ranked 45th while GameApi is ranked 58th. The most important reason people chose S2 engine HD is:
It is possible to edit weather conditions (rain, thunders,wind, clouds, night and day cycle) by simply moving some sliders.
Specs
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Pros

Pro All combinations of the features are working
In order to enable creation of 100 different kind of games, GameApi features are fully supporting all combinations of the existing features. This allows for huge amount of flexibility. The API carefully documents all dependencies between independent plugin-like modules and allows for combining their features in c++ code.

Pro Selection of 2d and 3d algorithms and techniques that construct opengl vertex arrays
The library is based on constructing opengl vertex arrays for 2d and 3d features including changing mesh colours, attaching textures, building larger worlds from smaller meshes, selection of predefined meshes like spheres, cubes and cones, Model loading from .obj files. Font support, gradient and other 2d bitmap operations, creation of 3d objects from bitmaps, Manipulation of points, lines, faces.
Pro Built-in weather system
It is possible to edit weather conditions (rain, thunders,wind, clouds, night and day cycle) by simply moving some sliders.
Pro UI is intuitively laid out
Important functions can easily be seen and accessed, making the engine quite user friendly, even for beginners or those who are new to the engine.
Pro Great rendering
S2ENGINE HD rendering power is comparable to more famous engines like Cryengine and UE4.
Pro Complete terrain editor
S2ENGINE HD is integrated with a complete terrain editor that lets you sculpt, paint, add vegetation, and add splines like roads, rivers, fences, etc.
Pro Easy to use
Essential functions are very easy to use and understand.
Cons

Con In alpha
GameApi has just recently moved from pre-alpha to alpha status.
Con Still in early access
Might be quite buggy.
