JustDown

Overview

JustDown is a 3D game demo I wrote from scratch in C++ and OpenGL 3.3 with no engine underneath. It is loosely Only Up! in reverse: the player descends instead of climbs. Everything an engine usually hides, the renderer, shaders, post-processing, physics and level loading, I built by hand. That was the whole point.

Technical Highlights

• From-scratch OpenGL renderer. Manages the VAO/VBO setup, two vertex layouts (standard and parallax), framebuffers, and a uniform-location cache that avoids redundant lookups every frame. See Model.h.
• Steep parallax mapping. The fragment shader ray-marches a height map in tangent space with an adaptive layer count (8 at normal incidence, up to 32 at grazing angles) and interpolates between crossing samples to remove banding. See Parallax/FragmentShader.fs.
• HDR bloom with ping-pong buffers. A bright-pass extraction into RGB16F targets, a ten-pass separable Gaussian blur alternating between two framebuffers, then an additive composite. See PostProcess.h.
• Per-face TBN construction. Tangent and bitangent vectors are computed analytically from UV deltas and packed as extra vertex attributes, so every point light works correctly in tangent space. See Model.h.
• Physics with fall damage. Gravity-integrated movement, AABB collision, and floor-versus-wall discrimination from the contact normal, with fall damage scaled by drop height past a threshold. See Player.h.
• Declarative scene format. A plain-text .scene file lists models, textures, transforms, skybox and spawn point, and the loader picks the right shader variant from the texture count, so levels change with no recompile. See Level0.scene.

Learnings

Building the low-level stack myself gave me a much clearer picture of what engines quietly handle, from GPU buffer management to the framebuffer juggling that post-processing needs. It remains one of the most technically valuable things I have built.