# Patent

## Title
Method and device for generating a super-resolution version of a low resolution input data structure

## Abstract

The invention relates to the improvement of the resolution of regularly sampled multi-dimensional signals, where a single low-resolution signal is available. These methods are generically referred to as example-based super-resolution or single-image super-resolution. The method for super-resolving a single image comprises three stages. First, an interpolation-based up-scaling of the input image is performed, followed by an equivalent low-pass filtering operation on the low-resolution image. The second stage comprises a search for low-frequency matches between an inspected patch in the high-resolution image and patches in a local neighborhood in the low-resolution low-frequency image, including partly overlapping patches, and accumulating the high-frequency contribution obtained from the low-resolution image. The third stage comprises adding the contributions of the low-frequency band of the high-resolution image and the extrapolated high-frequency band.

## Background
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/EP2013/050466, filed Jan. 11, 2013, which was published in accordance with PCT Article 21(2) on Jul. 18, 2013 in English and which claims the benefit of European patent application No. 12305046.0, filed Jan. 12, 2012.
FIELD OF THE INVENTION
This invention relates to a method for generating a super-resolution version of a low resolution input data structure, and to a corresponding device.
BACKGROUND
Super-resolution (SR) processing is known as an improvement of the resolution of regularly sampled multi-dimensional signals. Of special interest is the case where only a low-resolution signal is available. It is a significant difference whether a single low-resolution signal is available or a plurality of similar low-resolution signals, since in the latter case it is possible to exploit richer data values by combining the contributions of the several available signals. In the image processing literature, these methods are generically referred to as example-based super-resolution or, more precisely, single-image super-resolution. Although the following remarks are general and can be applied to signals of different dimensionality, the focus will be on the case of 2D image super-resolution.
Image super-resolution techniques have been well known for many years, starting with “Super Resolution from Image Sequences” by M. Irani and S. Peleg. Most commonly, these techniques relate to the estimation of a high-resolution image, given a set of noisy, blurred, low-resolution observations, such as consecutive images in a video sequence, using a reconstruction process that reverses the image formation model. Thus, sub-pixel motion between images, camera and post-processing blur and sub-sampling are reversed in order to fuse the available data and obtain a super-resolved image. Several globally-optimal iterative techniques are available, which basically differ in the assumed image prior model. This provides unique solutions to the otherwise ill-posed problem.
In general, the limiting factors of these techniques are in the estimation of the Point Spread Function (PSF) for image deblurring (often assumed to be Gaussian) and the registration (determination of the sub-pixel motion between images). Generally, SR techniques in the literature, e.g. Lucas-Kanade or Horn-Schunck, refer to classical Optical Flow (OF) estimation techniques for obtaining the registration. These work well in quasi-synthetic examples, but in practice the known solutions in OF estimation are unable to robustly register consecutive frames in video-sequences with sufficient accuracy when more general motion appears.
In “Fundamental Limits of Reconstruction-Based Superresolution Algorithms under Local Translation”, Z. Lin and H.-Y. Shum show that, under a wide range of natural conditions, this type of reconstruction-based SR algorithms have a fundamental limit in the maximal increase in resolution of around 1.6×. However, the article proves that, in synthetic scenarios, which is the commonly explored in most of the available publications, a much looser limit exists, which allows for resolution increases of up to 5.7×. This is due to the favorable conditions in terms of registration, when sub-pixel shifts are generally exact fractions of the pixel size.
An alternative type of SR algorithms attempts to increase the resolution of images by adequately enriching the input visual data (low-resolution images) with a-priori known examples of higher-resolution. These techniques are commonly referred to as example-based super-resolution (EBSR). In “Example-based super-resolution”, W. T. Freeman, T. R. Jones and E. G. Pasztor obtain suitable high-resolution examples from a sufficiently generic image-patch data-base, the high-frequency contents of which are averaged and conveniently fused with the low-frequency contents of the input image. However, the performance of the algorithm worsens as the target scene deviates from the cases included in the example data-base (when none of the known patches actually resembles that of the input image). In practice, enlarging the size of the data-base would incur an excessive computational cost in the search for the best matching training patches. So, this technique is not generically usable, but is focused on super-resolving images of a certain class.
In order to cope with this problem and behave adaptively to the contents to be magnified, other EBSR algorithms extract high-resolution examples from within the single input image, for which a pyramid representation of the image at different resolutions can be obtained at small downscaling factors. Then, for every patch (e.g. 5×5 pixels) in the input image, matching patches are searched across all or part of the image at different resolutions (levels in the pyramid) in order to perform per-patch data fusion similarly to reconstruction-based super-resolution. This technique is best represented by “Super-Resolution from a Single Image” by D. Glasner, S. Bagon and M. Irani, and “Space-Time Super-Resolution from a Single Video” by O. Shahar, A. Faktor and M. Irani, which is a follow-up for video super-resolution. The authors obtain a simultaneous increase in image resolution and frame rate, including removal of temporal aliasing, at the cost of an increase of the computational complexity due to 3D spatio-temporal search across video frames at several spatial and temporal scales. This renders the approach unusable for real-time operation with current computing capabilities. This approach is also used in WO2010/122502 A1.
Other known approaches suffer also from being costly and in general not indicated for real-time approaches, or tending to produce some unrealistic-looking edges by imposing excessive contrast, or tending to generate over-smoothing in textured areas, which in a general case produces unnaturally looking images.
In “Image and Video Upscaling from Local Self-Examples” by G. Freedman and R. Fattal, the proposed strategy is to exploit self-similarity in a local neighborhood of each image patch. This is shown to provide results close to the full-image searches used in “Super-Resolution from a Single Image”, with the benefit of a reduced computation time. A drawback of this approach is that the highly sophisticated design of the space-variant filters used for separating high-frequency from low-frequency in the images is not done on the fly, which results in a limited set of selectable up-scaling factors.
SUMMARY OF THE INVENTION 
The present invention solves at least some of the above-mentioned problems. The invention relates to a method for the improvement of the resolution of regularly sampled multi-dimensional signals, where a single low-resolution signal is available. In the image processing literature, these methods are generically referred to as example-based super-resolution or, more precisely, single-image super-resolution. Although the methodology disclosed herein is general and can be applied to signals of different dimensionality, the following will focus on the case of 2D image super-resolution.
According to the invention, super-resolving a single image comprises three stages. First, an interpolation-based up-scaling of the input image is performed, followed by an equivalent low-pass filtering operation on the low-resolution image. The second stage comprises a search for low-frequency matches between an inspected patch in the high-resolution image and patches in a local neighborhood in the low-resolution low-frequency image (including partly overlapping patches), and accumulating the corresponding high-frequency contribution obtained from the low-resolution image. The third stage comprises adding the contributions of the low-frequency band of the high-resolution image and the extrapolated high-frequency band.
In principle, a method for generating a super-resolution version of a single low resolution digital input data structure S0 according to the present invention comprises steps of upscaling and low-pass filtering the single low resolution digital input data structure S0 to obtain a low-frequency portion L1 of an upscaled high resolution data structure, and separating the low resolution digital input data structure S0 into a low-frequency portion L0 and a high-frequency portion H0. A high-frequency portion H1,init of the upscaled high resolution data structure is created, which is initially empty. Then, for each of a plurality of patches of the low-frequency portion L1 of the upscaled high resolution data structure, a best matching block in the low-frequency portion L0 of the low resolution digital input data structure is searched, and its corresponding block in the high-frequency portion H0 of the low resolution digital input data structure is determined. The determined block from the high-frequency portion H0 of the low resolution digital input data structure is then added to the high-frequency portion H1,acc of the upscaled high resolution data structure, at the position that the above-mentioned patch in the low-frequency portion L1 of the upscaled high resolution data structure has. Finally, the resulting high-frequency portion H1,acc of the upscaled high resolution data structure is normalized to an average value of the contributing pixels, and, in one embodiment, high-pass filtered. The normalized, high-pass filtered high-frequency portion H1 of the upscaled high resolution data structure is added to the low-frequency portion L1 of the upscaled high resolution data structure, which results in an improved super-resolution version S1 of the single low resolution digital input data structure S0.
It is noted that for better readability the term “block” is used herein for a group of adjacent values in a low resolution data structure, while the term “patch” is used for a group of adjacent values in a high resolution data structure. However, a block and a patch have the same size (i.e. number and shape of adjacent values) and are substantially the same.
The present invention also relates to an apparatus for performing super-resolution processing of a low resolution input data structure S0 of digital data, comprising a first low-pass filter Fl,0 for filtering the input data structure S0, wherein a low-frequency input data structure L0 is obtained, a subtraction unit (e.g. adder, subtractor, comparator or differentiator) for calculating a difference between the input data structure S0 and the low-frequency input data structure L0, whereby a high-frequency input data structure H0 is generated, an upscaler for upscaling the input data structure S0, a second low-pass filter Fl,1 for filtering the upscaled input data structure, wherein a low-frequency upscaled data structure L1 is obtained, a first determining unit for determining in the low-frequency upscaled data structure L1 a first patch at a first position, a search unit for searching in the low-frequency input data structure L0 a first block that matches the first patch best, and a second determining unit for determining the position of said first block within the low-frequency input data structure L0, a selector for selecting a second block in the high-frequency input data structure H0 at the determined position (i.e. at the position that was determined for the first block within the low-frequency input data structure L0), an accumulator for accumulating (e.g. adding up) pixel data of the selected second block to a second patch, the second patch being a patch in a high-frequency upscaled data structure at the first position, a control unit for controlling repetition of the processing for a plurality of patches in the low-frequency upscaled data structures L1 and H1,acc, a normalizing unit for normalizing (i.e. averaging) the accumulated pixel values in the high-frequency upscaled data structure, whereby a normalized high-frequency upscaled data structure H1 is obtained, and a combining unit for combining (e.g. adding) the normalized high-frequency upscaled data structure H1 with the low-frequency upscaled data structure L1, whereby a super-resolved data structure S1 is obtained. The high-frequency upscaled data structure (H1,acc) is initially empty. In an embodiment, the normalizing comprises, for a current pixel, dividing the accumulated value of the current pixel by the number of pixels that have contributed to the accumulated value of the current pixel.
The present invention also relates to a computer readable medium having executable instructions to cause a computer to perform a method for performing super-resolution processing of a low resolution input data structure S0 of digital data, comprising steps of filtering the input data structure S0 by a first low-pass filter, wherein a low-frequency input data structure L0 is obtained,
calculating a difference between the input data structure S0 and the low-frequency input data structure L0, whereby a high-frequency input data structure H0 is generated, upscaling the input data structure S0, and filtering the upscaled input data structure by a second low-pass filter, wherein a low-frequency upscaled data structure L1 is obtained,

determining in the low-frequency upscaled data structure L1 a first patch at a first position, searching in the low-frequency input data structure L0 a first block that matches the first patch best, and determining the position of said first block within the low-frequency input data structure L0, selecting a second block in the high-frequency input data structure H0 at the determined position (i.e. the position that was determined for the first block within the low-frequency input data structure L0), accumulating (i.e. adding up) pixel data of the selected second block to a second patch, the second patch being a patch in a high-frequency upscaled data structure at the first position, wherein the high-frequency upscaled data structure is initially empty,

repeating the steps of determining a new patch in the low-frequency upscaled data structure L1, searching in the low-frequency input data structure L0 a block that matches the selected patch best, selecting a corresponding block in the high-frequency input data structure H0 and accumulating pixel data of the selected corresponding block to a patch in the high-frequency upscaled data structure at the position of said new patch,

normalizing (i.e. averaging) the accumulated pixel values in the high-frequency upscaled data structure, whereby a normalized high-frequency upscaled data structure H1 is obtained, and adding the normalized high-frequency upscaled data structure H1 to the low-frequency upscaled data structure L1, whereby a super-resolved data structure S1 is obtained.

An advantage of the invention is that it is able to produce higher-resolution renditions of any 1D, 2D or 3D digital input data structure (e.g., any digital image) with (in at least one embodiment) any desired non-integer image up-scaling factor. Furthermore, this is done in a general manner, free from the introduction of arbitrary image priors, beyond the assumption that the image must show local self-similarity at different resolution levels. This has proven to hold for general images.
Another advantage of the invention is that, due to the employed self-averaging, less noise is introduced in the upscaled data structure than with conventional methods. A further advantage of the invention is that it works with a single-image, but advantageously does not require a data-base, code book or similar, and not any training or training data, while conventional single-image methods require a data-base for trained retrieving of high-frequency examples.
Advantageous embodiments of the invention are disclosed in the dependent claims, the following description and the figures.



BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the invention are described with reference to the accompanying drawings, which show in
FIG. 1 a flow-chart of a method for performing super-resolution processing;
FIG. 2 synthesis of the high-frequency band of the super-resolved image by extrapolation of the high-frequency information of similar patches at the original resolution scale;
FIG. 3 exemplary usage and positions of a search window;
FIG. 4 selection of successive patches in a 2D input data structure, including overlap, and the principle of determining a matching block for successive patches;
FIG. 5 selection of successive patches in a 1D input data structure, including overlap, and the principle of determining a matching block for successive patches;
FIG. 6 formation of the up-scaled low-frequency band L1 of the high-resolution image and two-band analysis (L0, H0) of the low-resolution input image S0;
FIG. 7 a conceptual block diagram of the process for synthesizing the high-frequency band H1 of the super-resolved image;
FIG. 8 the fusion of the interpolated low-frequency band L1 and the extrapolated high-frequency band H1 for generating the super-resolved image S1;
FIG. 9 a block diagram of a device;
FIG. 10 a spectrum of an image upscaled according to the invention, compared with a spectrum of a conventionally upscaled image; and
FIG. 11 three principle stages of the invention.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows, in an embodiment of the present invention, a flow-chart of a method for performing super-resolution processing of a low resolution input data structure S0 of digital 1D, 2D or 3D data. In this embodiment, the method comprises steps of filtering 170 the input data structure S0 by a first low-pass filter Fl,0, wherein a low-frequency input data structure L0 is obtained,
calculating in an adder/subtractor 180 a difference between the input data structure S0 and the low-frequency input data structure L0, whereby a high-frequency input data structure H0 is generated,
upscaling 120 the input data structure S0, and filtering 130 the upscaled input data structure by a second low-pass filter Fl,1, wherein a low-frequency upscaled data structure L1 is obtained,
determining in the low-frequency upscaled data structure L1 a first patch Pn,L1 at a first position,
searching 151,152,154 in the low-frequency input data structure L0 a first block Bn,L0 that matches the first patch Pn,L1 best, and determining the position of said first block Bn,L0 within the low-frequency input data structure L0,
selecting 155 a second block Bn,H0 in the high-frequency input data structure H0 at the determined position, accumulating 157 data values (e.g. pixel data) of the selected second block Bn,H0 to a second patch Pn,H1, the second patch being a patch in a high-frequency upscaled data structure H1,acc at the first position (that was determined above for the first patch Pn,L1),

repeating 150 the steps of determining a new patch Pn,L1 in the low-frequency upscaled data structure L1, searching 151,152,154 in the low-frequency input data structure L0 a block Bn,L0 that matches the selected patch Pn,L1 best, selecting 155 a corresponding block Bn,H0 in the high-frequency input data structure H0 and accumulating 157 pixel data of the selected corresponding block Bn,H0 to a patch Pn,H1 in the high-frequency upscaled data structure H1,acc at the position of said new patch Pn,L1, and

normalizing 190 the accumulated pixel values in the high-frequency upscaled data structure H1,acc, whereby a normalized high-frequency upscaled data structure H1 is obtained. Finally, a super-resolved data structure S1 is obtained by adding the normalized high-frequency upscaled data structure H1 to the low-frequency upscaled data structure L1.

In some embodiments, the upscaled input data structure after filtering 130 by the second low-pass filter Fl,1 is downscaled 140 by a downscaling factor d, with n>d. Thus, a total non-integer upscaling factor n/d is obtained for the low-frequency upscaled data structure L1. The high-frequency upscaled data structure H1,init (or H1 respectively) has the same size as the low-frequency upscaled data structure L1. The size of H1 may be pre-defined, or derived from L1. H1 is initialized in an initialization step 160 to an empty data structure H1,init of this size.
FIG. 2 shows the principle of the synthesis of the high-frequency band H1 of a super-resolved (i.e. high resolution) image by extrapolation of the high-frequency information of similar patches at the original resolution scale H0. Note that, if in the following description with respect to FIG. 2-FIG. 7 the high-frequency high-resolution data structure H1 is mentioned, actually the unfiltered, non-normalized high-frequency high-resolution data structure H1,acc is meant.
The low-frequency band of the high-resolution image L1 is first divided into small patches Pn,L1 (e.g. 5×5 pixels) with a certain overlap. The choice of the amount of overlap trades-off robustness to high-frequency artifacts (in the case of more overlap) and computation speed (in the case of less overlap). In one embodiment, an overlap of 20-30% in a each direction is selected, i.e. for adjacent patches with e.g. 5 values, 2 values overlap. In other embodiments, the overlap is higher, e.g. 30-40%, 40-50%, around 50% (e.g. 45-55%) or up to 90%. For an overlap below 20% of the patch size, the below-described effect of the invention is usually lower.
As mentioned above and further described below, the final high-frequency band H1 is obtained after normalizing by the number of patches contributing to each pixel, thus resulting in an average value. The larger the overlap between patches is, the better is the suppression of high-frequency artifacts resulting from the high-frequency extrapolation process, and the more values are accumulated.
Then, for each low-frequency high-resolution patch Pn,L1, a best match in terms of mean absolute difference (MAD, known from motion estimation) is obtained after an exhaustive search in a local search window (e.g. 11×11 pixels) over the low-frequency band L0 of the low-resolution image. The best match is a block Pn,L0 from the low-frequency high-resolution image L0 that has the same size as the low-frequency high-resolution patch Pn,L1 (e.g. 5×5 pixels). More details about the search window are described below with respect to FIG. 4.
For understanding the next step, it is important to note that the low-resolution low-frequency data structure L0 has the same dimension as the low-resolution high-frequency data structure H0, and the high-resolution low-frequency data structure L1 has the same dimension as the high-resolution high-frequency data structure H1. as shown in FIG. 2. For every patch, the position of the matched low-frequency low-resolution patch Pn,L0 (within L0) is determined, and the corresponding low-resolution high-frequency patch Pn,H0 (within H0) at the position of the matched low-frequency low-resolution patch Pn,L0 is extracted. The extracted low-resolution high-frequency patch Pn,H0 from H0 is then accumulated on the high-frequency band of the high-resolution image H1, at the same position that the current patch Pn,L1 in the high-resolution low-frequency data structure L1 has. In detail, each value (e.g. pixel) of the extracted low-resolution high-frequency patch Pn,H0 from H0 is accumulated on the corresponding value (e.g. pixel) in the respective patch of the high-frequency band of the high-resolution image H1. In this way, the high-frequency band of the high-resolution image H1 is synthesized by patch-wise accumulation. The process of dividing the low-frequency band of the high-resolution image L1 in overlapping patches, finding the best low-frequency match and accumulating the corresponding high-frequency contribution is illustrated in FIG. 3, and is described below.
As a result, each value in the resulting (preliminary) high-frequency band of the high-resolution data structure H1 is a sum of values from a plurality of contributing patches. Due to the patch overlap in L1 (and consequently also in H1 since both have the same dimension), values from at least two patches contribute to many or all values in H1. Therefore, the resulting (preliminary) high-frequency band of the high-resolution data structure H1 is normalized 190. For this purpose, the number of contributing values from H0 for each value in the high-frequency high resolution data structure H1 is counted during the synthesis process, and each accumulated value in H1,acc is eventually divided by the number of contributions.
FIG. 3 shows, exemplarily, usage and positioning of a search window within the low-resolution low-frequency data structure L0. For a first patch P11,L1 in L1, a first best matching block P11,L0 is searched in L0 within a first search window W11. Both patches have the same size. The search window is larger than the patch by at least one value in each direction (except on edges, as for the first patch). In this example, the first best matching block P11,L0 is found in L0 in the upper left corner of the first search window W11. The further process for this patch and block is as described above. Then, subsequent patches are shifted horizontally and/or vertically, wherein each patch overlaps a previous patch.
In the example, a second patch P12,L1 is selected at a position that is shifted horizontally by a given patch advance. Patch advance is the difference between patch size and overlap. Patch advances in different dimensions (e.g. horizontal and vertical for 2D data structures) may differ, which may lead to different effects or qualities in the dimensions of the high-resolution output data structure, but they are usually equal. A new search window W12 is determined according to the new patch position. In principle, the search windows advance in the same direction as the patch, but slower. Thus, a current search window may be at the same position as a previous search window, as is the case here. However, since another patch P12,L1 is searched in the search window, the position of the best matching patch P12,L0 will usually be different. The best matching patch P12,L0 is then accumulated to the high-resolution high-frequency data structure H1 at the position of the low-frequency high-resolution patch P12,L1, as described above. Subsequent patches P13,L1, P14,L1 are determined and their best matches are searched in the same way. As shown in FIG. 3, the position of the best matching block within the search window is arbitrary and depends on the input data (e.g. the image content).
The above description is sufficient for a 1-dimensional (1D) data structure. For 2D data structures, the position of a further subsequent patch is found by vertical patch advance (this may or may not be combined with a horizontal patch advance). Also vertical patch advance includes an overlap, as mentioned above and shown in FIG. 3 for P21,L1, . . . , P23,L1.
The position of the search window is determined according to the position of the current patch. As shown in FIG. 3, the search windows W11, . . . , W22 of different patches overlap. Since L0 is a smaller data structure than L1, the search window advance in each dimension is very small. In one embodiment, the search windows are on the edge of L0 if their corresponding patch is on an edge of L1, and it is uniformly and/or proportionally moved in between these edges.
In one embodiment (not shown in FIG. 3), the center of the search window is set at a position that is substantially proportional to the center of the patch. E.g. where the center of a patch is at 3% of the high-resolution data structure L1, the center of the search window is set to be at approximately 3% (rounded) of the low-resolution data structure L0. In this case, for patches near an edge, the search window size may be reduced, or the search window may be shifted completely into the low-resolution data structure L0.
In general, the larger the search window, the more likely it is to find a very similar patch. However, in practice little difference in accuracy is to be expected by largely increasing the search window, since the local patch structure is more likely to be found only in a very local region in general natural images. Moreover, a larger search window requires more processing during the search.
FIG. 4 shows details of the selection of successive patches in an image (i.e. a 2D input data structure), overlap and the principle of determining matching blocks for successive patches. Exemplarily, patches and blocks have 5×5 pixels and search windows have 12×12 pixels. For a first patch P1,L1 in L1, a search window W1 is determined in L0, as described above. Within the search window W1, comparison of the first patch at different block positions is performed, and a block B1,L0 is determined that has the least mean absolute difference (MAD). This is the best matching block. Its position within the low-resolution low-frequency data structure L0 is determined, e.g. its upper left corner being in the third column and third row.
Then a corresponding patch at the same position in the high-frequency low-resolution image H0 is determined. Thus, it is a 5×5 pixel patch with its upper left corner being in the third column and third row. This patch is extracted from H0 and added to H1 at the position of the current low-frequency high-resolution patch P1,L1, i.e. at the upper left corner of H1 (see FIG. 4 a).
The second patch P2,L1 is selected according to the employed patch advance, as shown in FIG. 4 b). The patch advance is in this case two pixels in both dimensions, which means that due to the patch size of 5×5 pixels, the overlap is three. Thus, in this example, vertical overlap v, and horizontal overlap vh are equal. Due to the slower search window advance, the search window W2 is the same as for the previous patch. However, due to different pixel values (according to arbitrary image content), another best matching block B2,L0 within the search window is found. In the same manner as described above, its position is determined (e.g. upper left corner in the 7th column, 2nd row), the corresponding 5×5 block (with upper left corner in the 7th column, 2nd row) is extracted from H0, and the extracted block from H0 is added to the high-frequency high-resolution image H1 at the position of the second patch P2,L1, i.e. with its upper left corner at the first row, third column. Thus, a particular pixel that belongs to two or more different patches, is accumulated from corresponding pixels of to the best matching blocks. I.e., exemplarily, a particular pixel s in the 4th column, 5th row of the high-resolution high-frequency image H1 (corresponding to the position in L1 shown in FIG. 4) has, at the current stage of the process as described, a value that is accumulated from a pixel at the 6th column, 7th row (from the best-matching block B1,L0 of the first patch) and from a pixel at the 8th column, 6th row (from the best-matching block B2,L0 of the second patch).
As mentioned above, the search window advances usually only after a plurality of patches have been processed. As shown exemplarily in FIG. 4 c) for the above-described configuration, it takes three patch advances (i.e. the 4th patch) before the search window W3 is shifted by one pixel in horizontal direction. Further, it is noted here that the sequential order of various dimensions of the patch advance (and thus search window advance) makes no difference. Thus, the patch depicted in FIG. 4 d) may be processed after previous patches have shifted until the right-hand edge of L1, but it may also be processed directly after the first patch as shown in FIG. 4 a).
FIG. 5 shows a corresponding example for a 1D data structure. Also in this case, a first patch (with values denoted x) at a position #1 . . . #4 of a low-frequency high-resolution data structure L1 is located within a search window w1 in the low-frequency low-resolution data structure L0, e.g. values at positions #2 . . . #5. Thus, values of the corresponding H0 (not shown) at the positions #2 . . . #5 are added to H1 (not shown) at the positions #1 . . . #4. In a second step, a second patch at a position #3 . . . #6 of L1 is located within a second search window w2 in L0, e.g. at positions #3 . . . #6 of L0 (positions #2 . . . #5 of the search window). Thus, values of the corresponding H0 (not shown) at the positions #3 . . . #6 are added to H1 (not shown) at the positions #2 . . . #5, and so on.
The same principle as described above for 1D and 2D data structures can also be applied to any multi-dimensional data structures, including 3D data structures.
As mentioned above, the disclosed method for super-resolving a single image is composed of three stages. As the first stage of the invention, FIG. 6 shows the principle of the formation of the up-scaled low-frequency band L1 of the high-resolution image and two-band analysis (L0, H0) of the low-resolution input image S0. The goal of this first stage of the method is to obtain the low-frequency band of the high-resolution image L1, with an up-scaling factor that in one embodiment is fractional, and a low-resolution image L0 with the same normalized bandwidth, besides the residual high-frequency component H0 of the low-resolution image. A two-band analysis of the low-resolution input image S0 into a low-frequency portion L0 and a high-frequency portion H0 is performed. The cut-off normalized frequency of the low-resolution image is equivalent to that of the high-resolution image. In one embodiment, separable filters are used in order to avoid large convolutions with square Point Spread Functions (PSFs). This means that both the interpolating high-resolution filter Fl,1 and the low-pass low-resolution filter Fl,0 can be fully expressed as a 1-dimensional filter for which, if desired, the PSF can be computed as the tensor product of the corresponding coefficients vector (Fl,12D=Fl,1Fl,1T and correspondingly for Fl,0).
In the following, one embodiment for the design of the filters is described.
The design of the two filters shown in FIG. 6 is mainly determined by the rational up-scaling factor n/d and an arbitrary choice of the order N0 of the low-resolution FIR filter Fl,0. As usual, the choice of the order is determined by the available computing time for convolution. Generally, values in the order of 8 . . . 16 should suffice for providing a steep enough transition band. By choosing even values for the order N, resulting in N+1 filter coefficients, additional phase shifts are avoided and a more accurate high-frequency band is obtained. Both low-pass filters Fl,1 and Fl,0 are real and have linear phase, i.e. they are finite impulse response (FIR) filters. The normalized gain of every filter at the corresponding cut-off frequency is defined as −6 dB. The coefficients are defined as a discrete sin c function with a Hamming window of length N+1. The filter magnitude is generally defined as the scaling at the center of the low-pass band after windowing.
High-Resolution Interpolating Filter
With these rules in mind, the first filter to design is the high-resolution interpolating filter Fl,1. Given the desired order N for the low-resolution filter Fl,0, the rational up-scaling factor n/d and the design rules from the previous paragraph, the only missing parameters are the scaling σ1 (in order to cope with the n zeros that have been inserted between known samples), the order for the high-resolution filter N1 and its cut-off normalized frequency Ω1. These parameters are σ1=n, N1=N0n and Ω1=min(1/n, 1/d))=1/n. The cut-off frequency obeys to the requirement of eliminating the spectral replicas originated by the insertion of zeros between existing samples.
Low-Resolution Equivalent Low-Pass Filter
In this case, no zeros have been introduced between existing samples, so the magnitude of this filter is σ0=1. The order of the filter has already been arbitrarily defined and the cut-off frequency is set to Ω0=d/n. This value compensates for the decimation after the interpolating filter applied to the high-resolution image.
With this filter design, the analyzed low-frequency components of the low-resolution input image match, in terms of normalized bandwidth, the low-frequency component of the desired high-resolution image. Conversely, the analyzed high-frequency component of the low-resolution image can be used for estimating the missing high-frequency band of the high-resolution image.
The purpose of the second stage of the invention is to synthesize the high-frequency band of the high-resolution image by exploiting local self-similarity in the input image. This is done is a per-small patch basis; the method will in general benefit from using smaller magnification factors, due to the availability of a wider frequency bandwidth (increased information) for the analyzed low-frequency component of the low-resolution image, which provides a better selectivity for image patches with similar low-frequency contents. This is schematically illustrated in FIG. 7. Further details of the method are discussed in the following
FIG. 7 shows exemplarily a conceptual block diagram 700 of the process for synthesizing the high-frequency band of the super-resolved image (H1, initially set to 0), which is done in principle by extrapolating the high-frequency band of the low-resolution image H0. The signals W1(t) and W0(t) are spatial windows with positions varying in time accordingly to the current image patches being processed, and symbolize the patch advance and the search window advance, respectively. That is, e.g. the advance and overlap of the small patches (e.g. 5×5 pixels) into which the low-frequency band of the high-resolution image L1 is divided, can be understood as a window W1(t) that advances at a first speed within the low-frequency high-resolution data structure L1, and at the same within the high-frequency bands of the high-resolution data structure H1. The advance of the search window over the low-frequency band of the low-resolution image L0 is modeled by the time-varying window W0(t), which is in the same manner applied to the high-frequency band of the low-resolution image H0. A search unit 710 performs an exhaustive search for a best matching block (i.e. the one having minimum SAD) within the search window. Its position is applied to the high-frequency band of the low-resolution image H0, as described above. Since the search usually has a certain duration, this delay is compensated by delay compensation units 720,730. The corresponding patch in the low-resolution high-frequency data structure H0 is extracted in an extraction unit 740 and accumulated to the high-resolution high-frequency data structure H1 in an accumulation unit 750. Similar embodiments can be derived for data structures with different dimensions than 2D, such as 1D or 3D.
The third stage is the formation of the final high-resolution image. The goal of this stage is to properly fuse the low-frequency band of the high-resolution image L1 with the normalized high-frequency band of the high-resolution image H1. The normalized high-frequency high-resolution band H1 can be high-pass filtered prior to the addition with the low-frequency high-resolution band L1. This high-pass filtering is advantageous in order to ensure spectral compatibility, but can be omitted when L1 and H1 have substantially no overlapping frequencies (cf. FIG. 8 b).
FIG. 8 shows exemplarily the fusion of the low-frequency high-resolution band L1 and the normalized high-frequency high-resolution band H1 for generating the super-resolved image S1. The normalized high-frequency band H1 is filtered using a high-pass filter 800 in order to ensure the spectral compatibility with the low-frequency band.
High-Resolution High-Pass Filter
The filter Fh,1 is designed in the same fashion as the filters Fl,0, Fl,1 in the first stage. In this case, the goal is to obtain a high-pass filter with a cut-off frequency Ω1,h=d/max (n,d)=d/n. Its order is set to a scaled version of the low-resolution filter order: N1,h=round(N0n/d), and its magnitude σ1,h=1. The final coefficients of the separable high-pass filter are set to a Kronecker delta aligned with the center of the Hamming window minus the coefficients of the complementary low-pass filter with the same cut-off frequency. That is, the high-pass filter is defined as an all pass-filter (set of coefficients equals a Kronecker delta) minus a low-pass filter with the same cut-off frequency as the desired high-pass filter. This is graphically shown in FIG. 8 b), where the left-hand side is the desired frequency response HP of the high-pass filter and the right-hand side is the difference of the responses of an all-pass filter AP and the above described low-pass filter LP.
As has become clear from the above description, the low-frequency band of the high-resolution image L1 is obtained in principle by interpolation, while the high-frequency band of the high-resolution image H1 is obtained in principle by extrapolation.
FIG. 9 shows an apparatus for performing super-resolution processing of a low resolution input data structure S0 of digital data, comprising a first low-pass filter 970 for filtering the input data structure S0, wherein a low-frequency input data structure L0 is obtained, an adder, subtractor or differentiator 980 for calculating a difference between the input data structure S0 and the low-frequency input data structure L0, whereby a high-frequency input data structure H0 is generated, an upscaler 920 for upscaling the input data structure S0, a second low-pass filter 930 for filtering the upscaled input data structure, wherein a low-frequency upscaled data structure L1 is obtained, a first determining unit 951 for determining in the low-frequency upscaled data structure L1 a first patch at a first position, a search unit 952 for searching in the low-frequency input data structure L0 a first block that matches the first patch best, and a second determining 954 unit for determining the position of said first block within the low-frequency input data structure L0, a selector unit 955 for selecting a second block in the high-frequency input data structure H0 at the determined position (i.e. at the position that was determined for said first block within the low-frequency input data structure), an accumulator 957 for accumulating (i.e. adding up) pixel data of the selected second block to a second patch, the second patch being a patch in a high-frequency upscaled data structure at the first position that is initially empty, a control unit 950 for controlling repetition of the processing for a plurality of patches in the low-frequency upscaled data structure L1, a normalizing unit 990 for normalizing (i.e. averaging) the accumulated pixel values in the high-frequency upscaled data structure, whereby a normalized high-frequency upscaled data structure H1 is obtained, a high-pass filter 995 for filtering the normalized high-frequency upscaled data structure H1, and a combining unit 999 for combining (e.g. pixel-wise adding) the normalized, high-pass filtered high-frequency upscaled data structure H1 to the low-frequency upscaled data structure L1, whereby a super-resolved data structure S1 is obtained. Various memories MemL0, MemL1, MemH0, MemH1 with appropriate sizes can be used for intermediate storage, which may however be implemented as one single or more physical memories. In principle, the normalizing (or averaging) comprises, for a current pixel, dividing the accumulated value of the current pixel by the number of pixels that have contributed to the accumulated value of the current pixel. However, any normalizing method that leads to substantially equivalent results can be used.
Further embodiments, some of which are also shown in FIG. 1 and/or FIG. 9, are described below.
In one embodiment, the method further comprises a step of determining 151,152 a first search window W1 in the low-frequency input data structure L0, wherein the first search window W1 covers an area around a block at a position that corresponds to said first position in the high-frequency upscaled data structure L1, and wherein the searching 152,154 in the low-frequency input data structure L0 is performed only within the first search window W1. The step of determining 151,152 a search window W1 in the low-frequency input data structure L0 is repeated for each new patch in the low-frequency upscaled data structure L1.
In one embodiment, the area that is covered by the search window comprises a plurality of values in each direction of the low-frequency upscaled data structure L0 around the block at the position corresponding to said first position in the high-frequency upscaled data structure L1.
In one embodiment, each new patch Pn,L1 in the low-frequency upscaled data structure L1 overlaps with at least one previously processed patch.
In one embodiment, the low-frequency upscaled data structure L1 is obtained by upscaling 120 the input data structure S0 by an upscaling factor n, filtering 130 the upscaled input data structure by said second low-pass filter Fl,1 and downscaling 140 the filtered upscaled input data structure in a downscaling unit 940 by a downscaling factor d, with n>d. Thus, a final non-integer upscaling factor n/d is obtained.
In one embodiment, the first low-pass filter Fl,0 and the second low-pass filter Fl,1 are equivalent filters (i.e., with respect to normalized cut-off frequency).
In one embodiment, the first low-pass filter Fl,0 has characteristics of an order N0, a magnitude of σ0=1 and a normalized cut-off frequency of Ω0=d/n, and the second low-pass filter Fl,1 has characteristics of an order N1=nN0, a magnitude of σ1=n and a normalized cut-off frequency of Ω1=1/n.
In one embodiment, the method further comprises a step of filtering the high-frequency upscaled data structure H1,acc with a high-pass filter Fh,1. The high-pass filter Fh,1 has a normalized cut-off frequency of Ω1,h=d/max{d,n}=d/n, an order of N1,h=round(N0*n/d) and a magnitude of σ1,h=1.
In one embodiment, the method further comprises steps of determining a new patch Pn,L1 in the low-frequency upscaled data structure L1, searching 152,154 in the low-frequency input data structure L0 a block Bn,L0 that matches the selected patch Pn,L1 best, selecting 155 a corresponding block Bn,H0 in the high-frequency input data structure H0 and accumulating 157 pixel data of the selected corresponding block Bn,H0 to a patch Pn,H1 in the high-frequency upscaled data structure H1,acc at the position of said new patch Pn,L1 are repeated for all patches until the complete low-frequency upscaled data structure L1 is covered.
In one embodiment, the method further comprises a step of counting the number of contributions per pixel in the high-frequency upscaled data structure H1,acc, i.e. the number of blocks from the high-frequency input data structure H0 that contributed to a pixel of the high-frequency upscaled data structure H1,acc. The step of normalizing 190 comprises then dividing the accumulated value per pixel in H1,acc by the number of contributions that is obtained by the counting step.
In one embodiment, the input data structure is a 2D digital image. In another embodiment, the input data structure is a 3D digital image. A digital image may generally be part of a digital video sequence.
In one embodiment, the input data structure comprises digital 2D data, and each block and each patch comprise at least 5×5 values, the search window covers at least 9×9 values and each patch overlaps at least one earlier processed patch by at least 2 values.
In one embodiment, the apparatus further comprises at least one memory MemL0, MemL1, MemH0, MemH1 for intermediate storage of at least one of the low-frequency input data structure L0, the low-frequency upscaled data structure L1, the high-frequency input data structure H0 and the high-frequency upscaled data structure H1.
In one embodiment, the apparatus further comprises within the search unit 952 a search window determining unit for determining a search window W1 in the low-frequency input data structure L0, wherein the search window W1 covers an area around a block at a position that corresponds to said first position in the high-frequency upscaled data structure L1, and wherein the search unit 952 searches in the low-frequency input data structure L0 only within the first search window W1.
In one embodiment, the apparatus further comprises a counter 953 for counting the number of contributions per pixel in the high-frequency upscaled data structure H1,acc. The normalizing unit 990 performs an operation of dividing the accumulated value per pixel by the number of contributions.
FIG. 10 shows, in a) and b), an image and its original spectrum. Further, FIG. 10 c) shows the spectrum of the image after it was upscaled according to the invention, and FIG. 10 d) the spectrum of the image after it was conventionally upscaled using the known bi-cubic interpolation. As can be recognized, the clipped spectrum of the conventionally upscaled image is improved, which is in this example visible in additional values along the diagonals. In other words, it is an advantage of the present invention that the spectrum of a data structure that was upscaled according to the invention comes closer to the original spectrum, than the spectrum of a data structure that was upscaled using a conventional method. The frequency spectrum shows clearly how the disclosed method is able to plausibly extrapolate the missing high frequencies of the up-scaled image (they can be observed in sharper contours in the up-scaled images, which result in more visually appealing images), whereas bi-cubic interpolation introduces a large amount of aliasing artifacts
The disclosed method has been implemented and tested on a number of publicly available low-resolution input images for which higher resolution versions are to be estimated. In one embodiment, it is generally applied only to the luminance channel of the input images, leaving the color up-scaling to faster algorithms like bi-cubic interpolation. In the example of FIG. 10, a super-resolved image has been obtained by iteratively applying small up-scaling factors of 1.5× (n=3, d=2) three times to the input image.
FIG. 11 shows the three principle stages comprised in the disclosed method for super-resolving a single image. In a first stage 1110, an interpolation-based up-scaling of the input image is performed, followed by an equivalent low-pass filtering operation on the low-resolution image. The second stage 1120 comprises a search for low-frequency matches between an inspected patch in the high-resolution image and patches in a local neighborhood in the low-resolution low-frequency image, including partly overlapping patches, and accumulating the high-frequency contribution obtained from the low-resolution image. The third stage 1130 comprises normalizing and high-pass filtering, and adding the contributions of the low-frequency band of the high-resolution image and the extrapolated, high-pass filtered, normalized high-frequency band.
It should be noted that although shown simply as a digital image, other types of digital data structures may be constructed other than digital images, as would be apparent to those of ordinary skill in the art, all of which are contemplated within the present invention. Similar principles can be applied to other image processing tasks, like image denoising or other restoration procedures, and also for obtaining super-resolved signals of different nature and dimensionality, such as audio signals.
The disclosed method works with a single-image, but advantageously without requiring a data-base for retrieving adequate examples containing high-frequency portions (i.e. details).
As another advantage, the disclosed method is comparatively computationally efficient (it requires only a single-image, and the main processing step is a small-scale local search), flexible in the definition of the up-scaling factor (it enables rational up-scaling factors and straightforward FIR filter design) and can also be generalized for processing signals of different nature (no prior assumptions on the signal model, beyond local self-similarity, are required).
A further advantage of the invention is that only a single upscaling procedure of the input data structure is employed, and the extrapolation is made from the input data structure at its original resolution. Thus, the amount of artifacts introduced by upscaling is minimized, which is particularly advantageous for rational upscaling factors, and a broad high-frequency band is available for augmenting the information in the high-frequency high-resolution data structure.
A further advantage of the invention is that explicit, simple rules for designing the filters are provided, and that the filters need to be designed only once, since they are space-invariant.
A further advantage of the invention is that, due to the spatial averaging, the high-frequency high-resolution data structure H1 is more robust to noise and other artifacts than others that are obtained with conventional methods.
A further advantage of the invention is that the procedure for fusing the interpolated low-frequency high-resolution band L1 and the extrapolated high-frequency high-resolution band H1 takes the spectral coherence between them into consideration. This is achieved by appropriate design of the high-pass filter for the high-frequency high-resolution data structure.
While there has been shown, described, and pointed out fundamental novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the apparatus and method described, in the form and details of the devices disclosed, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. Exemplarily, although the present invention has been disclosed with regard to square blocks and patches, one skilled in the art would recognize that the method and devices described herein may be applied to blocks and patches of other shapes and/or sizes, e.g. rectangle shapes or free-form shapes, 4×4, . . . , 16×16 squares etc. Further, although the present invention has been disclosed with regard to spatial resolution, one skilled in the art would recognize that the method and devices described herein may, mutatis mutandis, also be applied to temporal resolution. It is expressly intended that all combinations of those elements that perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated.
It will be understood that the present invention has been described purely by way of example, and modifications of detail can be made without departing from the scope of the invention.
Each feature disclosed in the description and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Features may, where appropriate be implemented in hardware, software, or a combination of the two, e.g. on graphics hardware (GPU). Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.
CITED REFERENCES


    “Super Resolution from Image Sequences”, M. Irani and S. Peleg (Int. Conf. on Pattern Recognition, 1990)
    Fundamental Limits of Reconstruction-Based Superresolution Algorithms under Local Translation”, Z. Lin and H.-Y. Shum (IEEE Trans. On Pattern Analysis and Machine Intelligence, 2004)
    “Example-based super-resolution”, W. T. Freeman, T. R. Jones and E. G. Pasztor (IEEE Computer Graphics and Applications, 2002)
    “Super-Resolution from a Single Image”, D. Glasner, S. Bagon and M. Irani (IEEE Int. Conf. on Computer Vision, 2009)
    “Space-Time Super-Resolution from a Single Video”, O. Shahar, A. Faktor and M. Irani (IEEE Conf. on Computer Vision and Pattern Recognition, 2011)
    “Image and Video Upscaling from Local Self-Examples”, G. Freedman and R. Fattal (ACM Trans. On Graphics, 2010)

## Claims

1. A method for performing super-resolution processing of a low resolution input data structure of digital 1D, 2D or 3D data, comprising:
filtering the input data structure by a first low-pass filter, wherein a low-frequency input data structure is obtained;
calculating a difference between the input data structure and the low-frequency input data structure, whereby a high-frequency input data structure is generated;
upscaling the input data structure, and filtering the upscaled input data structure by a second low-pass filter, wherein a low-frequency upscaled data structure is obtained;
determining in the low-frequency upscaled data structure a first patch at a first position;
searching in the low-frequency input data structure for a first block that matches the first patch best, and determining the position of said first block within the low-frequency input data structure;
selecting a second block in the high-frequency input data structure at the position that was determined for said first block within the low-frequency input data structure;
accumulating, by pixelwise addition, pixel data of the selected second block to a second patch, the second patch being a patch in a high-frequency upscaled data structure at the first position, wherein the high-frequency upscaled data structure is initially empty;
determining a new patch in the low-frequency upscaled data structure, searching in the low-frequency input data structure for a block that matches the new patch best, selecting a corresponding block in the high-frequency input data structure and accumulating pixel data of the selected corresponding block to a patch in the high-frequency upscaled data structure at the position of said new patch;
normalizing the accumulated pixel values in the high-frequency upscaled data structure, whereby a normalized high-frequency upscaled data structure is obtained and wherein the step of normalizing comprises, for a current pixel, dividing the accumulated value of the current pixel by the number of contributions of pixels to the accumulated value of the current pixel; and
adding the normalized high-frequency upscaled data structure to the low-frequency upscaled data structure, whereby a super-resolved data structure is obtained.

2. Method according to claim 1, further comprising determining a first search window in the low-frequency input data structure, wherein the first search window covers an area around a block at a position that corresponds to said first position in the high-frequency upscaled data structure, wherein the searching in the low-frequency input data structure is performed only within the first search window, and wherein the step of determining a search window in the low-frequency input data structure is repeated for each new patch in the low-frequency upscaled data structure.

3. Method according to claim 2, wherein the area that is covered by the search window comprises a plurality of values in each direction of the low-frequency upscaled data structure around said block at the position corresponding to said first position in the high-frequency upscaled data structure.

4. Method according to claim 1, wherein each new patch in the low-frequency upscaled data structure overlaps with at least one previously processed patch.

5. Method according to claim 1, wherein the low-frequency upscaled data structure is obtained by upscaling the input data structure by an upscaling factor n, filtering the upscaled input data structure by said second low-pass filter and downscaling the filtered upscaled input data structure by a downscaling factor d, with n>d, wherein a final non-integer upscaling factor n/d is obtained.

6. Method according to claim 1, wherein the first low-pass filter and the second low-pass filter are equivalent filters.

7. Method according to claim 1, wherein the first low-pass filter has characteristics of an order N0, a magnitude of σ0=1 and a normalized cut-off frequency of Ω0=d/n, and the second low-pass filter has characteristics of an order of N1=nN0, a magnitude of σ1=n and a normalized cut-off frequency of Ω1=1/n, wherein d is a downscaling factor and n is an upscaling factor.

8. Method according to claim 1, further comprising filtering the normalized high-frequency upscaled data structure with a high-pass filter, the high-pass filter having a normalized cut-off frequency of Ω1,h=d/n, an order of N1,h=round(N0*n/d) and a magnitude of σ1,h=1, wherein d is a downscaling factor, n is an upscaling factor and N0 is an order characteristic of the first low-pass filter.

9. Method according to claim 1, wherein determining a new patch in the low-frequency upscaled data structure, searching in the low-frequency input data structure for a block that matches the new patch best, selecting a corresponding block in the high-frequency input data structure and accumulating pixel data of the selected corresponding block to a patch in the high-frequency upscaled data structure at the position of said new patch are repeated for all patches until the complete low-frequency upscaled data structure is covered.

10. Method according to claim 1, further comprising counting the number of contributions per pixel in the high-frequency upscaled data structure, wherein the step of normalizing comprises dividing the accumulated value per pixel by the number of contributions obtained by said counting step.

11. Method according to claim 1, wherein the input data structure of digital 2D data is a digital image.

12. Method according to claim 1, wherein the input data structure comprises digital 2D data, and wherein each block and each patch comprises at least 5×5 values and each patch overlaps at least one earlier processed patch by at least 2 values.

13. An apparatus for performing super-resolution processing of a low resolution input data structure of digital data, comprising:
a first low-pass filter for filtering the input data structure, wherein a low-frequency input data structure is obtained;
an adder, subtractor or differentiator for calculating a difference between the input data structure and the low-frequency input data structure, whereby a high-frequency input data structure is generated;
an upscaler for upscaling the input data structure;
a second low-pass filter for filtering the upscaled input data structure, wherein a low-frequency upscaled data structure is obtained;
a determining unit for determining in the low-frequency upscaled data structure a first patch at a first position;
a search unit for searching in the low-frequency input data structure for a first block that matches the first patch best and determining the position of said first block within the low-frequency input data structure;
a selector unit for selecting a second block in the high-frequency input data structure at the position that was determined for said first block within the low-frequency input data structure;
an accumulator for accumulating, by pixelwise addition, pixel data of the selected second block to a second patch, the second patch being a patch in a high-frequency upscaled data structure at the first position, wherein the high-frequency upscaled data structure is initially empty;
a control unit for controlling:
the determining unit to determine a new patch in the low-frequency upscaled data structure,
the search unit to search in the low-frequency input data structure for a block that matches the new patch best and determine the position of said first block within the low-frequency input data structure,
the selector unit to select a corresponding block in the high-frequency input data structure, and
the accumulator to accumulate pixel data of the selected corresponding block to a patch in the high-frequency upscaled data structure at the position of said new patch;

a normalizing unit for normalizing the accumulated pixel values in the high-frequency upscaled data structure, whereby a normalized high-frequency upscaled data structure is obtained and wherein the normalizing comprises, for a current pixel, dividing the accumulated value of the current pixel by the number of contributions of pixels to the accumulated value of the current pixel;
and
a combining unit for combining the normalized high-frequency upscaled data structure to the low-frequency upscaled data structure, whereby a super-resolved data structure is obtained.

14. The apparatus according to claim 13, further comprising a high-pass filter for filtering the normalized high-frequency upscaled data structure, the high-pass filter having a normalized cut-off frequency of Ω1,h=d/n, an order of N1,h=round(N0*n/d) and a magnitude of σ1,h=1, wherein d is a downscaling factor, n is an upscaling factor and N0 is an order characteristic of the first low-pass filter.

15. The apparatus according to claim 13, further comprising a search window determining unit for determining a search window in the low-frequency input data structure, wherein the search window covers an area around a block at a position that corresponds to said first position in the high-frequency upscaled data structure, and wherein the search unit searches in the low-frequency input data structure only within the search window.

16. The apparatus according to claim 13, further comprising a counter for counting the number of contributions per pixel in the high-frequency upscaled data structure, wherein the normalizing unit performs an operation of dividing the accumulated value per pixel by the number of contributions.

17. The apparatus according to claim 13, wherein the control unit controls the determining unit, the search unit, the selector unit, and the accumulator to repeat their operations for all patches until the complete low-frequency upscaled data structure is covered.

18. A non-transitory computer readable medium having executable instructions to cause a computer to perform a method for performing super-resolution processing of a low resolution input data structure of digital data, comprising:
filtering the input data structure by a first low-pass filter, wherein a low-frequency input data structure is obtained;
calculating a difference between the input data structure and the low-frequency input data structure, whereby a high-frequency input data structure is generated, upscaling the input data structure, and filtering the upscaled input data structure by a second low-pass filter, wherein a low-frequency upscaled data structure is obtained;
determining in the low-frequency upscaled data structure a first patch at a first position;
searching in the low-frequency input data structure a first block that matches the first patch best, and determining the position of said first block within the low-frequency input data structure;
selecting a second block in the high-frequency input data structure at the position that was determined for the first block within the low-frequency input data structure;
accumulating by pixelwise addition pixel data of the selected second block to a second patch, the second patch being a patch in a high-frequency upscaled data structure at the first position, wherein the high-frequency upscaled data structure is initially empty;
determining a new patch in the low-frequency upscaled data structure, searching in the low-frequency input data structure for a block that matches the new patch best, selecting a corresponding block in the high-frequency input data structure and accumulating pixel data of the selected corresponding block to a patch in the high-frequency upscaled data structure at the position of said new patch;
normalizing by averaging the accumulated pixel values in the high-frequency upscaled data structure, whereby each accumulated pixel value is divided by a number of contributions of pixels having contributed to it, and whereby a normalized high-frequency upscaled data structure is obtained; and
adding the normalized high-frequency upscaled data structure to the low-frequency upscaled data structure, whereby a super-resolved data structure is obtained.

19. The medium according to claim 18, wherein the method for performing super-resolution processing further comprises filtering the normalized high-frequency upscaled data structure with a high-pass filter, the high-pass filter having a normalized cut-off frequency of Ω1,h=d/n, an order of N1,h=round(N0*n/d) and a magnitude of σ1,h=1, wherein d is a downscaling factor, n is an upscaling factor and N0 is an order characteristic of the first low-pass filter.

20. The medium according to claim 18, wherein determining a new patch in the low-frequency upscaled data structure, searching in the low-frequency input data structure for a block that matches the new patch best, selecting a corresponding block in the high-frequency input data structure and accumulating pixel data of the selected corresponding block to a patch in the high-frequency upscaled data structure at the position of said new patch are repeated for all patches until the complete low-frequency upscaled data structure is covered.