code 2 a 2 the bar codes being for example u201c 128 u201d bar codes of french standard nf en799 two of the three bar codes being surrounded by the outlines of respective rectangles 2 b which correspond to two regions of interest in this example by way of example and in order to enable the invention to be well understood both of the bar codes 2 a 1 and 2 a 2 are specific codes that are recognizable by the processor unit 125 for example the codes 2 a are designed to indicate that the zone within the rectangular outline 2 b that surrounds the specific bar code 2 a 1 is a zone to be included isolated in the filtered image i e said zone is not a filter zone in the filter mask whereas the specific code 2 a 2 is designed to indicate that the zone that surrounds it within the outline of the image in this example is a zone to be excluded filtered out in white in the filtered image i e a filter zone in the filter mask a zone identified by a specific code 2 a 1 inclusion code is thus a non filter zone whereas a zone identified by a specific code 2 a 2 is a filter zone in the image to be filtered in practice in the example shown in fig2 the filter mask.