Flicker compensation method based on scale-time equalization for videogrammetric measurement images in high speed wind tunnels

The complex illumination conditions in the high-speed wind tunnels can easily lead to the flicker of the videogrammetric measurement(VM) test image sequences, which affect the accuracy of the measurement result. As the flicker contains both the global change of the gray level caused by the illumination and the local change caused by the object motion, deformation and so on, it is difficult to apply the compensation methods based on affine transformation model (linear or non-linear) between pixels or between blocks of the images. Based on the scale space theory, the scale-time equalization(STE) method was proposed by Delon(2006) and applied to correct flicker in video and movie. The STE method performs the Gaussian convolution in the time dimension of each gray value in the histograms to obtain the target histogram of each image, and then uses histogram matching to get the flicker compensated images. Three experiments with different flicker and interference were conducted:the camera calibration plate image under the fluorescent lighting, the deformation test image of the wind tunnel model and the cavity oil film test image with diffusion. The experimental results show that the STE method is suitable for the global flicker compensation of the VM test image sequence. Compared with the model-based methods, the flicker compensation effect is not dependent on the reference image, and the robustness to the interference factors such as image jitter, local motion or deformation is strong. Meanwhile, the method has the advantages of simple procedures and small computation cost, and therefore it has high engineering application value.