Ray Marching

The Graphics Codex: Ray Marching

A good introduction of ray marching that progresses from naive ray marching to the sphere tracing algorithm. It then covers some common SDF primitives and operations, normal computation, and performance optimizations

Geometry/SDF
tutorial
c++
GLSL

Scratchapixel Volume Rendering

Scratchapixel's in-depth introduction to volumetric rendering using ray marching

Volumetric/Overview
tutorial
c++