OpenGL uses a right handed coordinated system (normalized device coordinates), which looks like the following:
OpenGL's clip space extensions from − 1.0 to 1.0 in all three axes with the z axis Pointing out of the screen
DirectX uses a left handed coordinated system (normalized device coordinates), which looks like the following:
DirectX's z axis Points into the screen , and the near plane is mapped to z= 0.0 ratio than z − 1.0.
Tangent Space, as the name suggests, is based on the tangent direction of each face. Normal mapping in tangent space is the most common form of normal mapping, most of which appear light blue purple. Among them, channel B represents the slope of the normal direction; The R channel represents the slope of the left and right tangent directions; The G channel represents the slope of the tangent direction up or down (OpenGl up, DirectX down).