Izvedba parametričnega nelinearnega filtra za iskanje kožnih značnic v digitalni sliki z FPSLIC tehnologijo

slovenian

2003

volume 33, issue 1

Strokovno društvo za mikroelektroniko, elektronske sestavne dele in materiale

This article is about development of mathematical model and hardware implementation of the digital filter for skin features identification. With increasing popularity of digital video the computer vision is becoming one of the common media in mainstream electronics. One of the general tasks that are required by consumer computer vision systems is image understanding like detection and tracking of people. Before those tasks can be performed, the images have to be preprocessed. Suchlike tasks can be very cumbersome for general purpose computers. In such cases the skin color can be a very comprehensive feature to achieve the goal. The core of color tracking is color based image segmentation. Introduced parametrical digital filter makes it possible to filter digital images in YCbCr color space and on its output it gives digital images in a binary mask fashion. As the primary purpose of this digital filter is to identify skin color tones in digital images that is the main preprocessing task in detection and tracking of skin color formations, the parametrical design of the filter also offers color detection at comprehensive sense. In contrast to other classical digital filters known for digital audio and video signal processing with convolution equation, this filter features a multitude of threshold functions as transfer function. Appropriated thresholds are defined in HSV color space for the value and saturation components where the hue attribute is reliable. Therefore we use a polyhedron with appropriate threshold values that correspond to the skin-colored clusters with well-defined saturation and value components, basedon a large sample set. Each pixel from the digital image that passes through the filter is labeled with appropriate binary information. This binary information creates a binary mask with the same size as the original image where a logical one points to skin-colored pixel. As the applicable level of shown digital filter extends to digital image preprocessing the low level hardware implementation is very suitable. The hardware implementation is shown with FPSLIC technology from Atmel that features high performance combination of FPGA programmable logic circuits and RISC microcontrollers. The given digital filter structure is suitable for low-level nontemporal preprocessing tasks that operate on pixel level. Shown architecture of the digital filter offers sufficient computing power or data bandwidth to satisfy the requirements for full resolution real-time image processing in standard PAL picture formats. For hardware implementation we developed a mathematical model that enables calculation of the digital filter parameters from given boundary values in HSV color space. The digital filter hardware is completely designed in FPGA part of FPSLIC circuit. The embedded FPSLIC RISC microcontroller features parameter computation within the given mathematical model. The appropriate boundary threshold values for skin color identificationin HSV color space are pass ed to the filter through asynchronous serial port and afterwards used for parameter computation. After calculation the computed parameters are directly applied to the digital structure of the filter by the embedded microcontroller. For parameter storage we use 8-bit wide registers that are implemented in the FPGA part of the FPSLIC. Such digital structure enables dynamical changing of the filter parameters as they are computed by the embedded microcontroller and their computation doesn't disturb the digital image filtering which is done by the FPGA logic. As it is shown in the given mathematical model we have expose the polyhedron threshold values to several conditional states that are computed bythree logical processing units. We have shown that six conditional states are needed to delineate the given threshold polyhedron in HSV color space. Each of those three processing units is performing two parameters computation so two of conditional states are processed by each unit. Through the parallel structure we get six computed conditional states for each image pixel that aregathered together for the binary pixellabel on the filter output by a simply logical function. As FPSLlC integrated circuits utilize many peripheral units with open connection possibilities it is simple to interface the digital filter to other devices like video decoders and development tools for digital signal processing. Through simulation and testing we found that the performance of digital filter mostly depends on lightning conditions of the captured scene. In thoseć cases where lightning conditions are worse the value threshold in HSV color space or the brightness parameters have to be appropriately corrected to increase the performance of the filter in identifying skin-colored features in digital image. In the future we will implement automatic tracing of the lightning conditions in captured scene and re-computation algorithm for specific brightness depending parameters.

18.12.2015