Color device characterization involves deriving a mathematical description of the device response to a known input. This is known as the forward characterization transform. In the final application, this transform must be inverted to generate a mapping that determines the device input required for a desired response. This paper focuses on the inverse characterization transform for hardcopy devices. This can be discussed for two cases:
(1) Devices employing 3 channels
A colorimetrically unique inverse mapping exists provided the input signal is within the achievable domain of the device. When the forward transform is described by an analytic model, the inverse can be obtained by search-based techniques. When the forward transform is obtained empirically, the inverse transform is estimated by 3-D fitting or interpolation methods.
(2) Devices employing > 3 channels.
The inverse mapping is not colorimetrically unique, and therefore ill-posed. Additional constraints must be incorporated to ensure uniqueness. As an example, the case of CMYK printer characterization will be discussed. Constraints via undercolor removal and gray component replacement will be presented. Other methods that explicitly constrain CMYK combinations based on criteria such moire minimization will also be described.
For both cases, the problem of out-of-domain mapping and noise considerations will be discussed.