Most digital cameras determine white balance primarily from the main image sensor data, not by simply forcing the brightest area to white. The camera analyzes the scene’s red, green, and blue content and estimates the color of the light source, then applies gains to the channels so neutral subjects look neutral.

Auto white balance usually relies on image-processing algorithms, often using assumptions like the presence of neutral tones or typical scene statistics. It is not just “make the brightest part white,” because bright areas may be colored light, reflections, or clipped highlights.

Some cameras may also use additional metering or ambient-light sensors to help scene analysis, but the core white-balance correction is typically based on the image data itself.

If accuracy matters, preset white balance, a custom white balance reading from a gray/white target, or shooting RAW gives better control than auto white balance alone.