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iNEDI - Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation
 

iNEDI - Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation

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In this paper we analyse the problem of general purpose image upscaling that preserves edge features and ...

In this paper we analyse the problem of general purpose image upscaling that preserves edge features and
natural appearance and we present the results of subjective and objective evaluation of images interpolated
using different algorithms. In particular, we consider the well-known NEDI (New Edge Directed Interpolation,
Li and Orchard, 2001) method, showing that by modifying it in order to reduce numerical instability and
making the region used to estimate the low resolution covariance adaptive, it is possible to obtain relevant
improvements in the interpolation quality. The implementation of the new algorithm (iNEDI, improved New
Edge Directed Interpolation), even if computationally heavy (as the Li and Orchard’s method), obtained, in
both subjective and objective tests, quality scores that are notably higher than those obtained with NEDI and
other methods presented in the literature.

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    iNEDI - Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation iNEDI - Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation Presentation Transcript

    • Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation Nicola Asuni Andrea Giachetti nicola.asuni@tecnick.com andrea.giachetti@univr.it UNIVERSITY OF CAGLIARI - ITALY UNIVERSITY OF VERONA – ITALY Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Department of Computer Science Vision, Image Processing & Sound Lab
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Summary Introduction interpolation and image upscaling, short literature review NEDI - New Edge Directed Interpolation how it works, limits iNEDI – improved NEDI modified method to reduce NEDI artifacts Experimental Results iNEDI provides the best results in a large set of objective and subjective tests. Conclusions N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 2
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Interpolation Constructing new data points from a discrete set of known data points. Construct a function which closely fits a discrete set of known data points (curve fitting or regression analysis). Generally, it is an ill-conditioned problem. The selection of the adequate interpolation method depends by the nature of know data points. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 3
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Digital Image Interpolation (1) Process that allows to obtain an High Resolution image form its Low Resolution counterpart. Images ©2005 Nicola Asuni - Alamy LR upsizing, resampling, zooming, magnification, HR resolution enhancement N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 4
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Digital Image Interpolation (2) LR HR The interpolation procedure tries to recover missing information by assuming that there is a known relationship between a low resolution image and the same image acquired with an high resolution sensor. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 5
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Why do we need to resample? Image upscaling through pixel interpolation is used in different fields and devices to create high resolution images with a "natural" appearance from low resolution acquired data. Applications of this procedure can be found in image viewing or processing software, photographic printing and Computer Graphics. Icons ©2005 www.tpdkdesign.net Real time algorithms can also be applied to increase the perceived quality of video streaming or textures in virtual navigation tools. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 6
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Interpolation Artifacts General upsampling algorithms do not add real information. Interpolation cannot reconstruct original high frequencies but edge geometries could be preserved. Sharpness and freedom from artifacts in edges are two critical factors in the perceived quality of images. aliasing (jaggies) blurring edge halo frequency artifacts N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 7
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Interpolation Methods Several algorithms have been proposed in literature to obtain better results and several patents have been obtained for "smart" interpolation techniques. Few systematic comparisons have been, however, presented and it is difficult to determine which method is the best suited for a selected application. Main categories Linear Filtering Non-linear methods Statical methods Edge-directed methods N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 8
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Linear Filtering The simplest image interpolation algorithms are based on linear filtering. OR NN BL BC These methods are computationally efficient and especially the bicubic interpolation provides visually good images, that do not appear, however, "natural" due to blur and jagged contours. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 9
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Non-Linear Methods Non linear methods are usually based on an implicit or explicit search of local image features and on a subsequent local adaptation of the interpolation function to the (low resolution) extracted features. Lu, X., Hong, P. S., and Smith, M. J. T. (2003). An efficient directional image interpolation method. In Proc. IEEE Int. Conf. Acoustics Speech Signal Processing, volume 3, pages 97–100. Schultz, R. R. and Stevenson, R. L. (1994). A bayesian approach to image expansion for improved definition. IEEE Trans. Image Processing, 3:233– 242. Takahashi, Y. and Taguchi, A. (2002). An enlargement method of digital images with the prediction of highfrequency components. In Proc. IEEE Int. Conf. Ac. Speech Signal Proc., volume 4, pages 3700–3703. Morse, B. and Schwartzwald, D. (2001). Image magnification using level- set reconstruction. In Proc. IEEE Conf. Computer Vision Pattern Recognition, volume 3, pages 333–340. Muresan, D. and Parks, T. (2004). Adaptively quadratic (aqua) image interpolation. IEEE Transactions on Image Processing, 13(5):690–698. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 10
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Statistical Methods Some authors have tried to exploit pixel or texture statistics or databases of example images to obtain good high resolution images. The huge variety of natural textures and scales makes, however, quite difficult a general purpose use of similar techniques, though they can be efficiently applied to particular tasks such as searching patterns like faces, trees, etc. Atkins, C. B., Bouman, C. A., and Allebach, J. P. (2001). Optimal image scaling using pixel classification. In Proc. IEEE Int. Conf. Image Processing, volume 3, pages 864–867. Freeman, W. T., Jones, T. R., and Pasztor, E. C. (2002). Example-based super-resolution. IEEE Computer Graphics and Applications, 22(2):56–65. Sun, J., Zheng, N., Tao, H., and Shum, H. (2003). Image hallucination with primal sketch priors. In Proceedings IEEE conf. on Computer Vision and Pattern Recognition, volume 2. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 11
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Edge-Directed Methods These methods try to improve the accuracy of the interpolation characterizing the edge features in a larger region around the point to interpolate. They aims at interpolating along edges rather than across them to prevent blurring. They usually produce images with sharper edges. J. Allebach and P. W. Wong, "Edge-directed interpolation," Proc. IEEE Int. Conf. Image Processing, vol. 3, pp. 707–710, September 1996. X. Li and M. T. Orchard, "New Edge-Directed Interpolation," IEEE Trans. on Image Processing, Vol. 10, No. 10, pp. 1521-1527, October 2001. NEDI seems to provide the best results for natural images, even in the case of large scale factors. This is why we start our analysis describing this technique and then proposing several improvements. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 12
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI (New Edge-Directed Interpolation) Edge-directed adaptative method for natural images. Its behavior depends on what it is interpolating. It uses bilinear interpolation for smooth regions. This method first estimates local covariance coefficients from a low-resolution image and then use these covariance estimates to adapt the interpolation at a higher resolution based on the geometric duality between the low-resolution covariance and the high-resolution covariance. The image size is doubled in each direction at each cycle. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 13
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI – 4th order interpolation (1) The darker circles represent the pixels from the original Low Resolution grid, the white circles represent the pixels that are to be interpolated and the central circle is the pixel to be interpolated in this first step. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 14
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI – 4th order interpolation (2) The central point is calculated using a fourth- order interpolation algorithm that simply sums the contribute of each diagonal neighbor multiplied by its alpha coefficient. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 15
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI – Estimation of Coefficients (1) NEDI estimates α coefficients by solving an unconstrained system of linear equations. The system is obtained by assuming that the coefficients linking each pixel with its four diagonal neighbors do not change with scale and that they are constant in a squared window W centered in the pixel to interpolate. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 16
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI – Estimation of Coefficients (2) The y vector contains the values of the pixels contained on the window W. The C matrix contains in each row the values of the four diagonal neighbors of each pixel listed on y vector. NEDI estimates α coefficients by minimizing the Mean Square Error (MSE). N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 17
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI – Second Step The second NEDI step fills the remaining gaps in the same way of the first step after a π/4 rotation of the grid. The darker circles represent the pixels from the original Low Resolution grid, the white circles represent the pixels that are to be interpolated and the gray circles represent the pixels interpolated on the first step. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 18
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI – Example 8X NEAREST NEIGHBOR BICUBIC NEDI Jagged contours are evident in nearest neighbor and bicubic interpolation. The NEDI interpolation presents sharp edges, even if introduces different artifacts and performs often worser than the bicubic method in quantitative comparisons. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 19
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science NEDI – Limits The assumption of local stationarity of the covariance (local regularity in curvature) is violated in several cases. Introduces several artifacts on high-frequency regions. The system is often ill-conditioned. The use of larger windows improves the conditioning of the CTC matrix but produces blurred images. Interpolated pixel values change with the global brightness. Very high computational complexity. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 20
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI (improved NEDI) iNEDI (improved New Edge-Directed Interpolation) is the proposed technique to fix/reduce the original NEDI problems and increase the interpolation accuracy. Main improvements: Non edge pixels handling Windows shape and dynamic windows Edge "segmentation" Value Adjust Matrix conditioning, error propagation and minimum norm solution N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 21
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI – Non edge pixels handling When the four pixels used to calculate the interpolated ones have a similar gray level, there is no need to compute the NEDI coefficients. If the covariance is stationary, a small error causes a bad conditioning of the solution. If the local gray level variation is below a fixed threshold NEDI uses the bilinear interpolation, while iNEDI uses bicubic interpolation. This choice gives a good trade-off between edge direction preservation, accuracy and speed, using higher values of the threshold (using iNEDI only for strong edges). N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 22
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI – Windows shape The squared window W used by NEDI introduces directional artifacts and makes the algorithm non isotropic. This has been solved by iNEDI using a circular window. To account for various frequencies regions, iNEDI can dynamically increase the window radius from a minimum to a maximum value while the normalized residual of the least squares fit decreases. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 23
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI – Edge "segmentation" iNEDI tries to exclude from the circular window all pixels that do not belong to the local edge using a sort of region growing method that excludes all uniform areas and pixels that are not connected to the edge. Two pixels are connected if they are adjacent and their values are included in a predefined range. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 24
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI – Value Adjust iNEDI increases the probability of obtaining a good interpolation, but there is still the possibility of having unwanted high frequencies. For this reason we put a further constraint by replacing any interpolated value outside the intensity range of the four neighbors with the closest of the values delimiting that range. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 25
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI – Matrix conditioning, error propagation and minimum norm solution The overconstrained system Cα = y is almost always ill-conditioned and a small error in y can cause a large error in the estimated α. A simple trick is to add a constant value to the gray levels, in order to have all values far from zero. This change is effective in reducing artifacts and wrong estimates. The solution to the least squares problem is not unique. On iNEDI we rewrite the system as α = C+y where C+=(CTC)-1CT is the Moore-Penrose pseudoinverse matrix, and find the minimum norm solution using the Singular Value Decomposition. In this way we avoid the direct inversion of CTC matrix. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 26
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI – Implementation for Testing iNEDI prototype has been implemented in matlab language (compatible with MATLAB® and Octave software applications) and released as a FLOSS by adopting GPL v.2 license. http://inedi.tecnick.com iNEDI code includes: Simple edge detection on threshold Padding procedures to preserve image borders Cropping and Rotation procedures RGB image handling N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 27
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science iNEDI vs NEDI – 16X Example NEDI iNEDI N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 28
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Experimental Results iNEDI has been widely tested and compared with other methods found in literature, as well as with the original NEDI. We have used both objective and subjective evaluation tests. Objective evalutation A simple test often used in literature consists of generating low resolution images by filtering and subsampling high resolution ones and then measure the difference between the differently re-upsized images and the original one. Subjective evalutation We have asked to a group of 24 people to sort from the worse to the best a set of 12 RGB images enlarged by a 8x factor with four different algorithms (iNEDI, NEDI, bicubic and bilinear). N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 29
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – Objective Metrics PSNR (Peak-to-Signal Noise Ratio) is used to compare the differences between interpolated and reference images. PSNR measures the accuracy of reconstruction. The constant MAXPIX is the end scale value of the pixel intensity. MSE is the Mean Square Error between pixel values of the original and interpolated image. Typical PSNR values are between 20 and 40 dB. Higher values indicates a better image reconstruction. Improvements of 0.1 dB are usually considered significative. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 30
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – Test images 9 8bit 1025x1025 natural images, subsampled at 256x256 and 128x128 and then upscaled respectively two times and four times using different algorithms. Different target images (512x512 and 511x511) were used to account the different image shifts introduced by the various techniques. We also crop a 50 pixel frame before comparison to account for the different behavior of the selected methods near the image border. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 31
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – 2X PSNR Measures iNEDI improvements vs NEDI are between 0.54 and 1.63 dB with an average value of 0.85 dB. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 32
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – 4X PSNR Measures iNEDI improvements vs NEDI are between 0.52 and 1.41 dB with an average value of 0.85 dB. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 33
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – PSNR Measures The iNEDI accuracy of the reconstruction is higher than those obtained with the other techniques. NOTE: The good results of the bicubic interpolation does not mean that it is surely better than other methods. Original NEDI, as well as the other edge based method tested, are effective in removing the typical artifacts of the bicubic and bilinear interpolation. The lower PSNR is probably due to the other kinds of artifacts affecting NEDI and the excessive smoothing of the other approaches. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 34
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – 8X Artificial Image +3.43 dB iNEDI removes the jagged lines effects of the linear methods and the directional artifacts of NEDI. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 35
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – Extended Comparison To compare the iNEDI technique with other techniques available on commercial software, we have extended the test provided by the SAR Image Processor Software: http://www.general-cathexis.com/interpolation.html The iNEDI algorithm provided a PSNR 1 dB higher than the best one of the 42 methods included (DDL with SuperRez postprocessing). N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 36
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – Subjective evaluation Traditional Signal-to-Noise Ratio measures cannot be compared with the perceived quality by humans. A group of 24 people have been asked to give a qualitative judgment on 12 color images originally of 80x60 pixels and enlarged of a factor 8 with iNEDI, NEDI, Bicubic and Bilinear interpolation. The qualitative judgment has been performed sorting the images from the worst (1) to the best (4). N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 37
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Results – Subjective measures The subjective results confirmed the results of the analysis based on the PSNR. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 38
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Conclusions Images enlarged with the proposed technique appear more natural and less smoothed than those obtained with other approaches presented in literature, and both psychological and quantitative tests confirm this fact. For selected applications, such as printing or off line extrapolation of high resolution textures from low resolution data, the relevant computational effort is not a problem, while for applications requiring a fast image processing, such as improving quality of video streaming, different methods should be applied, even if the algorithm can be optimized and parallelized. N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 39
    • UNIVERSITY OF CAGLIARI Faculty of Mathematical, Physics and Natural Sciences Department of Computer Science Thanks for your attention. Any Questions? N. Asuni, A. Giachetti: Accuracy Improvements and Artifacts Removal in Edge Based Image Interpolation VISAPP 2008 - 22/25 January, 2008 - Funchal, Madeira - Portugal 40