Printing calibration process and method, and printing systems configured to print therewith

1. A method for calibrating a printing process, comprising the steps of:

a) identifying a target tonal response curve for the printing process, the target tonal response curve having a function value for 0 that is non-zero or having a discontinuity between 0 and 10%;

b) deriving, using curve matching, an adjustment curve that when used in the printing process matches the identified target tonal response curve in step (a), the adjustment curve comprising a basic part, a highlight part that intersects with the basic part, the highlight part derived using a 1-dimensional function having one or more input parameters selected from a limited set of options, the adjustment curve characterized by:

a starting point having a first x-value and first y-value;

a highlight curve part having a first slope at the starting point and a first intersection of the highlight curve part with a basic curve part, the first intersection having a second x-value and second y-value, the first slope and the first intersection being among the input parameters selected from the limited set of options; and

the basic curve part having a midrange point in a middle-range of the adjustment curve, the midrange point having a third x-value and a third y-value respectively greater than the second x-value and second y-value, and having a second slope at the midrange point, the basic curve part selected from a predetermined family of adjustment curves limited in number;

c) adjusting one or more data values in a digital image input file by applying the selected adjustment curve to produce a digital image output file.

2. The method of claim 1 , wherein the adjustment curve further comprises a shadow part that intersects with the basic part, the shadow part derived using a 1-dimensional function having one or more input parameters selected from a limited set of options, the adjustment curve further characterized by:

a shadow curve part having a third slope at an end point and a second intersection of the basic curve part with the shadow curve part, the second intersection having a fourth x-value and a fourth y-value respectively greater than the third x-value and third y-value, the third slope and the second intersection being among the input parameters selected from the limited set of options; and

the end point having a fifth x-value and fifth y-value respectively greater than the x-values and y-values of all points on the adjustment curve.

3. The method of claim 1 , further comprising determining a function value of the starting point by determining a stable printing gray level.

4. The method of claim 3 , wherein determining the stable printing gray level comprises determining a lightest stable printing dot size by automatically measuring and comparing printed tone values for different dot sizes.

5. The method of claim 4 , wherein determining the stable printing gray level comprises automatically measuring and comparing printed tone values for different combinations of dot size and dot populations.

6. The method of claim 5 , further comprising implementing the lightest stable dot size in a screen and implementing a corresponding dot population as a minimum gray level in the adjustment curve.

7. The method of claim 2 , wherein the function value of the end-point is determined from the gray level that prints with a highest density.

8. The method of claim 1 , further comprising selecting a reference target tonal response curve for each process ink matching a print condition and reference profile, where the reference target tonal response curve is modified with a highlight point from a power function.

9. The method of claim 1 , further comprising selecting an unmodified reference target tonal response curve for each process ink matching a print condition and reference profile, and modifying the unmodified reference target tonal response curve based upon one or more extra points using a predetermined function.

10. The method of claim 9 , wherein the one or more extra points comprises a transition point of the screen.

11. The method of claim 10 , wherein the one or more extra points comprises a first point comprising a measured tone value of the minimal stable gray level, and a second point comprises the transition point of the screen, wherein the transition point comprises a measured tone value of an optimal transition point gray level with a Y-value corresponding to an automatically measured printed tone value of the optimal transition point, and an X-value selected so that the modified target tonal response curve has a first slope at the first point identical to a first slope at a beginning point (0,0) of the unmodified target tonal response curve.

12. The method of claim 10 , further comprising deriving the adjustment curve with the optimal transition point gray level given a relatively greater weight than the measured tone value of the minimal stable gray level in the curve matching step.

13. The method of claim 12 , wherein the adjustment curve is selected from a predetermined family of curves, the family comprising a set of predetermined basic curves continuous from 0% to 100%, modified with different highlight part shapes using Bezier spline functions, that allow a variable curve start point other than (0,0).

14. The method of claim 2 , wherein the adjustment curve is selected from a predetermined family of curves, the family comprising a set of predetermined basic curves continuous from 0% to 100%, modified with different shadow part shapes using Bezier spline functions, that allow a variable curve end point other than (100,100).

15. The method of claim 1 , wherein step (b) comprises any of the following steps:

i. calculating a theoretical set of adjustment points, the calculation comprising interpolating between measured values to match the target tonal response curve;

ii. determining the third x-value, third y-value, and second slope of the midtone point by selecting a curve from the family of curves that best matches said adjustment points in a midtone range U to V, where U>15% and V<85%;

iii. setting the first x-value, first y-value as the starting point identified by determining a stable printing gray level;

iv. determining the first slope at the starting point and determining the second x-value, second-y-value of the first intersection that when combined with the midtone point, best matches said adjustment points in a highlight range U to V, wherein U>starting point and V<50%;

v. setting the fifth x-value, fifth y-value to the end point identified by determining a gray level that prints with a highest density;

vi. determining the third slope at the end point and determining the fourth x-value, fourth y-value of the second intersection that when combined with the midtone point, best matches said adjustment points in a shadow range U to V, where U>50% and V<end point.

16. The method of claim 1 , wherein step (b) comprises any of the following steps:

i. calculating a theoretical set of adjustment points, the calculation comprising interpolating between measured values to match the target tonal response curve;

ii. setting the first x-value, first y-value to the starting point identified by determining a stable printing gray level;

iii. setting the fifth x-value, fifth y-value to the end point identified by determining a gray level that prints with a highest density;

iv. determining all other points and slopes by selecting a combination that best matches said adjustment points in a range U to V, where U>start point and V<end point.

17. The method of claim 1 , wherein the printing process comprises a multi-ink printing process, comprising selecting the adjustment curve for a first ink used in the multi-ink printing process and using the selected adjustment curve for a second ink used in the multi-ink printing process.

18. The method of claim 1 , wherein the printing process comprises a multi-ink printing process, comprising performing steps (a) and (b) to select a first adjustment curve for a first ink used in the multi-ink printing process, and repeating steps (a) and (b) to select a second adjustment curve different than the first adjustment curve for a second ink used in the multi-ink printing process.

19. The method of claim 1 , further comprising printing a printed image from the digital image output file.

20. Non-transitory machine-readable media comprising fixed machine readable instructions for causing a machine to perform the method of claim 18 .

21. The method of claim 1 , wherein the adjustment curve is selected from a predetermined family of curves, the family comprising a set of predetermined basic curves continuous from 0% to 100%, modified with different highlight part shapes using Bezier spline functions, that allow a variable curve start point other than (0,0).

22. The method of claim 1 , wherein the identifying comprises selecting the target tonal response curve from a set of standard tonal response curves or extracting the target tonal response curve from a reference profile to be matched.