Determining alignment using fiducials

A method to determine whether printed fiducials are aligned, the method comprising:

• printing, on a back side of a printable medium, a first fiducial, the first fiducial comprising a shape in a first colour and having a first dimension, and a background of print agent of a second colour, wherein the first dimension of the shape is substantially equal to or less than a corresponding dimension of the background;
• printing, on a front side of the printable medium, a second fiducial, wherein the second fiducial comprises a mirrored version of the shape in the first colour;
• back-illuminating the printable medium;
• scanning the printable medium to determine positions of the first and second fiducials;
• and
• determining, based on the determined positions of the first and second fiducials, whether the first and second fiducials are aligned.

A machine-readable medium comprising instructions which, when executed by a processor, cause the processor to:

• operate a print head to form, on a first side of a print substrate, a first registration mark in a first colour and having a first dimension, and a background of print agent of a second colour, wherein the first dimention of the first registration mark does not exceed a corresponding dimension of the background;
• operate a print head to form a second registration mark on a second side of the print substrate, wherein the second registration mark comprises an inverted representation of the first registration mark, without the background;
• operate a light source to emit light towards the print substrate;
• operate a sensor, during the emission of the light, to detect a position of the first registration mark and a position of the second registration mark on the print substrate;
• and
• determine, based on the detected positions of the first and second registration marks, a degree of alignment between the first and second registration marks.