  The SVOM mission under development will carry four instruments, and in
particular the coded-mask telescope named ECLAIRs, with a large field of view
of about 2 sr, operating in the 4-150 keV energy band. The trigger software on
board ECLAIRs will search for high-energy transients such as gamma-ray bursts
and peculiar behaviour (e.g. strong outbursts) from known X-ray sources, in
order to repoint the satellite to perform follow-up observations with the
onboard narrow field of view instruments. The image trigger, one of the two
algorithms implemented in the software on board ECLAIRs, produces images over
periods of exposure ranging from 20 seconds to 20 minutes during which the
Earth can cross the field of view. The CXB and contributions from known X-ray
sources are expected to dominate the ECLAIRs astrophysical and instrumental
background and must be taken into account and corrected prior to coded-mask
image deconvolution in order to optimise the sensitivity to faint transients.
To correct these background components, we implemented and studied a
traditional fitting method and a new method based on wavelet decomposition of
the detector image. In order to study and to assess the performance of these
methods, we performed a one-year simulation of the image trigger on board
ECLAIRs. From the images produced during this realistic observation scenario of
the SVOM mission, we also defined a way to analyse the sky images to search for
new sources. We present the algorithms behind the image trigger on board
SVOM/ECLAIRs. We show that the wavelet method we implemented provides similar
results in terms of cleaning performance compared to the traditional fitting
method, and has the benefit of not requiring any assumption on the shape of the
background on the detector. We also calibrate the detection threshold to be
adaptive and based on the quality of the reconstructed sky image.
