1. A method of identifying the shape of an object in an image, the object belonging to an object class, the method comprising the steps: obtaining a training set of images, each image depicting an object in the object class; identifying the shape of the objects in the training set of images; calculating a mean shape of the objects in the training set of images; calculating a covariance matrix of the shapes of the objects in the training set of images; identifying at least one second characteristic of the objects in the training set of images; calculating a mean for the second characteristic of objects in the training set of images; calculating a covariance matrix of the second characteristic of the objects in the training set of images; calculating a cross covariance matrix relating the shape of the objects to the second characteristic of the objects in the training set of images; calculating a conditional mean shape of the objects conditioned on the second characteristic of the objects in the training set of images; calculating a conditional covariance matrix of the shape of the objects conditioned on the second characteristic of the objects in the training set of images; calculating the eigenvalues and at least a subset of the corresponding eigenvectors of the conditional covariance matrix; obtaining a non-training set image containing an image of an object in the object class and including a value for the second characteristic for the non-training set image; calculating a conditional mean shape of the objects conditioned on the second characteristic of the objects in the training set of images and on the second characteristic value for the non-training set image; using the conditional mean shape and corresponding eigenvectors of the conditional covariance matrix to identify the shape of an object in the object class in the non-training set image.