With the face and both eyes detected, we'll perform face preprocessing by combining:
• Geometrical transformation and cropping: This process would include
scaling, rotating, and translating the images so that the eyes are aligned,
followed by the removal of the forehead, chin, ears, and background from
the face image.
• Separate histogram equalization for left and right sides: This process
standardizes the brightness and contrast on both the left- and right-hand
sides of the face independently.
• Smoothing: This process reduces the image noise using a bilateral filter.
• Elliptical mask: The elliptical mask removes some remaining hair and
background from the face image.
The following image shows the face preprocessing steps 1 to 4 applied to a detected
face. Notice how the final image has good brightness and contrast on both sides of
the face, whereas the original does not: