Detection of liquid

1 . 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.

2 . The method of claim 1 , wherein a top first order vertical derivative above the upper border zone is substantially null and is associated with an amount of liquid crossed by the inspection radiation being constantly substantially null above the surface of the liquid cargo.

3 . The method of claim 1 , further comprising:

for each determined zone, determining a lower second order vertical derivative of the signal levels, in each pixel of at least a lower border zone, the lower border zone straddling a lower border of the determined zone, the lower second order vertical derivative being determined from a lower vertical location in the lower border zone to an upper vertical location in the lower border zone; and

determining, in the determined zones, second selected determined zones where the determined lower second order vertical derivative has a positive peak at the lower border, and

wherein detecting the zones corresponding to the surface of the liquid cargo is further based on the second selected determined zones.

4 . The method of claim 3 , wherein a bottom first order vertical derivative is substantially null or slightly positive and is associated with an amount of liquid crossed by the inspection radiation not or slightly increasing from a lower location to an upper location, below the surface of the liquid cargo.

5 . The method of claim 3 , further comprising:

for each determined zone, determining a first order horizontal derivative of the lower second order vertical derivative in each pixel in the second selected zones; and

determining, in the second selected determined zones, fourth selected determined zones where the first order horizontal derivative of the lower second order vertical derivative is substantially null, and

wherein detecting the zones corresponding to the surface of the liquid cargo further based on the fourth selected determined zones.

6 . The method of claim 5 , wherein the first order horizontal derivative of the lower second order vertical derivative which is substantially null is associated with a horizontal surface of the liquid cargo.

7 . The method of claim 1 , wherein the first order horizontal derivative which is smaller than the predetermined threshold is associated with a horizontal surface of the liquid cargo.

8 . The method of claim 1 , further comprising:

for each determined zone, determining a first order horizontal derivative of the upper second order vertical derivative in each pixel in the first selected zones; and

determining, in the first selected determined zones, third selected determined zones where the determined first order horizontal derivative of the upper second order vertical derivative is substantially null, and

wherein detecting the zones corresponding to the surface of the liquid cargo is further based on the third selected determined zones.

9 . The method of claim 8 , wherein the first order horizontal derivative of the upper second order vertical derivative which is substantially null is associated with a horizontal surface of the liquid cargo.

10 . The method of claim 1 , wherein the inspection of the cargo comprises:

tilting the cargo in a first tilting orientation, and scanning the cargo tilted in the first tilting orientation; and

tilting the cargo in a second tilting orientation, the second tilting orientation being opposite to the first orientation, and scanning the cargo tilted in the second tilting orientation, and

wherein detecting the zones corresponding to the surface of liquid cargo comprises comparing the zones determined for each tilting orientation.

11 . The method of claim 1 , wherein the tilting of the cargo around the first horizontal direction is with a slope S with respect to a horizontal direction such that:

2°≤S≤25°.

12 . The method of claim 1 , wherein the first order vertical derivative which is positive is greater than a predetermined threshold.

13 . An apparatus comprising:

a processor; and

a memory storing instructions which, when executed by the processor, enable the apparatus to perform the method of claim 1 .