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Restoring severely out-of-focus blurred text images with Deep Image Prior

  • *Corresponding author: Leonardo A. Ferreira

    *Corresponding author: Leonardo A. Ferreira 
Abstract Full Text(HTML) Figure(13) / Table(6) Related Papers Cited by
  • Deblurring is a classical image processing problem with several techniques to solve it. However, increasingly complex methods are required as the degradation increases. Meanwhile, Deep Image Prior (DIP) is a technique, based on artificial neural networks, that does not depend on training data and presents promising results in several image processing tasks. In this work, we proposed the combination of DIP with a supervised convolutional neural network to deblur severely out-of-focus blurred text images from the Helsinki Deblur Challenge (HDC2021) dataset. We evaluated the deblurred text images using optical character recognition results and had a satisfactory performance up to the 16th highest blur category. We deblurred natural images of the same dataset to characterize our method as a general-purpose deblurring algorithm, recovering moderate details up to the 13th highest blur category. The experimental results were competitive against other state-of-art methods, showing the potential and robustness of our method. However, its high computational demands may hinder real-time applications.

    Mathematics Subject Classification: 68T07, 68U10.


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  • Figure 1.  Scheme of the DIP algorithm for deblurring. A noise tensor ($ \mathbf{N} $) is processed by a CNN ($ \phi_{ {\mathbf{ \pmb{\mathsf{ θ}} }} }(\cdot) $) which generates an output, which is given as input to the blur model ($ \varphi_{ {\mathbf{ \pmb{\mathsf{ α}} }} }(\cdot) $) to generate a blurred image. In the first stage, a loss function $ L_1(\cdot, \cdot) $ compares this blurred output to the input image of the deblurring algorithm ($ \mathbf{Y} $). An optimizer updates the parameters of the CNN and the blur model to minimize the loss value. Consequently, the CNN output becomes closer to the desired sharp image

    Figure 2.  (a) The initial uniform disk kernel. (b) Rings area, displayed by the regions between two consecutive dashed circles, totaling four rings beyond the kernel border and six rings within. (c) The remaining white circle is the core, and the dotted circle marks the kernel boundary. Optimization updates only the rings and the core of the kernel

    Figure 3.  Scheme of the proposed supervised CNN inference phase. First, the input full-size image is divided into patches (purple square). The patch is given as input to the supervised CNN and a central square (green dashed lines) is extracted from the corresponding output. Then, the final image is composed of this extracted square (green solid lines). By repeating this procedure throughout the image, with steps of $ 64 $ pixels between the extracted patches, the whole image is restored

    Figure 4.  Scheme of the third stage. At iteration $ 175 $, we calculate the average image from the last $ 50 $ DIP network outputs. The resulting image is the input to the supervised CNN and $ \tilde{\mathbf{X}}_3 $ is obtained. Further $ 50 $ iterations of optimizing $ L_3 $ are performed, in which the loss function compares the result of the supervised CNN to the DIP network output, and the final result is the mean of the method output observed during these 50 iterations

    Figure 5.  Deblurred images from category $ 8 $: Times New Roman

    Figure 6.  Deblurred images from category $ 8 $: Verdana

    Figure 7.  Deblurred images from category $ 8 $: Natural

    Figure 8.  Blur Category $ 12 $ examples. Rows from top to bottom: Times New Roman, Verdana, and Natural

    Figure 9.  Blur Category $ 15 $ examples. Rows from top to bottom: Times New Roman, Verdana, and Natural

    Figure 10.  Blur Category $ 19 $ examples. Rows from top to bottom: Times New Roman, Verdana, and Natural

    Figure 11.  Blur Category $ 14 $ examples. Rows from top to bottom: Times New Roman, Verdana, and Natural

    Figure 12.  Results of the blur category 8 with the initial radius of the blur kernel being 2 pixels greater than the estimated value

    Figure 13.  QR Code examples. From top to bottom: Sharp, blurred and DIP-I reconstructions

    Table 1.  Neural network architecture used for the DIP ($ \phi_{ {\mathbf{ \pmb{\mathsf{ θ}} }} } $)

    Parameter DIP
    Input size $ 320 \times 512 \times 32 $
    Output size $ 320 \times 512 \times 1 $
    No. of encoder layers 8
    No. of decoder layers 8
    No. of filters in each encoder layer 128 (all)
    No. of filters in each decoder layer 128 (all)
    Size of filters in each encoder/decoder layer $ 3 \times 3 $ (all)
    No. of skip layers 8
    No. of filters in each skip layer 32
    Size of filters in each skip layer $ 1 \times 1 $
    Activation function ReLU
    Activation function (last layer) Sigmoid
    Loss Function ($ L_1 $) See Equation (11)
    Optimizer Adam
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    Table 2.  Values of the estimated radius for each blur category used in this study

    Blur category Radius (pixels)
    8 13
    12 20
    15 26
    19 44
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    DownLoad: CSV

    Table 3.  Neural network architecture used for the supervised CNN ($ \phi^{sup}_{ {\mathbf{ \pmb{\mathsf{ θ}} }} }) $

    Parameter Supervised CNN
    Input size $ 96 \times 96 \times 1 $
    Output size $ 96 \times 96 \times 1 $
    No. of encoder layers 5
    No. of decoder layers 5
    No. of filters in each encoder layer [64, 64, 128, 128, 256]
    No. of filters in each decoder layer [128, 128, 64, 64, 1]
    Size of filters in each encoder/decoder layer $ 3 \times 3 $ (all)
    No. of skip layers 4
    Activation function ReLU
    Activation function (last layer) ReLU
    Loss Function ($ L_2 $) 1 - SSIM
    Optimizer Adam
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    Table 4.  Values of the parameters used for training the CNN

    Parameter Value
    Batch size 64
    Batches per epoch 500
    Max no. of epochs 30
    Tolerance (early stopping) 1 × 10−6
    No. of epochs (early stopping) 4
    L1 regularization 1 × 10−9
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    Table 5.  Quantity of correctly identified characters using OCR in the resulting images of each method. Presented as mean $ \pm $ standard deviation

    Method/Category 8 12 15 19
    Ground truth $ 93.8 \pm 9.0 $ $ 95.5 \pm 7.0 $ $ 96.5 \pm 6.7 $ $ 96.0 \pm 5.2 $
    DIP-O $ 58.9 \pm 25.9 $ $ 32.0 \pm 25.6 $ $ 45.6 \pm 23.0 $ $ 0.0 \pm 0.0 $
    DIP-I $ 80.9 \pm 18.7 $ $ 79.7 \pm 16.6 $ $ 73.4 \pm 21.5 $ $ 37.8 \pm 18.9 $
    DIP-M $ 81.9 \pm 19.1 $ $ 72.3 \pm 24.3 $ $ 78.3 \pm 14.9 $ $ 31.5 \pm 16.7 $
    DIP-P $ \mathbf{91.7 \pm 12.1} $ $ \mathbf{ 87.8 \pm 11.0} $ $ \mathbf{82.9 \pm 13.7} $ $ \mathbf{39.8 \pm 18.6} $
     | Show Table
    DownLoad: CSV

    Table 6.  FR-IQA measures in blur category $ 8 $

    Text Natural
    DIP-O $ 0.500\pm 0.021 $ $ 18.8\pm 0.3 $ $ 0.260\pm 0.105 $ $ 12.2\pm 1.7 $
    SelfDeblur $ 0.523\pm 0.012 $ $ 17.0\pm 0.4 $ $ 0.262\pm 0.102 $ $ 11.7\pm 1.3 $
    DeepRED $ 0.575\pm 0.008 $ $ 18.0\pm 0.3 $ $ \mathbf{0.292\pm 0.108} $ $ 12.0\pm 1.5 $
    DIP-I $ 0.613\pm 0.007 $ $ 20.2\pm0.4 $ $ 0.287\pm 0.112 $ $ \mathbf{12.2\pm 1.6} $
    DIP-M $ 0.613\pm 0.007 $ $ 20.5\pm0.4 $ $ 0.284\pm 0.112 $ $ \mathbf{12.2\pm 1.6} $
    DIP-P $ \mathbf{0.617\pm 0.006} $ $ \mathbf{22.5\pm 0.6} $ $ 0.261\pm 0.110 $ $ 8.5\pm 2.2 $
     | Show Table
    DownLoad: CSV
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