Normal Map (Normal Map / Normal Map) is a technical term that is extremely important in the rendering and optimization of game graphics (technical art). A technology that creates the illusion of three-dimensional shadows by pasting a pseudo "direction of unevenness (normal)" expressed in RGB values as a texture on the surface of a flat polygon and using lighting calculations.
Real-world analogy: The magic of light hitting a ``trompe l'oeil (3D art)'' drawn on flat paper
Normal Map is "an illusion magic that draws a special three-dimensional trompe l'oeil (normal texture) on a completely flat concrete floor and shines a light on it, making it look like the joints are actually concave". In reality, it is completely flat when you touch it with your finger (the number of vertices is small), but only the shadows change realistically according to the angle of light, so the player's brain believes that it is a ``real, rugged 3D shape.'' This allows you to create an overwhelming sense of reality without increasing the number of vertices.
Illustration: Normal Map (Normal Map / An infographic that clearly illustrates the basic processing flow and mechanism of normal map) in Japanese.
Detailed mechanism and operating principle
Bake the unevenness of the high polygon model as a blue-purple "Normal map (normal texture), paste it on the low polygon model, and reproduce the three-dimensional unevenness using GPU lighting calculations.