Scanline Rendering

Scanline Rendering

Scanline rendering is one of the oldest and most fundamental rendering algorithms. It works by processing the image one horizontal line at a time, from top to bottom. For each scanline, the renderer determines which polygons intersect that line, calculates the color at each pixel along the line, and writes the result. This approach is very efficient because it processes pixels in a predictable order.

The scanline algorithm starts by sorting all polygons by their Y coordinates. Then for each scanline, it maintains an active list of polygons that intersect that line. For each pixel on the scanline, it finds the closest polygon at that point, looks up the texture, calculates the lighting, and writes the pixel color. This is much faster than checking every polygon for every pixel.

Scanline rendering was the standard for real time graphics for many years. Early 3D games and hardware renderers used scanline algorithms because they were simple to implement in hardware and provided predictable performance. The original PlayStation and Nintendo 64 used scanline based rendering. Even today, some aspects of modern GPU rendering still use scanline concepts.

The limitation of scanline rendering is that it does not handle certain effects well. Transparency requires sorting polygons by depth, which is expensive. Reflections and shadows are difficult because the algorithm only processes what is visible from the camera. These effects require additional passes or different techniques. This is why modern renderers use ray tracing instead of pure scanline.

In modern VFX, pure scanline rendering is rarely used. It has been replaced by ray tracing and path tracing for final quality rendering. However, the scanline approach is still used in viewport previews and in some real time applications. Understanding scanline rendering helps you appreciate why certain rendering artifacts occur and why modern renderers work the way they do.