Techniques for Face Motion & Expression Analysis on Monocular Images - 3D Modeling and Animation: Synthesis and Analysis Techniques for the Human Body - page 205

Each of the modules may be more or less complex depending on the purpose of
the analysis (i.e., from the understanding of general behavior to exact 3D-motion
extraction). If the analysis is intended for later face expression animation, the
type of FA synthesis often determines the methodology used during expression
analysis. Some systems may not go through either the first or the last stages or
some others may blend these stages in the main motion & expression image
analysis . Systems lacking the pre-motion analysis step are most likely to be
limited by environmental constraints, like special lighting conditions or pre-
determined head pose. Those systems that do not perform motion interpreta-
tion do not focus on delivering any information to perform face animation
synthesis afterwards. A system that is thought to analyze video to generate face
animation data in a robust and efficient way needs to develop all three modules.
The approaches currently under research and that will be exposed in this chapter
clearly perform the facial motion & expression image analysis and to some
extent the motion interpretation to be able to animate 3D models. Nevertheless,
many of them fail to have a strong pre-motion analysis step to ensure some
robustness during the subsequent analysis.
Processing Fundamentals
Pre-Processing Techniques
The conditions under which the user may be recorded are susceptible to change
from one determined moment to the next one. Some changes may come from the
hardware equipment used, for instance, the camera, the lighting environment,
etc. Furthermore, although only one camera is used, we cannot presuppose that
the speaker's head will remain motionless and looking straight into the camera
at any time. Therefore, pre-processing techniques must help to homogenize the
analysis conditions before studying non-rigid face motion.
Camera calibration
Accurate motion retrieval is highly dependent on the precision of the image data
we analyze. Images recorded by a camera undergo different visual deformations
due to the nature of the acquisition material. Camera calibration can be seen as
the starting point of a precise analysis.