tailieunhanh - COLOR MANAGEMENT- P10

COLOR MANAGEMENT- P10: ICC White Papers are one of the formal deliverables of the International Color Consortium, the other being the ICC specification itself – ISO 15076: Image technology color management – Architecture, profile format, and data structure. The White Papers undergo an exhaustive internal development process, followed by a formal technical review by the membership and a ballot for approval by the ICC Steering Committee. | 254 Profile Construction and Evaluation Inverting the LUT-Based Transform For most applications the BToAx and AToBx transforms in a profile should invert accurately so that when an image is roundtripped convert first in one direction and then back in the other all the final in-gamut colors are a close approximation to the starting values allowing for minor differences arising from interpolation errors and round-off. This allows an AToBx transform to be used to preview the effect of a conversion using a BToAx tag. Simulation re-rendering and re-purposing all require the BToAx and AToBx tags to invert accurately. Below we will assume that an AToBx CLUT has been generated from a uniform sampling of the device encoding and the associated measurements and we will consider the methods of generating its inverse the BToAx CLUT. In the simplest case the function used to calculate the AToBx output table is linear and analytically invertible. This might be the case for a 3 x 3 matrix for example. When this inverse function is used to compute the inverse transform the results will be sufficiently accurate for most purposes. Alternatively for three-component data the output values might be computed by polynomial regression and while the resulting coefficients do not lend themselves to a simple inversion the regression can be recalculated with the sense inverted. Other functions cannot be inverted so readily. An alternative method is to find the direct inverse of the CLUT as described by Bala 2 . The goal of the method is to use the mapping in one direction to compute the mapping in the inverse direction. The input cube is partitioned into tetrahedra and the output table is similarly tessellated such that each tetrahedron in the input table is mapped uniquely to an output tetrahedron. Any point in the output encoding can now be mapped to the input encoding by a process of locating it within a tetrahedron in the output encoding and using tetrahedral interpolation to find its