A method for correcting tone-level non-uniformities in a digital printing system, comprising:
- a) providing digital image data for a test target including a set of uniform test patches, each test patch having an associated pixel value and extending in a cross-track direction;
- b) printing the test target using the digital printing system to provide a printed test target;
- c) using a digital image capture system to capture an image of the printed test target;
- d) using a data processing system to automatically analyze the captured image to determine a measured tone-level as a function of a cross-track position for each of the test patches;
- e) comparing the measured tone-levels to nominal tone-levels for each test patch to determine measured tone-level errors as a function of the cross-track position for each of the test patches;
- f) determining a tone-level correction function responsive to the measured tone-level errors, wherein the tone-level correction function specifies tone-level corrections to be applied as a function of cross-track position, pixel value and time;
- g) storing a representation of the tone-level correction function in a digital memory, wherein the representation of the tone-level correction function includes a set of one-dimensional feature vectors corresponding to a decomposition of the tone-level correction function into a summation of outer vector products of the one-dimensional feature vectors;
- h) receiving digital image data for an input digital image to be printed by the digital imaging system, wherein the digital image data specifies input pixel values for an array of image pixels;
- and
- i) determining corrected image data by:
- using the stored representation of the tone-level correction function to determine a tone-level correction value for each image pixel responsive to the input pixel value of the image pixel and the cross-track position of the image pixel;
- and
- modifying the input pixel value for each image pixel responsive to the determined tone-level correction value to provide a corrected pixel value;
- wherein the decomposition of the tone-level correction function has the form:
- Δ
- c
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- wherein Δc(x,v,t) is the tone-level correction function, if(x) are one-dimensional cross-track feature vectors taken along the cross-track axis of the tone-level correction function, kf(v) are one-dimensional pixel value feature vectors taken along the pixel value axis of the tone-level correction function, lf(t) are one-dimensional temporal feature vectors taken along the time axis of the tone-level correction function, x is the cross-track position, v is the pixel value, t is the time value, f is a feature vector index, n is the total number of feature vectors, and ∘ is the outer vector product operator.