**4. Results**

Figure 30 shows the dependences of the objective comparison of distances (PSNR) between the original value of the brightness of the pixel with the values obtained after applying the appropriate filters. The data in the chart are summarized.

**Figure 30.** Results of objective assessment (PSNR) of symmetry of researched operations.

The experiment was performed as follows. The original image was illuminated at 50 lux, 70 lux, 100, 260, and 2300 lux. These images are presented in Figures 2, 6, 10, 14, 18, 22 and 26, respectively. Canny, Sobel, and Prewitt filters were used for these images. These filtered images were compared with each other to assess the best performance of the filter and to make recommendations for the use of certain filters. Diagram 1 shows three graphs of the relationship between the signal-to-noise ratio of the filtered images. The blue color shows the values of the signal-to-noise ratio between img\_sobel and img\_sobel\_x. This value takes the value of 7.23 and is the largest value in this chart, and therefore, they are the most similar to each other. The second value is the result of comparing img\_sobel and img\_sobel\_y and takes the value of 6.92. We also observe high PSNR values compared to others, which explains that very similar transformations were used. The other values are a comparison between the pairs img\_sobel and img\_cany and img\_sobel and img\_prewitt\_x, and img\_sobel and img\_prewitt\_y. Because filtering was applied by other filters, the images are less similar, and that makes sense. Charts of other colors, similar to img\_sobel\_x and img\_sobel\_y, work similarly. Slightly higher values between the pairs img\_sobel\_x and img\_prewitt\_y are explained by the better finding of contours in the image, which should be taken into account when choosing one filtering method.

We performed an experiment in which the edges of the source objects were marked on the input image in one of the raster graphics editors. Figure 31a shows the image taken at 50 lux, Figure 31b shows the filtered image of the Sobel operator, Figure 31c shows the Canny operator, and Figure 31d shows Prewitt's operator. As a result of filtering, more edges from the original image were formed in Prewitt and Canny, which is seen subjectively. An experiment was performed to compare the input image with the filtered images using PSNR, and the results are shown:

Input image, img\_sobel—11.7;

Input image, img\_canny—13.76;

Input image, img\_prewitt's—14.67.

This is best performed by Prewitt's operator, as the PSNR values are the highest.

**Figure 31.** (**a**) Input image; (**b**) Sobel operator; (**c**) Canny operator; (**d**) Prewitt's operator.
