Background removal relies on color keying algorithms, a technique with deep roots in film and television production where it is known as chroma keying or green screen technology. The fundamental concept is straightforward: identify pixels that match a target color within a defined tolerance range and replace them with transparency. However, the implementation details determine whether results look professional or crude.

Color keying works by computing the distance between each pixel's color and the target background color in a color space. In RGB space, the Euclidean distance between two colors is the square root of the sum of squared differences in each channel: distance = sqrt((R1-R2)^2 + (G1-G2)^2 + (B1-B2)^2). Pixels with distances below the tolerance threshold are removed. However, RGB Euclidean distance does not align well with human color perception, so more sophisticated implementations use perceptually uniform color spaces like CIE Lab where equal distances correspond to equal perceived color differences.

The alpha channel is the technical mechanism that enables transparency in digital images. While RGB channels store color information, the alpha channel stores opacity for each pixel as a value from 0 (fully transparent) to 255 (fully opaque). PNG and WebP formats support full alpha channels, enabling smooth semi-transparent edges where pixels gradually transition from opaque (the subject) to transparent (the removed background). This graduated transparency at edges is crucial for natural-looking compositing because real-world edges are never perfectly sharp at the pixel level due to anti-aliasing, motion blur, and optical effects.

Chroma subsampling is a related concept from video compression that affects background removal quality. In formats like JPEG, color information is stored at lower resolution than brightness information because human vision is less sensitive to color detail. The notation 4:2:0 means color is sampled at half the horizontal and half the vertical resolution of brightness. This means that at color boundaries, the exact transition between background and subject colors may be blurred in the color channels even when the brightness edge is sharp. This is why background removal from JPEG images sometimes produces color fringing along edges, and why starting with PNG or uncompressed sources produces cleaner results. Professional chroma keying in film production uses specially lit green or blue screens because these colors are maximally different from human skin tones, reducing the chance of accidentally keying out parts of the subject.