## 1 Introduction  
Currently, internal combustion engines need to comply with severe regulations concerning fuel economy and pollutant emission. Focusing on spark ignition (SI) engines, actual trends to obtain a cleaner and more efficient combustion are turbocharging, water injection, high exhaust gas recirculation (EGR) dilution, and lean combustion [1; 2; 3; 4; 5]. Because of all these features, it is critical to ensure an effective ignition able to start a robust combustion; conventional spark plugs show their limits [6; 7] and an excessive cycle-to-cycle variability (CCV) is linked to the use of conventional igniters in these conditions. Actually, CCV depends on different factors and processes: manifold phenomena, fuel injection, in-cylinder motion, ignition, and gas exchange. They can be independent of each other or partly interact to generate the global engine CCV. To reduce variability, an increase in the dischargedenergy of the spark igniter is a possible solution; despite slightly improving combustion robustness, it dramatically affects the spark plug lifetime [8]. Similar issues can be found with other spark-based solutions, like multiple sparks, high-energy discharge, and distributed discharge [9]. Another way is based on low-temperature plasma (LTP) igniters, such as nanosecond-pulsed discharge [6; 10], radio-frequency (RF) corona discharge [11; 12], or dielectric-barrier discharge [12; 13]. Such kinds of plasma are featured with a strong temperature gap between electrons and heavy species [6]. LTP igniters were found to be capable of ensuring a higher lean/diluted combustion robustness. At the same time, electrode erosion or fouling--typical of spark igniters--was found to be minimized [6; 12; 14; 15; 16]. These advantages are related to the three combustion enhancement pathways: thermal, kinetic, and transport [17]. Among all the LTP systems, RF corona was intensively studied in recent years [18; 19; 20; 21], especially in optical engines [11; 14; 22], to characterize its ability to speed up the early flame kernel generation [15]. Independently of the engine application, optical techniques are increasingly used and improved to understand in-cylinder phenomena: trapped mass at intake valve closing [23], Sauter mean diameter for gasoline direct injection (GDI) [24], interaction between turbulent field and flames [25], and flame front characteristics (e.g., maximum propagation speed, circularity factor) [26]. In most of the cases, a very good match between optical and thermodynamic data was found, especially in terms of mass fraction burned and/or volume fraction burned [15; 27]. Optical techniques are also used in simplified configurations, such as constant-volume chambers, e.g., to determine the amount of energy released in air by a spark discharge [28]. Focusing on flame front analysis, the most critical step is the segmentation, i.e., the automatic partitioning of an image into significant regions [29]. For combustion applications, there are only two regions of interest, i.e., the burned and the unburned area. The process of binarization is extremely dependent on the threshold level, which can be defined in several ways. [PERSON]'s method [30], for example, minimizes intra-class intensity variance. [PERSON] et al. [31] proposed a semiautomatic method in which the initial binarization is implemented using the threshold of the previous image; the new threshold is calculated again as a linear function of the average intensity in this pre-estimated burned area. Unlike [PERSON]'s method, this was found to be effective for a large span of in-cylinder conditions, from stoichiometric to lean mixtures, and from the dark first frames of combustion onset to the brightest ones [11; 15; 32; 33]. Nevertheless, none of the previous works and methods were specifically dedicated to corona streamer detection and analysis. As the typical discharge duration is tens (or few hundreds, at most) of microseconds [32], an original methodology, consisting of a high-speed image processing workflow, was developed in this work.