Simulate the OpenGL Triangle Pipeline

OpenGL's Programmable Pipeline

OpenGL is a cross-platform API for 2D and 3D graphics. Despite being over 30 years old, its pipeline stages remain relevant because Vulkan, D3D12, and Metal expose the same flow with lower-level control.

The programmable pipeline stages:

1. Vertex Specification: upload vertex data via VBOs; optional EBO for indexed drawing
2. Vertex Shader: transform positions to clip space, pass varyings forward
3. Primitive Assembly: group vertices into triangles (every 3 vertices = 1 triangle)
4. Rasterization: convert triangles to fragments, interpolate attributes
5. Fragment Shader: per-fragment color, texture sampling, lighting
6. Per-Sample Operations: depth test, blending, framebuffer writes

Stateful Binding Before Draw

OpenGL is stateful: you bind buffers, shaders, textures, and uniforms, then call glDrawElements. For example, a single triangle draw might bind a VAO, set the MVP uniform, activate a texture unit, and issue one draw call producing thousands of fragments from three vertices.

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Modern APIs replace implicit global state with explicit pipeline state objects, but the stage sequence is identical. Debugging "why is my triangle invisible" almost always traces back to one of these stages.

For This Exercise

You will simulate the OpenGL pipeline for a single colored triangle, printing what happens at each stage. This task asks you to trace vertex transformation, attribute interpolation, and final fragment color. Understanding this pipeline is the prerequisite for every Vulkan exercise in this subtrack.