In computer vision tasks, various types of objects exhibit distinct characteristics in images. By learning and synthesizing the commonalities present in the training set, the neural network effectively performs tasks associated with diverse objects. However, when the training set is incomplete—specifically, when certain classes are missing—it becomes challenging for the network to learn the features of these absent classes during testing. Consequently, the coverage of the training data must be evaluated prior to training the network. This study uses synthetic aperture radar (SAR) aircraft detection as an example to illustrate the importance of evaluating dataset coverage, introduce evaluation methods, and propose solutions for incomplete dataset. Variations in SAR target features occur when the relative observation angle of SAR changes, causing changes in the brightness of the target scattering points. SAR images can exhibit significantly different characteristics even for similar targets. Based on this characteristic, the aircraft in SAR images are classified into eight angle-based classes. If the training set includes fewer than eight angle types for aircraft (at least one), the network will be unable to detect aircraft from all eight angles in the test set. To tackle potential issues arising from incomplete training sets, the following solutions are proposed: Firstly, a clustering algorithm is employed to classify the labeled data more accurately by considering the differences in the heat maps of various feature data. Next, the average heat map is extracted for each data class, overlaid, and compared with the average heat map of the complete test set to identify any missing data types. Finally, the training set is supplemented with the appropriate data based on the identified missing data types. Experimental results using partial data from the SAR-AIRcraft-1.0 dataset demonstrate the effectiveness of the proposed method.
A device is built to create virtual scenarios to realize intelligent perceptual tests of machine vision. The device consists of multiple projectors and uses light field superposition to build a multi-channel scenario projector, based on which any application scenario can be virtually reproduced on a two-dimensional plane. Currently, the device can cover 430~700 nm, and the effective number of controllable pixels is more than 1 million. We perform spectral segmentation of different channels, which can enhance the freedom of spectral combination for 2D scene reproduction. The application of the above multi-channel projector for intelligent perception is investigated using number plate character recognition as a case study. An image dataset of license plate character recognition for testing and evaluation was designed, and experiments on license plate character perception were carried out to demonstrate the application of the multi-channel projector in intelligent perception. The multi-channel projector-based test device and the basic test procedure, with reasonable scene matching and customization, can be used for testing and evaluating the intelligent perception performance of machine vision systems in a variety of scenes.
KEYWORDS: Cameras, Imaging systems, Machine vision, Light sources and illumination, Digital cameras, Image resolution, Spatial resolution, Signal to noise ratio
This paper preliminarily investigates the performance of machine vision system from the perspective of contrast and object detection process. We set up a testing system using a lightbox, transmissive/reflective test charts, a photometer, and two cameras. The photometer was used to obtain the standard luminance of the test chart. First, we obtain the DN response characteristics for luminance of two cameras with different dynamic ranges (72 dB and 123.6 dB). Based on this result the relationship between luminance domain and camera domain contrast ratio is provided. Distribution of the signal and contrast in the camera domain under low luminance conditions show the advantage of 16bit camera over 8bit camera. To link the camera's performance in practical scene, we conducted imaging tests of reflective resolution targets under various illuminance levels. We observed that the contrast and imaging quality of resolution targets by the camera at critical states can help establish correlation between single-metrics and scene-based imaging recognition performance evaluation.
Synthetic Aperture Radar (SAR) emits microwave electromagnetic pulses and detects remote targets through the backscattered echo signals. With the continuous advancement of technology, high-resolution SAR imaging can accurately observe various targets with small sizes, dense connections, and diverse shapes. The steep terrain or tall objects in SAR images display prominent shadows due the obstruction of ground-facing electromagnetic waves resulting in weak echo signals in the shadowed areas. Small targets can generate discrete shadows with contours similar to optical contours in high-resolution SAR images when the radar observing angle is appropriate. This characteristic, which shares similarities with the target contour, can be utilized to build the correlation between SAR images and other modal images such as optical images. This study uses aircraft as an example to validate the feasibility of this approach. It trains the YOLOv5 object detection network on the Remote Sensing Object Detection (RSOD) dataset of optical airport images, which is then utilized to detect the shadows of aircraft in high-resolution SAR images, indirectly achieving aircraft detection.
Firstly, the shadows in SAR images have an inverse relationship with the signal of the aircraft body in optical images in terms of grayscale. Therefore, it is possible to simply invert the grayscale of one of them. In this study, SAR images were chosen for grayscale inversion. After a simple grayscale inversion, the network detected a significant number of aircraft in the image at a confidence level of 0.2, while only part of aircraft were detected in the image without inversion. Besides, a series of adjustments were made to the brightness and contrast of the grayscale inverted image in order to find the optimal setting for aircraft detection. After scanning and adjusting the brightness and contrast, the network detects a certain number of additional aircraft in the grayscale inverted images at a confidence level of 0.2. The maximum number of aircraft detections was achieved at a specific filtering spatial frequency after applying filtering with different spatial frequencies. The overall detection result achieved an accuracy of over 80%.The maximum number of aircraft detections was achieved at a specific filtering spatial frequency after applying filtering with different spatial frequencies. The overall detection result achieved an accuracy of over 80%.
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