  The Virtual Image Correlation method applies for the measurement of
silhouettes boundaries with sub-pixel precision. It consists in a correlation
between the image of interest and a virtual image based on a parametrized
curve. Thanks to a new formulation, it is shown that the method is exact in 1D,
insensitive to local curvature and to contrast variation, and that the bias
induced by luminance variation can be easily corrected. Optimal value of the
virtual image width, the sole parameter of the method, and optimal numerical
settings are established. An estimator is proposed to assess the relevance of
the user-chosen curve to describe the contour with a sub-pixel precision.
Analytical formulas are given for the measurement uncertainty in both cases of
noiseless and noisy images and their prediction is successfully compared to
numerical tests.
