Focus Measure Operator Combining Cosine Transform and Laplacian Operator

Focus evaluation is a numerical analysis method that searches and obtains the optimal image point position based on the defocus sequence images. Its resolution directly determines the reconstruction accuracy of the three-dimensional focus shape restoration technology. This paper proposes a high-resolution focus evaluation method that combines the focus evaluation results of the two methods based on the spatial Laplacian operator and frequency domain discrete cosine transform. First, the discrete cosine transform operator was used to perform focus evaluation on the images. Moreover, the result of the symmetric transformation of the Laplacian operator focus evaluation value was used as the weighting factor. The weighting factor was then used to correct the discrete cosine transform evaluation value to increase the response to the image focus feature and suppress the response to the image defocus feature. By analyzing the defocus optical system, the defocus sequence images were generated by method of calculating the defocus point spread function, and the effectiveness of this method was validated by simulation. Finally, an experimental platform for various defocusing scenes was constructed using various imaging lenses and targets. The proposed focus method and the five traditional typical methods were used to compare and analyze the focus of the acquired defocus sequence images. The results show that the proposed focus evaluation method can achieve excellent evaluation results in both small and large defocus images, and the peak sensitivity and steepness are significantly improved. The proposed method has better resolution and response sensitivity of focus evaluation and can effectively improve the accuracy of three-dimensional focus profile restoration.