Resampling filters for scalable video coding with phase offset adjustment and signaling of same

Upsampling filters for use in scalable video coding may be selected from a set of filters each with a different phase. In order to accommodate a phase offset introduced from downsampling required to maintain proper luma/chroma color space positions after upsampling, an offset parameter may be used in computing the filter index. Moreover, a different offset may be used for each filter index. These offsets in effect provide a re-mapping of the filter indices. By remapping the filter indices in this manner the performance of the upsampling process can be improved and errors introduced by rounding or which are caused by the finite precision of the process used to compute the filter indices can be taken into account.

TECHNICAL FIELD

The present invention relates to a sampling filter process for scalable video coding. More specifically, the present invention relates to re-sampling using video data obtained from an encoder or decoder process, where the encoder or decoder process can be MPEG-4 Advanced Video Coding (AVC) or High Efficiency Video Coding (HEVC).

BACKGROUND

Scalable video coding (SVC) refers to video coding in which a base layer, sometimes referred to as a reference layer, and one or more scalable enhancement layers are used. For SVC, the base layer can carry video data with a base level of quality. The one or more enhancement layers can carry additional video data to support higher spatial, temporal, and/or signal-to-noise SNR levels. Enhancement layers may be defined relative to a previously coded layer.

The base layer and enhancement layers can have different resolutions. Upsampling filtering, sometimes referred to as resampling filtering, may be applied to the base layer in order to match a spatial aspect ratio or resolution of an enhancement layer. This process may be called spatial scalability. An upsampling filter set can be applied to the base layer, and one filter can be chosen from the set based on a phase (sometimes referred to as a fractional pixel shift). The phase may be calculated based on the ratio between base layer and enhancement layer picture resolutions.

SUMMARY

Embodiments of the present invention provide methods, devices and systems for deriving upsampling filters for use in scalable video coding. The upsampling filters that are used may be selected from a set of filters each with a different phase. In order to accommodate a phase offset introduced from downsampling required to maintain proper luma/chroma color space positions after upsampling, an offset parameter may be used in computing the filter index. Moreover, a different offset may be used for each filter index. These offsets in effect provide a re-mapping of the filter indices. By remapping the filter indices in this manner the performance of the upsampling process can be improved and errors introduced by rounding or which are caused by the finite precision of the process used to compute the filter indices can be taken into account.

DETAILED DESCRIPTION

Overview of Upsampling Process

An example of a scalable video coding system using two layers is shown inFIG. 1. In the system ofFIG. 1, one of the two layers is the Base Layer (BL) where a BL video is encoded in an Encoder E0, labeled100, and decoded in a decoder D0, labeled102, to produce a base layer video output BL out. The BL video is typically at a lower quality than the remaining layers, such as the Full Resolution (FR) layer that receives an input FR (y). The FR layer includes an encoder E1, labeled104, and a decoder D1, labeled106. In encoding in encoder E1104of the full resolution video, cross-layer (CL) information from the BL encoder100is used to produce enhancement layer (EL) information. The corresponding EL bitstream of the full resolution layer is then decoded in decoder D1106using the CL information from decoder D0102of the BL to output full resolution video, FR out. By using CL information in a scalable video coding system, the encoded information can be transmitted more efficiently in the EL than if the FR was encoded independently without the CL information. An example of coding that can use two layers shown inFIG. 1includes video coding using AVC and the Scalable Video Coding (SVC) extension of AVC, respectively. Another example that can use two layer coding is HEVC.

FIG. 1further shows block108with a down-arrow r illustrating a resolution reduction from the FR to the BL to illustrate that the BL can be created by a downsampling of the FR layer data. Although a downsampling is shown by the arrow r of block108FIG. 1, the BL can be independently created without the downsampling process. Overall, the down arrow of block108illustrates that in spatial scalability, the base layer BL is typically at a lower spatial resolution than the full resolution FR layer. For example, when r=2 and the FR resolution is 3840×2160, the corresponding BL resolution is 1920×1080.

The cross-layer CL information provided from the BL to the FR layer shown inFIG. 1illustrates that the CL information can be used in the coding of the FR video in the EL. In one example, the CL information includes pixel information derived from the encoding and decoding process of the BL. Examples of BL encoding and decoding are AVC and HEVC. Because the BL pictures are at a different spatial resolution than the FR pictures, a BL picture needs to be upsampled (or re-sampled) back to the FR picture resolution in order to generate a suitable prediction for the FR picture.

FIG. 2illustrates an upsampling process in block200of data from the BL layer to the EL. The components of the upsampling block200can be included in either or both of the encoder E1104and the decoder D1106of the EL of the video coding system ofFIG. 1. The BL data at resolution x that is input into upsampling block200inFIG. 2is derived from one or more of the encoding and decoding processes of the BL. A BL picture is upsampled using the up-arrow r process of block200to generate the EL resolution output y′ that can be used as a basis for prediction of the original FR input y.

The upsampling block200works by interpolating from the BL data to recreate what is modified from the FR data. For instance, if every other pixel is dropped from the FR in block108to create the lower resolution BL data, the dropped pixels can be recreated using the upsampling block200by interpolation or other techniques to generate the EL resolution output y′ from upsampling block200. The data y′ is then used to make encoding and decoding of the EL data more efficient. For example, the difference between y′ and y can be used for coding.

FIG. 3shows a general block diagram for implementing an upsampling process ofFIG. 2for embodiments of the present invention. The upsampling or re-sampling process can be determined to minimize an error E (e.g. mean-squared error) between the upsampled data y′ and the full resolution data y. The system ofFIG. 3includes a select input samples module300that samples an input video signal. The system further includes a select filter module302to select a filter from the subsequent filter input samples module304to upsample the selected input samples from module300.

In module300, a set of input samples in a video signal x is first selected. In general, the samples can be a two-dimensional subset of samples in x, and a two-dimensional filter can be applied to the samples. The module302receives the data samples in x from module300and identifies the position of each sample from the data it receives, enabling module302to select an appropriate filter to direct the samples toward a subsequent filter module304. The filter in module304is selected to filter the input samples, where the selected filter is chosen or configured to have a phase corresponding to the particular output sample location desired.

The filter input samples module304can include separate row and column filters. The selection of filters is represented herein by the P as filters h[n; p], where p is a phase index that runs from 0 to (P−1). That is, if, for instance, P=10, then there are a family of 10 filters h[n; 0], h[n; 1] . . . h[n; 9]. Each filter can have N+1 coefficients e.g., a filter with phase index p=3 has the coefficients h[0; 3], h[1; 3] . . . h[N; 3]. As used herein a family of P filters will be denoted as h[n,p], whereas a particular filter having a selected phase will be denoted as h[n], where the filter has N+1 coefficients. The output of the filtering process using the selected filter h[n] on the selected input samples produces output value y′.

FIG. 4shows details of components for the select sample module302ofFIG. 3(labeled302ainFIG. 4) and the filters module304ofFIG. 3(labeled304ainFIG. 4) for a system with fixed filters. For separable filtering the input samples can be along a row or column of data. To supply a set of input samples from select input samples module300, the select filter module302aincludes a select control400that identifies the input samples x[m] and provides a signal to a selector402that directs them through the selector402to a desired filter. The filter module304athen includes the different filters h[n;p] that can be applied to the input samples, where the filter phase can be chosen among P phases from each row or column element depending on the output sample n desired. As shown, the selector402of module302adirects the input samples to a desired column or row filter in304abased on the “Filter (n) SEL” signal from select control400. A separate select control400signal “Phase (p) SEL” selects the appropriate filter phase p for each of the row or column elements. The filter module304aoutput produces the output y′[n].

InFIG. 4, the outputs from individual filter components of h[n;p] are shown being added “+” to produce the output y′[n]. This illustrates that each box, e.g. h[0;p], represents one coefficient or number in a filter with phase index p. Therefore, the filter represented by a phase index p includes all N+1 coefficients in h[0,p], . . . , h[N;p]. This is the filter that is applied to the selected input samples to produce an output value y′[n], for example, y′[0]=h[0,p]*x[0]+h[1,p]*x[1]+ . . . +h[N,p]*x[N], requiring the addition function “+” as illustrated. As an alternative to adding inFIG. 4, the “+” could be replaced with a solid connection and the output y′ [n] would be selected from one output of a bank of P filters representing the P phases, with the boxes h[n:p] in module304arelabeled, for example, as h[n;0], h[n,1], . . . , h[n,P−1] and now each box would have all the filter coefficients needed to form y′ [n] without the addition element required.

Although the filters h[n:p] in module304aare shown as having fixed phases, they can be implemented using a single filter with the phase being selected and adaptively controlled. The adaptive phase filters can be reconfigured, for example, by software. The adaptive filters can thus be designed so that each filter h[n] corresponds to a desired phase. The filter coefficients h[n] for a given filter can be signaled in the EL from the encoder so that the decoder can reconstruct a prediction to the FR data.

Phase selection for the filters h[n:p] enables recreation of the FR layer from the BL data. For example, if the BL data is created by removing every other pixel of data from the FR, to recreate the FR data from the BL data, the removed data must be reproduced or interpolated from the BL data available. In this case, depending on whether even or odd indexed samples are removed, the appropriate filter h[n;p] with a phase represented by a phase index p can be used to interpolate the new data. The selection of P different phase filters from the filters h[n:p] allows the appropriate phase shift to be chosen to recreate the missing data depending on how the BL data is downsampled from the FR data.

Separable Column and Row Filtering

As previously mentioned, the resampling filters can be one-dimensional or two-dimensional filters. Generally, a one-dimensional filter is separately applied to the rows and columns of the video signal and, although the same filter is generally used for the columns and for the rows. For the re-sampling process, in one embodiment the filters applied can be separable, and the coefficients for each horizontal (row) and vertical (column) dimension can be signaled or selected from a set of filters. The processing of row or columns separably allows for flexibility in filter characteristics (e.g. phase offset, frequency response, number of taps, etc.) in both dimensions while retaining the computational benefits of separable filtering. In addition, however, it may be advantageous to employ different filters for the rows and columns since the characteristics of the data may differ along the rows relative to the columns.

Upsampling with Phase Offset Adjustment

In order to interpolate pixel values in the EL, it is necessary to find a pixel in the BL that corresponds to a pixel in the EL. To accomplish this, the upsampling process requires knowledge of the alignment between pixels in the BL and the EL. Since the pixels in the two layers may not be exactly aligned, the offset between them is expressed in terms of a fractional pixel shift, which is referred to as a phase offset. The phase offset cannot be expressed with infinite precision. Rather, in Scalable HEVC (SHEVC) Test Model 1.0 (SHM1.0) a phase resolution of 1/16 is employed, That is, the offset between pixels in the BL and EL can be interpolated to within a 1/16 of a pixel. In the upsampling process each phase offset corresponds to a different upsampling filter and thus in SHM1.0 16 filters are used to represent the 16 phase offsets. While a phase resolution of 1/16 will be used by way of example, in the following discussion, those of ordinary skill in the art will recognized that the techniques described herein are equally applicable to other phase resolutions as well.

In SHM1.0 a set of 16 fixed filters with different phase offsets in the unit interval can be specified. These filters are indexed where larger filter indices are used for larger phase offsets. In order to accommodate a phase offset introduced from downsampling required to maintain proper luma/chroma color space positions after upsampling, co-pending U.S. Appl. Ser. No. 14/250,349 proposes that an offset parameter be signaled and used in computing the filter index separate from the normative offset. As discussed therein, different phase offsets may be signaled for the luma and chroma color spaces and for each of the horizontal and vertical dimensions. However, the same phase offset is used for each filter index used in upsampling for a given luma/chroma color space and a given dimension. In other words, this approach assumes that for a given luma/chroma color space and a given dimension each pair of corresponding pixels in the BL and the EL are offset by the same amount.

For a variety of reasons the pairs of corresponding pixels in the BL and the EL may not be offset by the same amount, even for a given luma/chroma color space and a given dimension. Accordingly, the use of a single phase offset for each filter index may not always be satisfactory. In such cases it may be preferable to apply a different offset to each filter index. These offsets in effect provide a re-mapping of the filter indices. The increased flexibility of this approach can improve performance and compensate for errors introduced by rounding or the finite precision in the computation of the filter indices based on the scalability ratio and/or downsampling phase offsets.

Signaling of Phase Offset Shift

The phase offsets that are selected for each filter index can be signaled using any suitable syntax. In some embodiments the signals may occur at the picture parameter set (PPS) level or PPS extension. Note that signaling may occur at other places within the PPS. Alternatively, signaling can be specified at other levels, e.g., the sequence parameter set (SPS), video parameter set (VPS), slice, prediction unit (PU), etc. Further, although offset adjustment is being accounted for in luma/chroma phase positions, similar phase compensation can be made for color spaces, cropping, etc.

An illustrative example of the syntax elements that may be employed in the context of SHM1.0 whenever CL prediction is enabled is as follows.

luma_offset_flag[0] equal to 1 indicates that the filter index offsets for upsampling the rows of the luma component are signaled. luma_offset_flag[0] equal to 0 indicates that the filter index offsets for upsampling the rows of the luma component are not signaled.

luma_offset_flag[1] equal to 1 indicates that the filter index offsets for upsampling the columns of the luma component are signaled. luma_offset_flag[1] equal to 0 indicates that the filter index offsets for upsampling the columns of the luma component are not signaled.

chroma_offset_flag[0] equal to 1 indicates that the filter index offsets for upsampling the rows of the chroma component are signaled. chroma_offset_flag[0] equal to 0 indicates that the filter index offsets for upsampling the rows of the chroma component are not signaled.

chroma_offset_flag[1] equal to 1 indicates that the filter index offsets for upsampling the columns of the chroma component are signaled. chroma_offset_flag[1] equal to 0 indicates that the filter index offsets for upsampling the columns of the chroma component are not signaled.

luma_filter_offset[i][j] specifies the offset value used to modify the luma filter index j in the upsampling process along direction i. This is a signed value between −15 to +15 (given a scaled grid size of 16×).

chroma_filter_offset[i][j] specifies the offset value used to modify the chroma filter index j in the upsampling process along direction i. This is a signed value between −15 to +15 (given a scaled grid size of 16×).

FIG. 5shows a table that includes an example of the syntax that may be used for signaling the phase shift offsets using the syntax elements defined above. In this example the column denoted Descriptor specifies the format that is to be used in signaling the values of the various syntax elements. In the example of the table shown inFIG. 5, the syntx specifies a re-mapping of filter indices for a given picture. It is also possible to signal more than one re-mapping for different picture types (or slices, PU, etc.) using similar additional syntax elements. One example where this might be appropriate is for the case of field pictures.

Luma and Chroma Sample Interpolation Process

The above syntax is proposed for the Joint Collaborative Team on Video Coding (JCT-VC), SHVC Test Model (SHM 1) Section G.6.2 entitled “Derivation process for reference layer sample location used in resampling,” and in particular see J. Chen, J. Boyce, Y. Ye, M. Hannuksela, “Draft of SHVC Test Model Description,” JCTVC-L1007, January 2013. The proposed text for this particular version of the document SHVC G.6.2 includes information helpful in understanding the syntax, so in some implementations it may be modified as follows:

For the SHVC text in G.6.2, the inputs to this process are:

a variable cIdx specifying the color component index, and

a sample location (xP, yP) relative to the top-left sample of the color component of the current picture (i.e., the pixel position in the EL) specified by cIdx.

The output of this process for a 16× reference layer picture (i.e., the BL) is a sample location (xRef16, yRef16) specifying the reference layer sample location in units of 1/16-th sample relative to the top-left sample of the reference layer picture.

The variables xRef and xPhase for the luma component are derived as follows:

Alternatively, the variables xRef and xPhase for the luma component can be derived as follows:

Similarly, the variables yRef and yPhase for the luma component are derived as follows:

The alternative approach for deriving the variables xRef and xPhase for the luma component shown above may also be used to derive the variables yRef and yPhase for the luma component.

The sample location (xRef16, yRef16) for the chroma component are derived in a manner similar to the luma component. In particular, the variables xRef and xPhase for the chroma component are derived as follows:

The variables yRef and yPhase are derived as follows:

Note that in the coding of the offsets, e.g. luma_filter_offset[i][j], other entropy coding methods can be used instead of the example shown in the table ofFIG. 5. In addition, only the offsets used for a given level (e.g. PPS or slice) need be signaled.

FIG. 6is a flowchart illustrating one example of a method for coding scalable video. At block500, the cross-layer prediction is examined to determine if luma and chroma phase offsets are to be employed. If so, the method proceeds to step510in which sampling signals from a first coding layer are received. Input samples of the video signal in the first coding layer are selected for coding video with a base resolution at block520. At block530a filter is selected from a first series of filters for processing a luma component of the selected input samples for an upsampling process. Each of the filters in the first series has a different phase index. A different phase offset is generated for each of the phase indices. The different phase offsets are used to select one of the filters in the first series. The selected input samples of the luma component are filtered with the selected filter at block540. The resulting output signal is provided in step550to a second coding layer that codes the luma component of the video with an enhanced resolution having a higher resolution than a base resolution.

Next, a chroma component of the selected input samples are processed. At block560a filter is selected from a second series of filters for processing the chroma component of the selected input samples for the upsampling process. Each of the filters in the second series has a different phase index. A different phase offset is generated for each of the phase indices. The different phase offsets are used to select one of the filters in the second series. The selected input samples of the chroma component are filtered with the selected filter at block570. The resulting output signal is provided in step580to the second coding layer that codes the chroma component of the video with an enhanced resolution having a higher resolution than a base resolution.

The encoding method described above provides a number of advantages. For instance, as previously mentioned, this technique can compensate for rounding or finite precision errors when computing filter indices. Additionally, arbitrary phase offsets per filter and color can be used in each dimension. Also, better matching of filters and phases to scalability ratios other than 2×, 1.5× can be achieved, as well as better matching of filters and phases to downsampling phase offsets.

Illustrative Operating Environment

FIG. 7is a simplified block diagram that illustrates an example video coding system10that may utilize the techniques of this disclosure. As used described herein, the term “video coder” can refer to either or both video encoders and video decoders. In this disclosure, the terms “video coding” or “coding” may refer to video encoding and video decoding.

As shown inFIG. 7, video coding system10includes a source device12and a destination device14. Source device12generates encoded video data. Accordingly, source device12may be referred to as a video encoding device. Destination device14may decode the encoded video data generated by source device12. Accordingly, destination device14may be referred to as a video decoding device. Source device12and destination device14may be examples of video coding devices.

Destination device14may receive encoded video data from source device12via a channel16. Channel16may comprise a type of medium or device capable of moving the encoded video data from source device12to destination device14. In one example, channel16may comprise a communication medium that enables source device12to transmit encoded video data directly to destination device14in real-time.

In this example, source device12may modulate the encoded video data according to a communication standard, such as a wireless communication protocol, and may transmit the modulated video data to destination device14. The communication medium may comprise a wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or other equipment that facilitates communication from source device12to destination device14. In another example, channel16may correspond to a storage medium that stores the encoded video data generated by source device12.

In the example ofFIG. 7, source device12includes a video source18, video encoder20, and an output interface22. In some cases, output interface22may include a modulator/demodulator (modem) and/or a transmitter. In source device12, video source18may include a source such as a video capture device, e.g., a video camera, a video archive containing previously captured video data, a video feed interface to receive video data from a video content provider, and/or a computer graphics system for generating video data, or a combination of such sources.

Video encoder20may encode the captured, pre-captured, or computer-generated video data. The encoded video data may be transmitted directly to destination device14via output interface22of source device12. The encoded video data may also be stored onto a storage medium or a file server for later access by destination device14for decoding and/or playback.

In the example ofFIG. 7, destination device14includes an input interface28, a video decoder30, and a display device32. In some cases, input interface28may include a receiver and/or a modem. Input interface28of destination device14receives encoded video data over channel16. The encoded video data may include a variety of syntax elements generated by video encoder20that represent the video data. Such syntax elements may be included with the encoded video data transmitted on a communication medium, stored on a storage medium, or stored a file server.

Display device32may be integrated with or may be external to destination device14. In some examples, destination device14may include an integrated display device and may also be configured to interface with an external display device. In other examples, destination device14may be a display device. In general, display device32displays the decoded video data to a user.

Video encoder20includes a resampling module25which may be configured to code (e.g., encode) video data in a scalable video coding scheme that defines at least one base layer and at least one enhancement layer. Resampling module25may resample at least some video data as part of an encoding process, wherein resampling may be performed in an adaptive manner using resampling filters. Likewise, video decoder30may also include a resampling module35similar to the resampling module25employed in the video encoder20.

Video encoder20and video decoder30may operate according to a video compression standard, such as the High Efficiency Video Coding (HEVC) standard. The HEVC standard is being developed by the Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Motion Picture Experts Group (MPEG). A recent draft of the HEVC standard, referred to as “HEVC Working Draft 7” or “WD 7,” is described in document JCTVC-11003, Bross et al., “High efficiency video coding (HEVC) Text Specification Draft 7,” Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, 9th Meeting: Geneva, Switzerland, Apr. 27, 2012 to May 7, 2012.

Additionally or alternatively, video encoder20and video decoder30may operate according to other proprietary or industry standards, such as the ITU-T H.264 standard, alternatively referred to as MPEG-4, Part 10, Advanced Video Coding (AVC), or extensions of such standards. The techniques of this disclosure, however, are not limited to any particular coding standard or technique. Other examples of video compression standards and techniques include MPEG-2, ITU-T H.263 and proprietary or open source compression formats and related formats.

Video encoder20and video decoder30may be implemented in hardware, software, firmware or any combination thereof. For example, the video encoder20and decoder30may employ one or more processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, or any combinations thereof. When the video encoder20and decoder30are implemented partially in software, a device may store instructions for the software in a suitable, non-transitory computer-readable storage medium and may execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Each of video encoder20and video decoder30may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device.