Comment by newzombie

This is the kind of 3d engines I was making as a teen. The way it works is that you iterate over x screen-space, get a "ray"/"slice", then you iterate over z camera-space (goes inward) using that ray. From there, you have what it takes to sample a heightmap. Draw a vertical line. There is plenty of room for optimizations and it's very good for learning to code.