tinyrenderer

This isn't a tool you install; it's a course. Tinyrenderer teaches you how 3D rendering actually works by having you build a software renderer from scratch in C++. No GPU, no OpenGL, no libraries. Just math and pixels. You start with drawing lines, move to triangles, add textures, implement lighting, build a z-buffer, and by the end you've written a basic 3D engine that can render textured, lit models. The whole thing is about 500 lines of code. Each lesson has theory, code, and visual output so you can see what each step does.

Completely free. The lessons are in the GitHub wiki. The code is public. No paid tier, no course fee, no upsell. One of the most popular computer graphics educational resources on GitHub, and it's been a go-to in graphics education for years.

The catch: this is not for beginners who've never coded. You need to be comfortable with C++ (or at least C-like syntax) and basic linear algebra (vectors, matrices, dot products). If that sounds intimidating, start with a gentler intro to graphics programming first. This teaches software rendering, useful for understanding how GPUs work, but you won't use this approach in production. For actual 3D work, you'll move to OpenGL, Vulkan, or a game engine.