Linear interpolator for color correction

A linear interpolating apparatus for color signals in a memory of a picture reproducing machine, comprising means for storing appropriate values of color picture output signals corresponding to certain stepped values of color input signals in a memory addressed in a three-dimensional fashion, and means for interpolating values of color output signals at points which are between said values by:

• means for dividing up the cubic interpolation unit of the memory which is constituted by a single step of each of the color input signals into a plurality of tetrahedra whose vertices are either vertices of the cubic unit, centers of its faces, or its center point;
• means for calculating the color output signal at each vertex of these tetrahedra which is a center of a face of the cubic unit, if any, by averaging the values of the color output signal at the four vertices which are corners of said face, and at the center point of the cubit unit by averaging the values of the color output signal at all eight of the vertices of the cubic unit;
• means for determining which of these tetrahedra includes the interpolation point at which the value of the color output signal is to be interpolated;
• and determining which deriving the interpolated value of the interpolation point as a weighted sum the values at the four vertices of the determined tetrahedron, the value at each vertex being given a weight corresponding to the ratio of the volume of a second tetrahedron whose vertices are the interpolation point and the other three vertices of the determined tetrahedron to that of the determined tetrahedron.

A linear interpolating method for color signals in a memory comprising the steps of:

• producing color separated signals (B), (R), and (G) by photoelectrically scanning on original picture;
• converting said color separated signals to those of logarithmic values;
• converting said logarithmic value signals to digital signals and for separating each of said digital signals into two sets of four bit address signals, one set of which is an address signal for calling color corrected signals of four bits of the higher order (for R, G and B) and the other set of which is for the lower order bits (for r, g and b);
• comparing the relative magnitudes of said lower four bit address signals and generating an output signal which is a function of said relative magnitudes;
• monitoring the output signals and determining, as a function of said relative magnitudes, to which address four bit higher order color corrected signals the quantity one is to be added;
• preliminarily storing said color corrected signals corresponding to all combinations of said four bits in the higher order address signals;
• calculating four sets of coefficients (1-r, 1-g, 1-b), (r-g, r-b, g-b, g-r, b-r, b-g), (g-b, b-g, b-r, r-b, r-g, g-r) and (b, r, g), from the lower order four bit signals and for selecting four coefficient signals from said four sets of coefficients as a function of said output signals;
• multiplying each of said coefficient signals by each of the corresponding color corrected signals stored in said memory;
• and
• adding higher order bits obtained by the multiplication of said signals in said multipliers.