Acquisition et reproduction d'images couleur : approches colorimétrique et multispectrale

The goal of the work reported in this dissertation is to develop methods for the acquisition and reproduction of high quality digital colour images. To reach this goal it is necessary to understand and control the way in which the different devices involved in the entire colour imaging chain treat colours. Therefore we addressed the problem of colorimetric characterisation of scanners and printers, providing efficient and colorimetrically accurate means of conversion between a device-independent colour space such as the CIELAB space, and the device-dependent colour spaces of a scanner and a printer. First, we propose a new method for the colorimetric characterisation of colour scanners. It consists of applying a non-linear correction to the scanner RGB values followed by a 3rd order 3D polynomial regression function directly to CIELAB space. This method gives very good results in terms of residual colour differences. The method has been successfully applied to several colour image acquisition devices, including digital cameras. Together with other proposed algorithms for image quality enhancements it has allowed us to obtain very high quality digital colour images of fine art paintings. An original method for the colorimetric characterisation of a printer is then proposed. The method is based on a computational geometry approach. It uses a 3D triangulation technique to build a tetrahedral partition of the printer colour gamut volume and it generates a surrounding structure enclosing the definition domain. The characterisation provides the inverse transformation from the deviceindependent colour space CIELAB to the device-dependent colour space CMY, taking into account both colorimetric properties of the printer, and colour gamut mapping. To further improve the colour precision and colour fidelity we have performed another study concerning the acquisition of multispectral images using a monochrome digital camera together with a set of K > 3 carefully selected colour filters. Several important issues are addressed in this study. A first step is to perform a spectral characterisation of the image acquisition system to establish the spectral model. The choice of colour chart for this characterisation is found to be very important, and a new method for the design of an optimised colour chart is proposed. Several methods for an optimised selection of colour filters are then proposed, based on the spectral properties of the camera, the illuminant, and a set of colour patches representative for the given application. To convert the camera output signals to device-independent data, several approaches are proposed and tested. One consists of applying regression methods to convert to a colour space such as CIEXYZ or CIELAB. Another method is based on the spectral model of the acquisition system. By inverting the model, we can estimate the spectral reflectance of each pixel of the imaged surface. Finally we present an application where the acquired multispectral images are used to predict changes in colour due to changes in the viewing illuminant. This method of illuminant simulation is found to be very accurate, and it works well on a wide range of illuminants having very different spectral properties. The proposed methods are evaluated by their theoretical properties, by simulations, and by experiments with a multispectral image acquisition system assembled using a CCD camera and a tunable filter in which the spectral transmittance can be controlled electronically.