The main statement of this thesis is: knowledge of an object’s colour, like skin colour changes under different illumination conditions, can be used to develop more robust techniques against illumination changes. Faces were selected as the objects to be studied because they are common and important in very many applications. To prove the statement, this thesis employs three different phases: 1) collecting facial skin data under different illumination conditions, 2) analysing the data, and 3) applying the obtained knowledge. The following list shows the novel contributions and their support for the main statement:
A method for evaluating colour camera performance (Paper I) is developed for studying metamerism in human and camera vision systems and is used in evaluating cameras. This information can also be used as a criterion for selecting a camera or between human vision and device colour spaces.
A unique Physics-based Face Database (Paper II) is introduced for face related colour research. Its novelty lies in the combination of face images and spectral data related to the formation of those images as well as the procedure for studying the illumination effects on the images. In the procedure, the camera was first white balanced to one of the light sources and then images were taken under this light source and under other light sources with different spectral power distribution. This was repeated for four different light sources. The purpose of the database is to collect knowledge about facial skin colour appearance under known illumination and camera white balancing conditions.
A novel method for skin colour correction is presented for face images with clipping (Paper III). For this method, the knowledge obtained from the database is shown to be useful for its development.
Creation of a chromatic constraint which does not only cover different illumination conditions but also takes into account the effect of different camera calibrations (Paper IV). This constraint offers information about possible skin chromaticities perceivable by a colour camera with a certain illumination range and white balancing conditions.
Use of basis functions obtained from skin colour signals for creating the chromatic constraint (Paper V). This is a spectral based method for obtaining the information about skin chromaticities under different conditions. In addition, it makes it easy to simulate outputs for different cameras.
A study on how skin colour behaves in different colour spaces and evaluation of their usefulness (Paper VI). The purpose is to analyse different colour spaces for chromatic constraint based applications.
A novel Face Video Database (Paper VII) which contains videos with drastic colour changes taken in real environments and face images under known illumination is suggested for the testing and developing of algorithms. The videos have been taken with several different cameras. In addition, the face localizations are available in each frame. The goal is once again data collection.
Implementing a chromatic constraint as a part of different face tracking methods to make possible adaptive skin colour modelling (Paper IV and Paper VIII). Here it is shown that the knowledge obtained about skin chromaticities can be used to provide robustness against illumination change.
Visualizing how different skin models can be used for segmenting faces in videos (Paper VIII). This is another example of how the chromaticity constraint provides improved results against illumination change.