Shape and colour measurement of colourful objects by fringeprojection

摘要

In this paper, we present a novel method to measure shape and colour information of a colourful object byprojecting separate red, green and blue colour fringe patterns onto the object surface. With regard to the objectsurface's colour, the modulation at each pixel position in the three colour channels has different values. Forexample, when projecting blue fringe patterns onto a red point, the corresponding pixel has too low a fringemodulation to accurately calculate the phase (shape) information; but with red fringe patterns a high fringemodulation is obtained. Therefore, phase information of the red point can be calculated by projecting red fringepatterns. For each object point, by comparing the modulation values from the three colour channels, it is possible tochoose the channel having maximum modulation, and hence phase information can be reliably obtained by thephase-shifting algorithm. The fringe order information is obtained by using the optimum three-frequency selectionmethod, so there is a maximum reliability in determining the fringe order and the 3-D shape of an object with stepor large slopes on the surface. Since three colour channels are used, colour information of the object surface can beextracted with high dynamic range from the same fringe patterns. Chromatic aberration between colour channels isunavoidable and can be eliminated by a software-based method. Using the recently developed colour fringeprojection system, separate colour fringe patterns are projected onto a mug having different colour patterns, acolourful box and plate, and a colour checker card to test the proposed method. The results show the range ofcolours that can be measured and that shape and colour information of colourful objects can be reliably obtained.