Some question about the Equation(4) of the paper

[hetolin]

Hi, thanks for sharing your code. I have some question about the the L_rot of Eq(4) in your paper. To calculate the error between two rotation matrix, most paper use the equation of theta = arccos((trace(R_gt^T · R_est)-1) /2) to evaluate the angular distance. I cannot find something related mathematic literature refers to this equation of Equation(4) in your paper, could you please share some details or related mathematic literature explaining this Eq(4)? I have struggled to understand this formula for a long time.