Rasterization

In simple terms

Rasterization is figuring out which pixels are covered by a shape and what colour each one should be. Given a triangle in 3D space, the rasterizer projects it onto the screen and decides which pixels are inside the projection.

More detail

A simplified rasterization pipeline for a single triangle:

1. Vertex shading — transform each vertex from model space to screen space; compute per-vertex attributes (colour, UVs, normals).
2. Primitive assembly — group vertices into triangles.
3. Clipping — discard or split triangles that fall outside the view.
4. Triangle setup — compute edge equations and depth gradients.
5. Rasterization proper — for each pixel inside the triangle, generate a fragment.
6. Fragment / pixel shading — compute the final colour, sampling textures, evaluating lights.
7. Depth test / blending — keep the nearest fragment; blend transparency.

Modern GPUs run this pipeline in massive parallel and at billions of triangles per second. APIs include Vulkan, DirectX 12, Metal, WebGPU.

Rasterization is fast but it answers only “what is at this pixel?” without thinking about light bouncing around. Ray tracing answers questions of visibility and lighting more accurately at a much higher cost. Modern engines blend the two: rasterize the bulk and ray-trace reflections, shadows, or ambient occlusion selectively.

Why it matters

Real-time graphics — games, mapping, design tools, virtual production — is rasterization. It is the technique that turns geometry into the pixels you see at 60 or 120 frames per second.

Real-world examples

• A AAA game renders millions of triangles per frame via rasterization.

• A SaaS dashboard with a 3D chart in WebGL uses rasterization under the hood.

• The font on this page is rasterized from vector outlines.

• Real-time path-traced lighting (Cyberpunk 2077’s Overdrive mode, Alan Wake 2) finally let games go beyond rasterization for primary visibility — but only with hardware ray-tracing acceleration.

Common misconceptions

• “Rasterization is being replaced by ray tracing.” Hybrid pipelines dominate. Pure ray tracing at game frame rates is still a high-end-GPU luxury.
• “More polygons = better image.” Past a screen-pixel-density threshold, additional geometry stops being visible.

Learn next

The output unit: pixel. The accuracy-first alternative: ray tracing (under graphics).