Detection of liquid

A method for detecting zones corresponding to a surface of liquid cargo in an inspection image of inspected cargo, the method comprising:

• obtaining inspection data associated with pixels of zones of interest in the inspection image generated from inspection of the cargo using penetrating radiation transmitted through the cargo, the pixels having signal levels and wherein the inspection of the cargo comprises:
• tilting the cargo around a first horizontal direction perpendicular to a vertical direction corresponding to a direction of gravity;
• and
• scanning the tilted cargo in a second horizontal direction perpendicular to the first horizontal direction;
• determining a vertical derivative field from the obtained inspection data by:
• determining a first order vertical derivative of the signal levels in each pixel of the zones of interest, from a lower vertical location in the zones of interest to an upper vertical location in the zones of interest;
• determining a horizontal derivative field from the obtained inspection data by:
• determining a first order horizontal derivative of the signal levels in each pixel of the zones of interest;
• and
• determining, based on the determined vertical derivative field and on the determined horizontal derivative field, zones where:
• the first order vertical derivative is positive;
• and
• the absolute value of the first order horizontal derivative is smaller than a predetermined threshold, wherein the first order vertical derivative which is positive is associated with an amount of liquid crossed by the inspection radiation decreasing from the lower vertical location to the upper vertical location, in the detected zone, at the surface of the liquid cargo;
• and
• for each determined zone, determining an upper second order vertical derivative of the signal levels, in each pixel of at least an upper border zone, the upper border zone straddling an upper border of the determined zone, the upper second order vertical derivative being determined from a lower vertical location in the upper border zone to an upper vertical location in the upper border zone;
• determining, in the determined zones, first selected determined zones where the determined upper second order vertical derivative has a negative peak at the upper border, and
• detecting zones corresponding to a surface of liquid cargo based at least on the determined zones, wherein detecting the zones corresponding to the surface of the liquid cargo is further based on the first selected determined zones.