I can easily believe he wrote this in OpenGL in a month and a half. OpenGL is not as hard as people think, you don't draw 2D images in the hopes of contriving a 3D illusion, rather you literally specify vertices in 3D (provided you have a Shader) and then the shader converts them to 2D onscreen c...
Long story short: John Carmack did everything, a large amount of games simply used modified id Tech engines, (Half Life, Call of Duty, MOHAA etc,), even the source engine (Half Life 2, Team Fortress 2, Counter Strike Source) is a heavily modified quake engine. And John Carmack worked on all those...
I can easily believe he wrote this in OpenGL in a month and a half. OpenGL is not as hard as people think, you don't draw 2D images in the hopes of contriving a 3D illusion, rather you literally specify vertices in 3D (provided you have a Shader) and then the shader converts them to 2D onscreen c...