When a continuous shape — a diagonal line, a circle, a curve — is mapped onto a discrete pixel grid, the result inevitably contains staircase-like artifacts called aliasing or "jaggies." Anti-aliasing mitigates this by blending edge pixels with colors intermediate between the foreground and background, creating the illusion of smooth edges at the cost of introducing new colors into the image.
How It Works
The basic principle is supersampling: render the scene at a higher resolution, then downsample. Each output pixel becomes the average of multiple sub-pixels, producing smooth gradients at edges. In practice, modern implementations use more efficient approaches:
- MSAA (Multi-Sample) — Samples multiple points per pixel only at polygon edges. Efficient in GPU hardware.
- FXAA (Fast Approximate) — Post-processing filter that detects edges in the final image and blurs them. Fast but can soften textures.
- Subpixel rendering — Exploits the physical layout of LCD subpixels (RGB stripes) to triple the horizontal resolution. Used extensively in font rendering.
The Pixel Art Exception
Anti-aliasing is one of the few rendering features that must be actively suppressed in certain contexts. Pixel art is designed to work with the grid — every pixel is a deliberate placement, and the hard edges are an aesthetic feature, not a flaw. Applying anti-aliasing to pixel art blurs the precise edges that the artist carefully crafted.
This is why browsers provide the CSS property image-rendering: pixelated and the Canvas API offers imageSmoothingEnabled = false. Both instruct the renderer to use nearest-neighbor interpolation when scaling images, preserving the hard pixel boundaries that define pixel art's visual character.
Impact on Palette Analysis
Anti-aliasing has a direct impact on color palette extraction. A pure black-and-white image contains exactly 2 unique colors. The same image with anti-aliased edges may contain dozens of intermediate grays introduced by the smoothing algorithm. When our analysis pipeline detects a small number of unique colors (≤256), it treats every value as intentional — but if those extra colors are anti-aliasing artifacts rather than deliberate choices, the palette count will be inflated.
