# Resolving Gas Bubbles Ascending in Liquid Metal from Low-SNR Neutron Radiography Images
## 7 Conclusions  
To summarize, we have demonstrated the new version of our image processing methodology for resolving gas bubbles travelling through liquid metal imaged using dynamic neutron radiography. The showcased components of our code, such as the multi-scale recursive interrogation filter (MRIF) and the underlying global and local image filters, as well as soft color tone map masking (SCTMM), proved effective for detecting bubbles and extracting their dynamic shapes from images with low SNR and CNR. Output quality was further improved by the implemented luminance map-based false positive filter that bolstered the MRIF's intrinsic false positive filtering function. It as shown by direct comparison that the new image processing code clearly outperforms the previous version used in [8; 43], while the outputs of both are still consistent. In addition, we have validated the new methods experimentally by imaging a reference body, both stationary and in motion, with a precisely machined spherical cavity. Results indicate that that local filtering performed by MRIF largely limits the shape detection errors: relative shape mismatch area and shape area difference with respect to reference shapes are within acceptable bounds of \(\sim\)14% (\(\sim\)20% with synthetic artefacts) and \(\sim\)10% (\(\sim\)14% with synthetic artefacts), respectively, (accounting for motion blur and the worst-case underestimation correction), while the asphericity of the detected shapes is rather negligible. As such, we find that applying the current methodology to the neutron radiography images obtained for our model systems with bubble chain flow is safe in that physically meaningful results with manageable errors can be expected. Note that we have also used the present image processing code to benchmark our object tracking code MHT-X [75]. In follow-up articles, we are going to process the data acquired in the previous and latest neutron imaging campaigns using the methods presented in this paper and our MHT-X code, and showcase the effects of applied horizontal and vertical magnetic field with different strengths on bubble chain flow in a rectangular liquid gallium vessel. Bubble trajectories (length, curvature, oscillation frequencies, envelopes, etc. ), velocity (both overall spectra and dynamics, including acceleration) and shapes (aspect ratio, tilt angle, etc., and dynamics thereof) will be assessed and compared for different flow conditions--in addition to magnetic field configurations, a range of gas flow rates will be considered. We will also attempt to perform dynamic mode decomposition for the bubble shapes extracted from neutron radiography images and compare the dynamics against simulations--this will be done using the methods recently developed in [55]. Finally, we expect that the developed image processing pipeline and/or separate elements thereof should be applicable beyond the current application and context, which we also plan to demonstrate in follow-up papers. This is especially true for MRIF which is modular in the sense that the global and local filters used herein could be substituted by any other methods (both simpler ones and more complex state-of-art) depending on the problem at hand. The image processing code is available on _GitHub_: Mihailis-Birjukovs/Low_C-SNR_Bubble_Detection (accessed: 16 October 2021). The code will be improved as outlined in Section 6. Conceptualization, M.B. ; data curation, M.B., P.T. and A.K. ; formal analysis, M.B. ; funding acquisition, K.T., A.J. and M.B. ; investigation, M.B., P.T., A.K., J.H. and M.K. ; methodology: M.B., P.T. and M.K. ; project administration, M.B. and A.J. ; resources: P.T., A.K., J.H. and D.J.G. ; software: M.B. ; supervision: K.T. and A.J. ; validation, M.B. ; visualization, M.B. ; writing--originaldraft, M.B. ; writing--review & editing, M.B., P.T., A.K., J.H., M.K., D.J.G., K.T. and A.J. All authors have read and agreed to the published version of the manuscript.