Multi-module printer calibration

A memory resource storing instructions that when executed cause a processing resource at a printing apparatus to effect a calibration process, the instructions comprising:

• a print module to print a first plurality of test patches for each print module of a plurality of print modules of the printing apparatus;
• a test patch measurement module to measure a color value associated with each of the first plurality of test patches;
• a clustering module to identify at least two clusters of print modules based on the measured color values, wherein each cluster comprises a sub-group of print modules having substantially similar performance;
• the print module further to print a second plurality of test patches comprising a predetermined number of shades for each primary color for each cluster of print modules, the second plurality of test patches being distributed across the print modules of the respective cluster, and
• a calibration module to analyse the extended calibration test patch set for each cluster to determine a calibration parameter for the printing apparatus.

A printing apparatus comprising:

• a plurality of print modules;
• and
• a controller coupled to the plurality of print modules, the controller to:
• cause the plurality of print modules to print a first plurality of test patches;
• determine a difference metric for each print module compared to a reference print module based on the printed first plurality of test patches;
• identify at least two clusters of print modules based on the determined difference metrics, wherein each cluster comprises a sub-group of print modules having substantially similar performance;
• cause each cluster of print modules to print a second plurality of test patches, the second plurality of test patches comprising a predetermined number of test patches, each print module of a cluster printing a subset of the predetermined number of test patches of the second plurality of test patches;
• and
• analyse the second plurality of test patches for each cluster to determine a calibration parameter for the printing apparatus.