Patent Publication Number: US-2023164338-A1

Title: Luma Mapping with Chroma Scaling for Gradual Decoding Refresh

Description:
TECHNOLOGICAL FIELD 
     An example embodiment relates generally to techniques in video coding, and, more particularly, to techniques for efficient management of luma mapping with chroma scaling processes. 
     BACKGROUND 
     In video coding design, such as Advanced Video Coding (AVC), High Efficiency Video Coding (HEVC), and/or Versatile Video Coding (VVC), a coded video sequence may include intra coded pictures and inter coded pictures. Intra coded pictures may typically use more bits than inter coded pictures, such that the transmission time of intra coded pictures increases the encoder to decoder delay. For low delay applications, it may be desirable that all coded pictures use a similar number of bits such that the encoder to decoder delay can be reduced to around one picture interval. Hence, intra coded pictures mat not be ideal for (ultra) low delay applications. However, an intra coded picture may be needed at a random access point. 
     Approaches such as Gradual Decoding Refresh (GDR), Gradual Random Access (GRA), and Progressive Intra Refresh (PIR) may alleviate the delay issue with intra coded pictures. Instead of coding an intra picture at a random access point, GDR progressively refreshes pictures by spreading intra coded areas over several pictures. However, GDR may suffer incompatibility and/or other issues with certain coding tools, such as luma mapping with chroma scaling (LMCS), a process that is applied as part of the decoding process which maps luma samples to particular values and may apply a scaling operation to the values of chroma samples. 
     BRIEF SUMMARY 
     In an embodiment, an apparatus is provided comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to access a picture to be decoded. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to determine whether the picture is within a gradual decoding refresh period. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to, in an instance in which the picture is within the gradual decoding refresh period, prevent a luma mapping with chroma scaling decoding process from being applied to the picture. 
     In some embodiments of the apparatus, the luma mapping with chroma scaling decoding process comprises all decoding processes associated with luma mapping with chroma scaling. In some embodiments of the apparatus, the luma mapping with chroma scaling decoding process comprises only chroma residual scaling decoding processes associated with luma mapping with chroma scaling. In some embodiments of the apparatus, the prevention of the luma mapping with chroma scaling decoding process is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the prevention of the luma mapping with chroma scaling decoding process is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, an apparatus is provided comprising means for accessing a picture to be decoded. The apparatus further comprises means for determining whether the picture is within a gradual decoding refresh period. The apparatus further comprises means for, in an instance in which the picture is within the gradual decoding refresh period, preventing a luma mapping with chroma scaling decoding process from being applied to the picture. In some embodiments, the luma mapping with chroma scaling decoding process comprises all decoding processes associated with luma mapping with chroma scaling. In some embodiments, the luma mapping with chroma scaling decoding process comprises only chroma residual scaling decoding processes associated with luma mapping with chroma scaling. In some embodiments of the apparatus, the prevention of the luma mapping with chroma scaling decoding process is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the prevention of the luma mapping with chroma scaling decoding process is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a computer program product is provided comprising a non-transitory computer readable storage medium having program code portions stored thereon, the program code portions configured, upon execution, to access a picture to be decoded. The program code portions are further configured, upon execution, to determine whether the picture is within a gradual decoding refresh period. The program code portions are further configured, upon execution, to, in an instance in which the picture is within the gradual decoding refresh period, prevent a luma mapping with chroma scaling decoding process from being applied to the picture. 
     In some embodiments of the computer program product, the luma mapping with chroma scaling decoding process comprises all decoding processes associated with luma mapping with chroma scaling. In some embodiments of the computer program product, the luma mapping with chroma scaling decoding process comprises only chroma residual scaling decoding processes associated with luma mapping with chroma scaling. In some embodiments of the computer program product, the prevention of the luma mapping with chroma scaling decoding process is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the computer program product, the prevention of the luma mapping with chroma scaling decoding process is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the computer program product, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a method is provided, the method comprising accessing a picture to be decoded. The method further comprises determining whether the picture is within a gradual decoding refresh period. The method further comprises, in an instance in which the picture is within the gradual decoding refresh period, preventing a luma mapping with chroma scaling decoding process from being applied to the picture. In some embodiments of the method, the luma mapping with chroma scaling decoding process comprises all decoding processes associated with luma mapping with chroma scaling. In some embodiments of the method, the luma mapping with chroma scaling decoding process comprises only chroma residual scaling decoding processes associated with luma mapping with chroma scaling. In some embodiments of the method, the prevention of the luma mapping with chroma scaling decoding process is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the method, the prevention of the luma mapping with chroma scaling decoding process is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the method, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, an apparatus is provided comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to access a picture within a gradual decoding refresh period to be decoded. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to determine whether a virtual boundary is within a current coding tree unit of the picture, the virtual boundary defining a separation of a clean area and a dirty area of the picture. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to, in an instance in which the virtual boundary is within the current coding tree unit, prevent a chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area. 
     In some embodiments of the apparatus, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, an apparatus is provided comprising means for accessing a picture within a gradual decoding refresh period to be decoded. The apparatus further comprises means for determining whether a virtual boundary is within a current coding tree unit of the picture, the virtual boundary defining a separation of a clean area and a dirty area of the picture. The apparatus further comprises means for, in an instance in which the virtual boundary is within the current coding tree unit, preventing a chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area. 
     In some embodiments of the apparatus, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a computer program product is provided comprising a non-transitory computer readable storage medium having program code portions stored thereon, the program code portions configured, upon execution, to access a picture within a gradual decoding refresh period to be decoded. The program code portions are further configured, upon execution, to determine whether a virtual boundary is within a current coding tree unit of the picture, the virtual boundary defining a separation of a clean area and a dirty area of the picture. The program code portions are further configured, upon execution, to, in an instance in which the virtual boundary is within the current coding tree unit, prevent a chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area. 
     In some embodiments of the computer program product, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the computer program product, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the computer program product, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a method is provided, the method comprising accessing a picture within a gradual decoding refresh period to be decoded. The method further comprises determining whether a virtual boundary is within a current coding tree unit of the picture, the virtual boundary defining a separation of a clean area and a dirty area of the picture. The method further comprises, in an instance in which the virtual boundary is within the current coding tree unit, preventing a chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area. 
     In some embodiments of the method, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the method, the prevention of the chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the method, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, an apparatus is provided comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to access a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within a clean area. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to determine whether one or more pixels within a neighboring pixel area are within a dirty area. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, prevent all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling. 
     In some embodiments of the apparatus, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, an apparatus is provided comprising means for accessing a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within a clean area. The apparatus further comprises means for determining whether one or more pixels within a neighboring pixel area are within a dirty area. The apparatus further comprises means for, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, preventing all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling. 
     In some embodiments of the apparatus, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a computer program product is provided comprising a non-transitory computer readable storage medium having program code portions stored thereon, the program code portions configured, upon execution, to access a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within a clean area. The program code portions are further configured, upon execution, to determine whether one or more pixels within a neighboring pixel area are within a dirty area. The program code portions are further configured, upon execution, to, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, prevent all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling. 
     In some embodiments of the computer program product, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the computer program product, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the computer program product, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a method is provided, the method comprising accessing a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within clean area. The method further comprises determining whether one or more pixels within a neighboring pixel area are within a dirty area. The method further comprises, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, preventing all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling. 
     In some embodiments of the method, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the method, the prevention of all pixels within the neighboring pixel area from being used for a chroma residual scaling process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the method, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, an apparatus is provided comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to access a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within a clean area. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to determine whether one or more pixels within a neighboring pixel area are within a dirty area. The at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, pad the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling. 
     In some embodiments of the apparatus, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, an apparatus is provided comprising means for accessing a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within a clean area. The apparatus further comprises means for determining whether one or more pixels within a neighboring pixel area are within a dirty area. The apparatus further comprises means for, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, padding the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma residual scaling decoding process associated with luma mapping with chroma scaling. 
     In some embodiments of the apparatus, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the apparatus, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the apparatus, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a computer program product is provided comprising a non-transitory computer readable storage medium having program code portions stored thereon, the program code portions configured, upon execution, to access a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within a clean area. The program code portions are further configured, upon execution, to determine whether one or more pixels within a neighboring pixel area are within a dirty area. The program code portions are further configured, upon execution, to, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, pad the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma residual scaling decoding process associated with luma mapping with chroma scaling. 
     In some embodiments of the computer program product, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the computer program product, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the computer program product, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
     In another embodiment, a method is provided, the method comprising accessing a current coding unit of a picture within a gradual decoding refresh period to be decoded, the current coding unit being within a clean area. The method further comprises determining whether one or more pixels within a neighboring pixel area are within a dirty area. The method further comprises, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, padding the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma residual scaling decoding process associated with luma mapping with chroma scaling. 
     In some embodiments of the method, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is applied regardless of whether the picture is within the gradual decoding refresh period. In some embodiments of the method, the padding of the neighboring pixel area from one or more reconstructed pixels in the clean area for a chroma scaling decoding process associated with luma mapping with chroma scaling is further based on an enabling value associated with a virtual boundary within the picture. In some embodiments of the method, usage of the enabling value associated with the virtual boundary is based on a second enabling value associated with luma mapping with chroma scaling (LMCS). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus described certain example embodiments of the present disclosure in general terms, reference will hereinafter be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG.  1    is a representation of example pictures within a GDR period wherein a clean area and a dirty area are separated by a virtual boundary in accordance with an example embodiment of the present disclosure; 
         FIG.  2    is a representation of an example coding tree unit in accordance with an example embodiment of the present disclosure; 
         FIG.  3    is a block diagram of an apparatus that may be specifically configured in accordance with an example embodiment of the present disclosure; 
         FIG.  4 A  is a flow diagram illustrating operations performed in accordance with an example embodiment; 
         FIG.  4 B  is a representation of an example usage of top and left neighboring blocks during combined inter intra prediction weight derivation; 
         FIG.  4 C  is a representation of an example usage of top and left neighboring blocks during combined inter intra prediction weight derivation; 
         FIG.  5 A  is a flow diagram illustrating operations performed in accordance with an example embodiment; 
         FIG.  5 B  is a representation of particular changes in a decoding process for the VVC Specification in accordance with an example embodiment; 
         FIG.  6 A  is a flow diagram illustrating operations performed in accordance with an example embodiment; 
         FIG.  6 B  is a representation of particular changes in a decoding process for the VVC Specification in accordance with an example embodiment; 
         FIG.  7 A  is a flow diagram illustrating operations performed in accordance with an example embodiment; and 
         FIG.  7 B  is a representation of particular changes in a decoding process for the VVC Specification in accordance with an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all, embodiments of the disclosure are shown. Indeed, some embodiments may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like reference numerals refer to like elements throughout. As used herein, the terms “data,” “content,” “information,” and similar terms may be used interchangeably to refer to data capable of being transmitted, received and/or stored in accordance with embodiments of the present disclosure. Thus, use of any such terms should not be taken to limit the spirit and scope of embodiments of the present disclosure. 
     Additionally, as used herein, the term ‘circuitry’ refers to (a) hardware-only circuit implementations (e.g., implementations in analog circuitry and/or digital circuitry); (b) combinations of circuits and computer program product(s) comprising software and/or firmware instructions stored on one or more computer readable memories that work together to cause an apparatus to perform one or more functions described herein; and (c) circuits, such as, for example, a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation even if the software or firmware is not physically present. This definition of ‘circuitry’ applies to all uses of this term herein, including in any claims. As a further example, as used herein, the term ‘circuitry’ also includes an implementation comprising one or more processors and/or portion(s) thereof and accompanying software and/or firmware. As another example, the term ‘circuitry’ as used herein also includes, for example, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, other network device (such as a core network apparatus), field programmable gate array, and/or other computing device. 
     Overview 
     As described above, the GDR approach may alleviate the delay issue with intra coded pictures. Instead of coding an intra picture at a random access point, GDR progressively refreshes pictures by spreading intra coded areas over several pictures of a GDR period. A typical picture within a GDR period consists of a clean area (e.g., an area that has undergone an intra refresh) and a dirty area (e.g., an area that has not yet undergone an intra refresh), wherein the clean area may contain a forced intra area next to dirty area for progressive intra refresh (PIR). 
     Picture Header of the Versatile Video Coding (VVC) specification includes virtual boundary syntax. With virtual boundary syntax included in Picture Header, a picture may have its own virtual boundaries. For example, a virtual boundary may be a horizontal or vertical line defining a boundary between a clean area and a dirty area. 
       FIG.  1    illustrates an example of vertical GDR with a vertical virtual boundary, in which a GDR period begins with picture POC(n) and ends with picture POC(n+N-1), including N pictures in total. The first picture POC(n) within the GDR period is referred to as the GDR picture. Forced intra coded areas  105  gradually spread over the N pictures within the GDR period from left to right, and the clean area  110  expends gradually from the picture POC(n) to the picture POC(N+n-1). The picture POC(n+N) is referred to as the recovery point picture. A virtual boundary  120  separates the clean area ( 110  and  105 ) and the dirty area  115  of the picture POC(n+1) of the GDR period. A current picture within a GDR period comprises a clean area and a dirty area, wherein the clean area may contain a forced intra area next to the dirty area for progressive intra refresh (PIR), as shown in picture POC(n+1) in  FIG.  1   . The virtual boundary  120  between the clean area and the dirty area may be signalled by virtual boundary syntax in Picture Header. In GDR, if an exact_match at the recovery point is required, coding units (CU) in a clean area cannot use any coding information (e.g., reconstructed pixels, code mode, motion vectors (MVs), reference index (refldx), etc.) from the dirty area. 
     Included in VVC is a coding tool called luma mapping with chroma scaling (LMCS). LMCS is a process that is applied as part of the decoding process that maps luma samples to particular values and may apply a scaling operation to the values of chroma samples. Under the current VVC Specification (B. Bross, J. Chen, S. Lin, and Y-K. Wang, “Versatile Video Coding”, JVET-Q2001-v13, January 2020, which is incorporated by reference herein, and hereinafter referred to as the “VVC Specification”), LMCS for coding units in a clean area may use the pixels in dirty area. Thus, exact_match may not be able to be achieved for GDR with LMCS. 
       FIG.  2    shows an example of how LMCS for CUs in clean area may utilize the reconstructed pixels in dirty area, wherein coding tree unit (CTU) size is set to 128 x128 pixels, and a virtual boundary passes through a current CTU and a current coding unit  205  of 32x32 within the current CTU is in a clean area. 
     In the VVC Specification, the syntax element sps_log2_ctu_size_minus5 is signaled, and the associated variables are calculated as follows: 
     CtbSizeY = 1 &lt;&lt; (sps_log2_ctu_size_minus5 + 5), sizeY = Min( CtbSizeY, 64 ) 
     For a current CU in a current CTU, an array, recLuma[i], i = 0, 1, ..., 2  ∗  sizeY-1, is defined as a collection of sizeY left reconstructed neighboring pixels and sizeY above reconstructed neighboring pixels, as shown in  FIG.  2    where sizeY = 64. 
     The average of array recLuma[i], i = 0, 1, ..., 2  ∗  sizeY-1, is calculated as: 
     
       
         
           
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     The variable invAvgLuma calculated above is then used to determine a scaling factor, varScale, for chroma residuals: 
     
       
         
           
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     With the scaling factor varScale, the chroma components (Cb and Cr) of the current CU are then reconstructed as follows, wherein the subscript rec represented reconstructed, pre represents prediction, and res represents residual: 
     
       
         
           
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     However, it may be possible that some elements of array recLuma[i], i = 0, 1, ..., 2 * sizeY-1, may be within a dirty area and thus the final reconstruction of chroma components of a current CU in a clean area may use the reconstructed neighboring luma pixels in the dirty area, which may cause leaks and result in exact_match not being achievable. For example, as illustrated in  FIG.  2   , for the current CU  205 , the above reconstructed neighboring luma pixels  210  are in a dirty area. 
     Example Apparatus 
     One example of an apparatus  300  that may be configured to carry out operations in accordance with an embodiment described herein is depicted in  FIG.  3   . As shown in  FIG.  3   , the apparatus includes, is associated with or is in communication with processing circuity  32 , a memory  34  and a communication interface  36 . The processing circuitry  32  may be in communication with the memory via a bus for passing information among components of the apparatus. The memory may be non-transitory and may include, for example, one or more volatile and/or non-volatile memories. In other words, for example, the memory may be an electronic storage device (e.g., a computer readable storage medium) comprising gates configured to store data (e.g., bits) that may be retrievable by a machine (e.g., a computing device like the processing circuitry). The memory may be configured to store information, data, content, applications, instructions, or the like for enabling the apparatus to carry out various functions in accordance with an example embodiment of the present disclosure. For example, the memory could be configured to buffer input data for processing by the processing circuitry. Additionally, or alternatively, the memory could be configured to store instructions for execution by the processing circuitry. 
     The apparatus  100  may, in some embodiments, be embodied in various computing devices. However, in some embodiments, the apparatus may be embodied as a chip or chip set. In other words, the apparatus may comprise one or more physical packages (e.g., chips) including materials, components and/or wires on a structural assembly (e.g., a baseboard). The structural assembly may provide physical strength, conservation of size, and/or limitation of electrical interaction for component circuitry included thereon. The apparatus may therefore, in some cases, be configured to implement an embodiment of the present disclosure on a single chip or as a single “system on a chip.” As such, in some cases, a chip or chipset may constitute means for performing one or more operations for providing the functionalities described herein. 
     The processing circuitry  32  may be embodied in a number of different ways. For example, the processing circuitry may be embodied as one or more of various hardware processing means such as a coprocessor, a microprocessor, a controller, a digital signal processor (DSP), a processing element with or without an accompanying DSP, or various other circuitry including integrated circuits such as, for example, an ASIC (application specific integrated circuit), an FPGA (field programmable gate array), a microcontroller unit (MCU), a hardware accelerator, a special-purpose computer chip, or the like. As such, in some embodiments, the processing circuitry may include one or more processing cores configured to perform independently. A multi-core processing circuitry may enable multiprocessing within a single physical package. Additionally, or alternatively, the processing circuitry may include one or more processors configured in tandem via the bus to enable independent execution of instructions, pipelining and/or multithreading. 
     In an example embodiment, the processing circuitry  32  may be configured to execute instructions stored in the memory device  34  or otherwise accessible to the processing circuitry. Alternatively, or additionally, the processing circuitry may be configured to execute hard coded functionality. As such, whether configured by hardware or software methods, or by a combination thereof, the processing circuitry may represent an entity (e.g., physically embodied in circuitry) capable of performing operations according to an embodiment of the present disclosure while configured accordingly. Thus, for example, when the processing circuitry is embodied as an ASIC, FPGA or the like, the processing circuitry may be specifically configured hardware for conducting the operations described herein. Alternatively, as another example, when the processing circuitry is embodied as an executor of instructions, the instructions may specifically configure the processor to perform the algorithms and/or operations described herein when the instructions are executed. However, in some cases, the processing circuitry may be a processor of a specific device (e.g., an image or video processing system) configured to employ one embodiment by further configuration of the processing circuitry by instructions for performing the algorithms and/or operations described herein. The processing circuitry may include, among other things, a clock, an arithmetic logic unit (ALU) and logic gates configured to support operation of the processing circuitry. 
     The communication interface  36  may be any means such as a device or circuitry embodied in either hardware or a combination of hardware and software that is configured to receive and/or transmit data, including media content in the form of video or image files, one or more audio tracks or the like. In this regard, the communication interface may include, for example, an antenna (or multiple antennas) and supporting hardware and/or software for enabling communications with a wireless communication network. Additionally, or alternatively, the communication interface may include the circuitry for interacting with the antenna(s) to cause transmission of signals via the antenna(s) or to handle receipt of signals received via the antenna(s). In some environments, the communication interface may alternatively or also support wired communication. As such, for example, the communication interface may include a communication modem and/or other hardware/software for supporting communication via cable, digital subscriber line (DSL), universal serial bus (USB) or other mechanisms. 
     According to some embodiments, the apparatus  300  may be configured in accordance with an architecture for providing for video encoding, decoding, and/or compression. In this regard, the apparatus  300  may be configured as a video coding device. For example, the apparatus  300  may be configured to code video in accordance with one or more video compression standards, such as, for example, the VVC Specification. While certain embodiments herein refer to operations associated with the VVC standard, it is to be appreciated that the processes discussed herein may be utilized for any video coding standard. 
     Example Processes for LMCS for GDR 
     In embodiments described herein, a new variable, WithinGdrPeriodFlag, may be introduced in Picture Header semantics as shown below in Table A1. 
     
       
         
          TABLE A1
           
               
             
               
                 recovery_poc_cnt specifies the recovery point of decoded pictures in output order. If the current picture is a GDR picture that is associated with the PH, and there is a picture picA that follows the current GDR picture in decoding order in the CLVS that has PicOrderCntVal equal to the PicOrderCntVal of the current GDR picture plus the value of recovery_poc_cnt, the picture picA is referred to as the recovery point picture. Otherwise, the first picture in output order that has PicOrderCntVal greater than the PicOrderCntVal of the current picture plus the value of recovery_poc_cnt is referred to as the recovery point picture. The recovery point picture shall not precede the current GDR picture in decoding order. The value of recovery_poc_cnt shall be in the range of 0 to MaxPicOrderCntLsb - 1, inclusive. When the current picture is a GDR picture, the variable GdrPicOrderCntVal, RpPicOrderCntVal are derived as follows: 
               
             
            
               
                 GdrPicOrderCntVal = PicOrderCntVal 
               
               
                 RpPicOrderCntVal = GdrPicOrderCntVal + recovery_poc_cnt (81) 
               
               
                 For the current picture, the variable WithinGdrPeriodFlag is derived as follows: 
               
               
                      if (GdrPicOrderCntVal &lt;= PicOrderCntVal &amp;&amp; PicOrderCntVal &lt; RpPicOrderCntVal) 
               
               
                             WithinGdrPeriodFlag = 1 
               
               
                      else 
               
               
                             WithinGdrPeriodFlag = 0 
               
               
                 NOTE 2 - When gdr enabled _flag is equal to 1 and PicOrderCntVal of the current picture is greater than or equal to RpPicOrderCntVal of the associated GDR picture, the current and subsequent decoded pictures in output order are exact match to the corresponding pictures produced by starting the decoding process from the previous IRAP picture, when present, preceding the associated GDR picture in decoding order. 
               
            
           
         
       
     
     Alternatively, WithinGdrPeriodFlag may be derived as shown below: 
     When the current picture is a GDR picture, the variable GdrPicOrderCntVal, RpPicOrderCntVal are derived as follows: 
     
       
         
           
             
               
                 GdrPicOrderCntVal = PicOrderCntVal 
               
             
             
               
                 RpPicOrderCntVal = GdrPicOrderCntVal + recovery_poc_cnt 
               
             
           
         
       
     
     For the current picture, the variable WithinGdrPeriodFlag is derived to be equal to 1 if there is any GDR picture that has GdrPicOrderCntVal less than or equal to PicOrderCntVal and RpPicOrderCntVal is greater than PicOrderCntVal, and derived to be equal to 0 otherwise. 
     The above derivations for concluding the GDR period and WithinGdrPeriodFlag are merely example embodiments, and other embodiments may be similarly realized. For example, some derivations above assume that recovery_poc_cnt being equal to 0 (e.g., the GDR picture being also the recovery point) is allowed. In an example, the derivations are adjusted for different semantics of recovery_poc_cnt, where it specifies the last picture within the GDR period rather than the recovery point picture, and thus it is also disallowed from having a GDR picture that is also the recovery point picture. In this case, for the current picture, the variable WithinGdrPeriodFlag is derived to be equal to 1 if there is any GDR picture that has GdrPicOrderCntVal less than or equal to PicOrderCntVal and RpPicOrderCntVal is greater than or equal to PicOrderCntVal, and derived to be equal to 0 otherwise. 
     In one embodiment, an encoder concludes whether a picture is within a GDR period, e.g. by deriving the WithinGdrPeriodFlag as described above. When a picture is concluded to be within a GDR period, the encoder turns LMCS off, e.g. by setting ph_lmcs_enabled_flag in the picture header equal to zero (0) or by setting slice_lmcs_enabled_flag in all slice headers of the picture equal to zero (0). 
     In one embodiment, an encoder concludes whether a picture is within a GDR period, e.g. by deriving the WithinGdrPeriodFlag as described above. When a picture is concluded to be within a GDR period, the encoder turns the chroma residual scaling of LMCS off, e.g. by setting ph_chroma_residual_scale_flag in the picture header equal to 0. 
       FIG.  4 A  shows a flowchart including operations of one embodiment in which a control is utilized for LMCS. At operation  401 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to access a picture to be decoded. For example, as described above, the picture may be included as part of a coded video sequence, and the apparatus  300  may be configured to access the picture during a coding process of the coded video sequence, such as during a decoding process. 
     At operation  402 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to determine whether the picture is within a gradual decoding refresh (GDR) period. In some embodiments, the determination as to whether the picture is within a GDR period may comprise analyzing a flag variable associated with the picture. As shown in  FIG.  4 B , an example Picture Header structure syntax includes a Boolean flag variable WithinGdrPeriodFlag. In this regard, if a picture is within a GDR period (WithinGdrPeriodFlag != 0), none of LMCS related syntax elements will be signalled and the LMCS related decoding process will be skipped, which may save bits and processing cycles that would have otherwise been implemented. As seen in  FIG.  4 B , for pictures within a GDR period comprising a WithinGdrPeriodFlag variable not equal to zero, the syntax flag ph_lmcs_enabled_flag is not signaled and it is inferred to be equal to zero. 
     Since the flag ph_lmcs_enabled_flag is equal to 0, the decoding process associated with LMCS will not be implemented for pictures with GDR periods. In this regard, at operation  403 , in an instance in which the picture is a picture within a GDR period, the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to prevent a luma mapping with chroma scaling decoding process from being applied to the picture within the GDR period. 
     In another embodiment, the LMCS decoding process may be limited to the chroma components only. In this embodiment, if a current picture is within a GDR period, the syntax flag ph_chroma_residual_scale_flag will not be signalled and the decoding process for chroma residual scaling associated with LMCS will be prevented from occurring. For example,  FIG.  4 C  shows another example Picture Header structure syntax. As shown, for pictures within a GDR period (e.g., when WithinGdrPeriodFlag is not equal to 0), the syntax flag ph_chroma_residual_scale_flag is not signaled and it is inferred to be equal to 0. Since the flag ph_chroma_residual_scale_flag is equal to 0, the decoding process associated with LMCS’s chroma scaling will not be implemented for pictures within the GDR period. In this regard, in some embodiments, the apparatus  300 , such as the processing circuitry  32 , memory  34 , and/or the like, may be configured to prevent only chroma residual scaling decoding processes associated with LMCS from being applied to the picture within the GDR period. 
     In another embodiment, for a current CU in a clean area of a current CTU with a virtual boundary passing through, if any of its sizeY above (or sizeY left) reconstructed neighboring pixels are in a dirty area, chroma residual scaling of LMCS may be prevented for the current CU within the clean area of the current CTU. Using  FIG.  2    as an example, chroma residual scaling of LMCS for the current CU  205  is prevented since the virtual boundary  220  passes through the current CTU and the current CU  205  within the current CTU is in the clean area of the current CTU. 
     As shown in  FIG.  5 A , at operation  501 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to access a picture within a GDR period to be decoded. As described above, the picture within the GDR period may be included as part of a coded video sequence, and the apparatus  300  may be configured to access the picture within a GDR period during a coding process of the coded video sequence, such as during a decoding process. At operation  502 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to determine whether a virtual boundary is within a current coding tree unit of the picture within the GDR period. In some embodiments, the virtual boundary may define a separation of a clean area and a dirty area of the picture within the GDR period. At operation  503 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to, in an instance in which the virtual boundary is within the current coding tree unit, prevent a chroma residual scaling decoding process associated with luma mapping with chroma scaling from being applied to one or more coding units within the clean area. 
     These operations may be performed through changes in the decoding process. Described below in Table A are changes for the section “picture reconstruction with luma dependent chroma residual scaling process for chroma samples” of the VVC Specification in accordance with certain embodiments described herein.  FIG.  5 B  further highlights the specific changes to the section proposed herein. 
     
       
         
          TABLE A
           
               
             
               
                 8.7.5.3 Picture Reconstruction With Luma Dependent Chroma Residual Scaling Process For chroma samples 
               
             
            
               
                 Inputs to this process are: 
               
               
                     a chroma location ( xCurr, yCurr ) of the top-left chroma sample of the current chroma transform block relative to the top-left chroma sample of the current picture, 
               
               
                     a variable nCurrSw specifying the chroma transform block width, 
               
               
                     a variable nCurrSh specifying the chroma transform block height, 
               
               
                     a variable tuCbfChroma specifying the coded block flag of the current chroma transform block, 
               
               
                     an (nCurrSw)x(nCurrSh) array predSamples specifying the chroma prediction samples of the current block, 
               
               
                     an (nCurrSw)x(nCurrSh) array resSamples specifying the chroma residual samples of the current block, 
               
               
                 Output of this process is a reconstructed chroma picture sample array recSamples. 
               
               
                 The variable sizeY is set equal to Min( CtbSizeY, 64 ). 
               
               
                 The reconstructed chroma picture sample recSamples is derived as follows for 
               
               
                 i = 0..nCurrSw - 1, j = 0..nCurrSh - 1: 
               
               
                     If one or more of the following conditions are true, recSamples[ xCurr + i ][ yCurr + j ] is set equal to Clip1( predSamples[ i ][ j ] + resSamples[ i ][ j ] ): 
               
               
                         ph_chroma_residual_scale_flag is equal to 0. 
               
               
                         slice_lmcs_enabled_flag is equal to 0. 
               
               
                         nCurrSw * nCurrSh is less than or equal to 4. 
               
               
                         tu_cbf_cb [ xCurr ][ yCurr ] is equal to 0 and tu_cbf_cr [ xCurr ][ yCurr ] is equal to 0. 
               
               
                         ( xCurrY, yCurrY ) = ( xCurr * SubWidthC, yCurr * SubHeightC ) and ( xCuCb, yCuCb ) is specified as top-left luma sample location of the coding unit that contains the luma sample at ( xCurrY / sizeY * sizeY, yCurrY / sizeY * sizeY ) and xVir is (VirtualBoundariesPosX[ 0 ] / sizeY * sizeY) and yVir is (VirtualBoundariesPosY[ 0 ] / sizeY * sizeY) and the following conditions are true. 
               
               
                             WithinGdrPeriodFlag is equal to 1, 
               
               
                             ph_chroma_residual_scale_flag is equal to 1, 
               
               
                             slice_lmcs_enabled_flag is equal to 1, and 
               
               
                             one of the two following conditions is true 
               
               
                                 (NumVerVirtualBoundaries &gt; 0 &amp;&amp; xCuCb &lt; VirtualBoundariesPosX[ 0 ] &lt; xCuCb + sizeY &amp;&amp; xVir &lt;= xCurrY &lt; VirtualBoundariesPosX[ 0 ]) 
               
               
                                 (NumHorVirtualBoundaries &gt; 0 &amp;&amp; yCuCb &lt; VirtualBoundariesPosY[ 0 ] &lt; yCuCb + sizeY &amp;&amp; yVir &lt;= yCurrY &lt; VirtualBoundariesPosY[ 0 ]). 
               
               
                     Otherwise, the following applies: 
               
               
                         The current luma location ( xCurrY, yCurrY ) is derived as follows: 
               
               
                         (xCurrY, yCurrY ) = (xCurr * SubWidthC, yCurr * SubHeightC )    (1234) 
               
               
                     The luma location (xCuCb, yCuCb ) is specified as the top-left luma sample location of the coding unit that contains the luma sample at ( xCurrY / sizeY * sizeY, yCurrY / sizeY * sizeY ). 
               
               
                     The variables availL and availT are derived as follows: 
               
               
                         The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the location (xCurr, yCurr ) set equal to ( xCuCb, yCuCb ), the neighbouring luma location ( xNbY, yNbY) set equal to (xCuCb - 1, yCuCb ), checkPredModeY set equal to FALSE, and cIdx set equal to 0 as inputs, and the output is assigned to availL. 
               
               
                         The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the location (xCurr, yCurr ) set equal to ( xCuCb, yCuCb ), the neighbouring luma location ( xNbY, yNbY) set equal to ( xCuCb, yCuCb - 1 ), checkPredModeY set equal to FALSE, and cIdx set equal to 0 as inputs, and the output is assigned to availT. 
               
               
                     The variable currPic specifies the array of reconstructed luma samples in the current picture. 
               
               
                     For the derivation of the variable varScale the following ordered steps apply: 
               
               
                     1. The variable invAvgLuma is derived as follows: 
               
               
                         The array recLuma[ i ] with i=0..( 2 * sizeY - 1 ) and the variable cnt are derived as follows: 
               
               
                         The variable cnt is set equal to 0. 
               
               
                         When availL is equal to TRUE, the array recLuma[ i ] with i = 0..sizeY - 1 is set equal to currPic[ xCuCb - 1 ][ Min( yCuCb + i, pic_height_in_luma_samples - 1 ) ] with i = 0.. sizeY - 1, and cnt is set equal to sizeY. 
               
               
                         When availT is equal to TRUE, the array recLuma[ cnt + i ] with i = 0.. sizeY - 1 is set equal to currPic[ Min( xCuCb + i, pic_width_in_luma_samples - 1 ) ][ yCuCb -1 ] with i = 0.. sizeY - 1, and cnt is set equal to ( cnt + sizeY ). 
               
               
                     The variable invAvgLuma is derived as follows: 
               
               
                         If cnt is greater than 0, the following applies: 
               
               
                                 invAvgLuma = Clip1             ∑             k = 0       cnt-1        recLuma     k     +       cnt   &gt;   &gt;                   &gt;   &gt;   Log2( cnt   ​    ))       (1235) 
               
               
                         Otherwise (cnt is equal to 0), the following applies: 
               
               
                         invAvgLuma = 1 &lt;&lt; (BitDepth - 1 )    (1236) 
               
               
                     2. The variable idxYInv is derived by invoking the identification of piece-wise function index process for a luma sample as specified in clause 8.8.2.3 with the variable lumaSample set equal to invAvgLuma as the input and idxYInv as the output. 
               
               
                     3. The variable varScale is derived as follows: 
               
               
                         varScale = ChromaScaleCoeff[ idxYInv ]    (1237) 
               
               
                     The reconstructed chroma picture sample array recSamples is derived as follows: 
               
               
                         If tuCbfChroma is equal to 1, the following applies: 
               
               
                         resSamples[ i ][ j ] = Clip3( -( 1 &lt;&lt; BitDepth ), ( 1 &lt;&lt; BitDepth ) - 1, resSamples[ i ][ j ] )    (1238) 
               
               
                         recSamples[ xCurr + i ][ yCurr + j ] = Clip1( predSamples[ i ][ j ] + (1239) Sign( resSamples[ i ][ j ]) * (( Abs( resSamples[ i ][ j ]) * varScale + ( 1 &lt;&lt; 10) ) &gt;&gt; 11 )) 
               
               
                         Otherwise (tuCbfChroma is equal to 0), the following applies: 
               
               
                         recSamples[ xCurr + i ][ yCurr + j ] = Clip1(predSamples[ i ][ j ] ) (1240) 
               
            
           
         
       
     
     In another embodiment, for a current CU in a clean area of a current CTU with a virtual boundary passing through, if any of its sizeY above (or sizeY left) reconstructed neighboring pixels are in a dirty area, the entire row of sizeY above (or column of sizeY left) reconstructed neighboring pixels may be treated as unavailable for chroma residual scaling of LMCS for the current CU. For example, in  FIG.  2   , since the above reconstructed neighboring pixels  210  for the current CU  205  are in a dirty area, the entire row of 64 above reconstructed neighboring pixels are treated as unavailable for chroma residual scaling of LMCS. In this regard, as shown in  FIG.  6 A , at operation  601 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to access a current coding unit of a picture with a GDR period to be decoded, wherein the current coding unit is within a clean area. As described above, the picture within the GDR period may be included as part of a coded video sequence. At operation  602 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to determine whether one or more pixels within a neighboring pixel area are within a dirty area. At operation  603 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to, in an instance in which one or more pixels within the top (or the left) neighboring pixel area are determined to be within the dirty area, prevent all of the pixels within the top (or the left) neighboring pixel area from being used for a chroma residual scaling decoding process associated with luma mapping with chroma scaling. 
     These operations may be performed through changes in the decoding process. Described below in Table B are changes for the section “picture reconstruction with luma dependent chroma residual scaling process for chroma samples” of the VVC specification in accordance with certain embodiments described herein.  FIG.  6 B  further highlights the specific changes to the section proposed herein. 
     
       
         
          TABLE B
           
               
             
               
                 8.7.5.3 Picture Reconstruction With Luma Dependent Chroma Residual Scaling Process for chroma samples 
               
             
            
               
                     Inputs to this process are: 
               
               
                     a chroma location ( xCurr, yCurr ) of the top-left chroma sample of the current chroma transform block relative to the top-left chroma sample of the current picture, 
               
               
                     a variable nCurrSw specifying the chroma transform block width, 
               
               
                     a variable nCurrSh specifying the chroma transform block height, 
               
               
                     a variable tuCbfChroma specifying the coded block flag of the current chroma transform block, 
               
               
                     an (nCurrSw)x(nCurrSh) array predSamples specifying the chroma prediction samples of the current block, 
               
               
                     an (nCurrSw)x(nCurrSh) array resSamples specifying the chroma residual samples of the current block, 
               
               
                 Output of this process is a reconstructed chroma picture sample array recSamples. 
               
               
                 The variable sizeY is set equal to Min( CtbSizeY, 64 ). 
               
               
                 The reconstructed chroma picture sample recSamples is derived as follows for 
               
               
                 i = 0..nCurrSw - 1, j = 0..nCurrSh - 1: 
               
               
                     If one or more of the following conditions are true, recSamples[ xCurr + i ][ yCurr + j ] is set equal to Clip1( predSamples[ i ][ j ] + resSamples[ i ][ j ] ): 
               
               
                         ph_chroma_residual_scale_flag is equal to 0. 
               
               
                         slice_lmcs_enabled_flag is equal to 0. 
               
               
                         nCurrSw * nCurrSh is less than or euqal to 4. 
               
               
                         tu_cbf_cb [ xCurr ][ yCurr ] is equal to 0 and tu_cbf_cr [ xCurr ][ yCurr ] is equal to 0. 
               
               
                     Otherwise, the following applies: 
               
               
                         The current luma location ( xCurrY, yCurrY ) is derived as follows: 
               
               
                         (xCurrY, yCurrY ) = (xCurr * SubWidthC, yCurr * SubHeightC )    (1234) 
               
               
                         The luma location (xCuCb, yCuCb ) is specified as the top-left luma sample location of the coding unit that contains the luma sample at ( xCurrY / sizeY * sizeY, yCurrY / sizeY * sizeY ) and xVir is (VirtualBoundariesPosX[ 0 ] / sizeY * sizeY) and yVir is (VirtualBoundariesPosY[ 0 ] / sizeY * sizeY). 
               
               
                         The variables availL and availT are derived as follows: 
               
               
                             The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the location (xCurr, yCurr ) set equal to ( xCuCb, yCuCb ), the neighbouring luma location ( xNbY, yNbY) set equal to (xCuCb - 1, yCuCb ), checkPredModeY set equal to FALSE, and cIdx set equal to 0 as inputs, and the output is assigned to availL. 
               
               
                             availL is set equal to FALSE if the following conditions are true 
               
               
                             WithinGdrPeriodFlag is equal to 1, 
               
               
                             ph_chroma_residual_scale_flag is equal to 1, 
               
               
                             slice_lmcs_enabled_flag is equal to 1, 
               
               
                             NumHorVirtualBoundaries &gt; 0 &amp;&amp; yCuCb &lt; VirtualBoundariesPosY[ 0 ] &lt; yCuCb + sizeY &amp;&amp; yVir &lt;= yCurrY &lt; VirtualBoundariesPosY[ 0 ]. 
               
               
                             The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the location (xCurr, yCurr ) set equal to ( xCuCb, yCuCb ), the neighbouring luma location ( xNbY, yNbY) set equal to ( xCuCb, yCuCb - 1 ), checkPredModeY set equal to FALSE, and cIdx set equal to 0 as inputs, and the output is assigned to availT. 
               
               
                             availL is set equal to FALSE if the following conditions are true 
               
               
                             WithinGdrPeriodFlag is equal to 1, 
               
               
                             ph_chroma_residual_scale_flag is equal to 1, 
               
               
                             NumHorVirtualBoundaries &gt; 0 &amp;&amp; yCuCb &lt; VirtualBoundariesPosY[ 0 ] &lt; yCuCb + sizeY &amp;&amp; yVir &lt;= yCurrY &lt; VirtualBoundariesPosY[ 0 ]. 
               
               
                     The variable currPic specifies the array of reconstructed luma samples in the current picture. 
               
               
                     For the derivation of the variable varScale the following ordered steps apply: 
               
               
                     4. The variable invAvgLuma is derived as follows: 
               
               
                         The array recLuma[ i ] with i=0..( 2 * sizeY - 1 ) and the variable cnt are derived as follows: 
               
               
                             The variable cnt is set equal to 0. 
               
               
                             When availL is equal to TRUE, the array recLuma[ i ] with i = 0..sizeY - 1 is set equal to currPic[ xCuCb - 1 ][ Min( yCuCb + i, pic_height_in_luma_samples - 1 ) ] with i = 0.. sizeY - 1, and cnt is set equal to sizeY. 
               
               
                             When availT is equal to TRUE, the array recLuma[ cnt + i ] with i = 0.. sizeY - 1 is set equal to currPic[ Min( xCuCb + i, pic_width_in_luma_samples - 1 ) ][ yCuCb - 1 ] with i = 0.. sizeY - 1, and cnt is set equal to ( cnt + sizeY ). 
               
               
                         The variable invAvgLuma is derived as follows: 
               
               
                             If cnt is greater than 0, the following applies: 
               
               
                                    invAvgLuma = Clip1             ∑             k = 0       cnt-1        recLuma     k     +       cnt   &gt;   &gt;   1                   &gt;   &gt;   Log2( cnt ))           (1235) 
               
               
                             Otherwise (cnt is equal to 0), the following applies: 
               
               
                             invAvgLuma = 1 &lt;&lt; (BitDepth - 1 )    (1236) 
               
               
                     5. The variable idxYInv is derived by invoking the identification of piece-wise function index process for a luma sample as specified in clause 8.8.2.3 with the variable lumaSample set equal to invAvgLuma as the input and idxYInv as the output. 
               
               
                     6. The variable varScale is derived as follows: 
               
               
                         varScale = ChromaScaleCoeff[ idxYInv ]    (1237) 
               
               
                     The reconstructed chroma picture sample array recSamples is derived as follows: 
               
               
                         If tuCbfChroma is equal to 1, the following applies: 
               
               
                             resSamples[ i ][ j ] = Clip3( -( 1 &lt;&lt; BitDepth ), ( 1 &lt;&lt; BitDepth ) - 1, resSamples[ i ][ j ] )    (1238) 
               
               
                             recSamples[ xCurr + i ][ yCurr + j ] = Clip1( predSamples[ i ][ j ] + Sign( resSamples[ i ][ j ]) * (( Abs( resSamples[ i ][ j ]) * varScale + (1 &lt;&lt; 10) ) &gt;&gt; 11))    (1239) 
               
               
                         Otherwise (tuCbfChroma is equal to 0), the following applies: 
               
               
                             recSamples[ xCurr + i ][ yCurr + j ] = Clip1(predSamples[ i ][ j ] )    (1240) 
               
            
           
         
       
     
     In another embodiment, for a CU in a clean area of a current CTU with a virtual boundary passing through, if one or more of its sizeY above (or sizeY left) reconstructed neighbouring pixels are in a dirty area, the reconstructed neighboring pixels in the dirty area may be padded from the reconstructed pixels in the clean area for chroma residual scaling of LMCS for the current CU. For example, in  FIG.  2   , since the above reconstructed neighboring pixels  210  for the current CU  205  are in a dirty area, they are padded from the reconstructed pixel 95. In this regard, as shown in  FIG.  7 A , at operation  701 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to access a current coding unit of a picture within a GDR period to be decoded, wherein the current coding unit is within a clean area. As described above, the picture within the GDR period may be included as part of a coded video sequence. At operation  702 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, configured to determine whether one or more pixels within a neighboring pixel area are within a dirty area. At operation  703 , the apparatus  300  includes means, such as the processing circuitry  32 , memory  34 , and/or the like, is configured to, in an instance in which one or more pixels within the neighboring pixel area are determined to be within the dirty area, pad the neighboring pixels in the dirty area from one or more reconstructed pixels in the clean area for a chroma residual scaling decoding process associated with luma mapping with chroma scaling. 
     These operations may be performed through changes in the decoding process. Table C describes changes for the section “picture reconstruction with luma dependent chroma residual scaling process for chroma samples” of the VVC specification in accordance with certain embodiments described herein.  FIG.  7 B  further highlights the specific changes to the section proposed herein. 
     
       
         
          TABLE C
           
               
             
               
                 8.7.5.3 Picture Reconstruction With Luma Dependent Chroma Residual Scaling Process for chroma samples 
               
             
            
               
                 Inputs to this process are: 
               
               
                     a chroma location ( xCurr, yCurr ) of the top-left chroma sample of the current chroma transform block relative to the top-left chroma sample of the current picture, 
               
               
                     a variable nCurrSw specifying the chroma transform block width, 
               
               
                     a variable nCurrSh specifying the chroma transform block height, 
               
               
                     a variable tuCbfChroma specifying the coded block flag of the current chroma transform block, 
               
               
                     an (nCurrSw)x(nCurrSh) array predSamples specifying the chroma prediction samples of the current block, 
               
               
                     an (nCurrSw)x(nCurrSh) array resSamples specifying the chroma residual samples of the current block, 
               
               
                 Output of this process is a reconstructed chroma picture sample array recSamples. 
               
               
                 The variable sizeY is set equal to Min( CtbSizeY, 64 ). 
               
               
                 The reconstructed chroma picture sample recSamples is derived as follows for 
               
               
                 i = 0..nCurrSw - 1, j = 0..nCurrSh- 1: 
               
               
                     If one or more of the following conditions are true, recSamples[ xCurr + i ][ yCurr + j ] is set equal to Clip1( predSamples[ i ][ j ] + resSamples[ i ][ j ] ): 
               
               
                         ph_chroma_residual_scale_flag is equal to 0. 
               
               
                         slice_lmcs_enabled_flag is equal to 0. 
               
               
                         nCurrSw * nCurrSh is less than or equal to 4. 
               
               
                         tu_cbf_cb [ xCurr ][ yCurr ] is equal to 0 and tu_cbf_cr [ xCurr ][ yCurr ] is equal to 0. 
               
               
                     Otherwise, the following applies: 
               
               
                         The current luma location ( xCurrY, yCurrY ) is derived as follows: 
               
               
                         (xCurrY, yCurrY ) = (xCurr * SubWidthC, yCurr * SubHeightC)    (1234) 
               
               
                     The luma location (xCuCb, yCuCb ) is specified as the top-left luma sample location of the coding unit that contains the luma sample at ( xCurrY / sizeY * sizeY, yCurrY / sizeY * sizeY ) and xVir is (VirtualBoundariesPosX[ 0 ] / sizeY * sizeY) and yVir is (VirtualBoundariesPosY[ 0 ] / sizeY * sizeY). 
               
               
                         The variables availL and availT are derived as follows: 
               
               
                             The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the location (xCurr, yCurr ) set equal to ( xCuCb, yCuCb ), the neighbouring luma location ( xNbY, yNbY) set equal to (xCuCb - 1, yCuCb ), checkPredModeY set equal to FALSE, and cIdx set equal to 0 as inputs, and the output is assigned to availL. 
               
               
                             The derivation process for neighbouring block availability as specified in clause 6.4.4 is invoked with the location (xCurr, yCurr ) set equal to ( xCuCb, yCuCb ), the neighbouring luma location ( xNbY, yNbY) set equal to ( xCuCb, yCuCb - 1 ), checkPredModeY set equal to FALSE, and cIdx set equal to 0 as inputs, and the output is assigned to availT. 
               
               
                     The variable currPic specifies the array of reconstructed luma samples in the current picture. 
               
               
                     For the derivation of the variable varScale the following ordered steps apply: 
               
               
                     7. The variable invAvgLuma is derived as follows: 
               
               
                         The array recLuma[ i ] with i=0..( 2 * sizeY - 1 ) and the variable cnt are derived as follows: 
               
               
                             The variable cnt is set equal to 0. 
               
               
                             When availL is equal to TRUE, 
               
               
                             If WithinGdrPeriodFlag is equal to 1, ph_chroma_residual_scale_flag is equal to 1, slice_lmcs_enabled_flag is equal to 1 and (NumHorVirtualBoundaries &gt; 0 &amp;&amp; yCuCb &lt; VirtualBoundariesPosY[ 0 ] &lt; yCuCb + sizeY &amp;&amp; yVir &lt;= yCurrY &lt; VirtualBoundariesPosY[ 0 ] ), 
               
               
                             the array recLuma[ cnt + i ] with i = 0..sizeY - 1 is set equal to currPic[ xCuCb - 1 ][ Min( yCuCb + i, VirtualBoundariesPosY[ 0 ] - 1) ] with i = 0..sizeY - 1, and cnt is set equal to ( cnt + sizeY ). 
               
               
                             Otherwise: 
               
               
                             the array recLuma[ i ] with i = 0.. sizeY - 1 is set equal to currPic[ xCuCb - 1 ][ Min( yCuCb + i, pic_height__in_luma_samples 1 ) ] with i = 0.. sizeY - 1, and cnt is set equal to sizeY. 
               
               
                         The variable invAvgLuma is derived as follows: 
               
               
                             If cnt is greater than 0, the following applies: 
               
               
                                     invAvgLuma = Clip1             ∑             k = 0       cnt-1        recLuma     k     +       cnt   &gt;   &gt;                   &gt;   &gt;   Log2( cnt ))             (1235) 
               
               
                             Otherwise (cnt is equal to 0), the following applies: 
               
               
                             invAvgLuma = 1 &lt;&lt; (BitDepth - 1 )      (1236) 
               
               
                     8. The variable idxYInv is derived by invoking the identification of piece-wise function index process for a luma sample as specified in clause 8.8.2.3 with the variable lumaSample set equal to invAvgLuma as the input and idxYInv as the output. 
               
               
                     9. The variable varScale is derived as follows: 
               
               
                         varScale = ChromaScaleCoeff[ idxYInv ]      (1237) 
               
               
                     The reconstructed chroma picture sample array recSamples is derived as follows: 
               
               
                         If tuCbfChroma is equal to 1, the following applies: 
               
               
                         resSamples[ i ][ j ] = Clip3( -( 1 &lt;&lt; BitDepth ), ( 1 &lt;&lt; BitDepth ) - 1, resSamples[ i ][ j ] )      (1238) 
               
               
                         recSamples[ xCurr + i ][ yCurr + j ] = Clip1( predSamples[ i ][ j ] + Sign( resSamples[ i ][ j ]) * (( Abs( resSamples[ i ][ j ]) * varScale + (1 &lt;&lt; 10) ) &gt;&gt; 11 ))      (1239) 
               
               
                         Otherwise (tuCbfChroma is equal to 0), the following applies: 
               
               
                         recSamples[ xCurr + i ][ yCurr + j ] = Clip1(predSamples[ i ][ j ] )      (1240) 
               
            
           
         
       
     
     In another embodiment, for a current CU in the clean area of a current CTU with a virtual boundary passing through, if one or more of its sizeY above (or sizeY left) reconstructed neighboring pixels are within the dirty area, they are treated as not available for chroma residual scaling of LMCS for the current CU. For example, referring back to  FIG.  2   , since the above reconstructed neighboring pixels  210  for the current CU  205  are within a dirty area, they are not used for chroma residual scaling of LMCS for the current CU  205 . 
     In another embodiment, any of the above-described embodiments involving decoding process changes can be applied for all virtual boundaries regardless of whether the picture is within a GDR period or not. The above-described embodiments involving decoding process changes hence apply by setting WithinGdrPeriodFlag equal to 1 unconditionally for the decoding process. 
     In another embodiment, any of the above-described embodiments involving decoding process changes can be applied for indicated virtual boundaries. An encoder may indicate that any of the above-described embodiments applies as if WithinGdrPeriodFlag were equal to 1 for the pictures within a GDR period (e.g., among pictures for which WithinGdrPeriodFlag is or would be equal to 1) and applies as if WithinGdrPeriodFlag were equal to 0 in pictures outside any GDR period (e.g., among pictures for which WithinGdrPeriodFlag is or would be equal to 0). However, an encoder may alternatively or additionally use the indications for purposes other than GDR; as one example, for projected 360° video with discontinuous projection faces, such as cube faces of a cubemap, bounded by virtual boundaries. An encoder indicates in a syntax structure, such as in a picture parameter set, and a decoder decodes from a syntax structure, such as from a picture parameter set, one or more syntax elements, such as a flag, controlling how any of the above-described embodiments involving decoding process changes applies. The syntax element(s), such as the flag, may collectively control all virtual boundaries within the scope of the syntax structure, or there may be separate syntax element(s) controlling one or more of the virtual boundaries separately. For example, any of the above-described embodiments may apply by setting WithinGdrPeriodFlag equal to the value of the flag in the syntax structure. For example, a ph_lmcs_over_virtual_boundaries_flag may be included in the picture parameter set syntax structure together with other syntax elements for virtual boundaries, e.g. as follows: 
     
       
         
          TABLE D
           
               
            
               
                     ... 
               
               
                     if( sps_virtual_boundaries_enabled_flag &amp;&amp; !sps_virtual_boundaries_present_flag) { 
               
               
                         ph_virtual_boundaries_present_flag        u(1) 
               
               
                         if( ph_virtual_boundaries_present_flag ) { 
               
               
                             ph_lmcs_over_virtual_boundaries_flag        u(1) 
               
               
                             ph_num_ver_virtual_boundaries        u(2) 
               
               
                             for( i = 0; i &lt; ph_num_ver_virtual_boundaries; i++ ) 
               
               
                                 ph_virtual_boundaries_pos_x[ i ]        u(13) 
               
               
                             ph_num_hor_virtual_boundaries        u(2) 
               
               
                             for( i = 0; i &lt; ph_num_hor_virtual_boundaries; i++ ) 
               
               
                                 ph_virtual_boundaries_pos_y[ i ]    u(13) 
               
               
                     } 
               
               
                     } 
               
               
                     ... 
               
            
           
         
       
     
     In another embodiment, any of the above-described embodiments involving decoding process changes can be applied for indicated virtual boundaries. An encoder may indicate that any of the above-described embodiments applies as if WithinGdrPeriodFlag were equal to 1 for the pictures within a GDR period (e.g., among pictures for which WithinGdrPeriodFlag is or would be equal to 1) and applies as if WithinGdrPeriodFlag were equal to 0 in pictures outside any GDR period (e.g., among pictures for which WithinGdrPeriodFlag is or would be equal to 0). However, an encoder may alternatively or additionally use the indications for purposes other than GDR; as one example, for projected 360° video with discontinuous projection faces, such as cube faces of a cubemap, bounded by virtual boundaries. An encoder indicates in a syntax structure, such as in a picture parameter set, and a decoder decodes from a syntax structure, such as from a picture parameter set, one or more syntax elements, such as a flag, controlling how any of the above-described embodiments involving decoding process changes applies. The syntax element(s), such as the flag, may collectively control all virtual boundaries within the scope of the syntax structure, or there may be separate syntax element(s) controlling one or more of the virtual boundaries separately. For example, any of the above-described embodiments may apply by setting WithinGdrPeriodFlag equal to the value of the flag in the syntax structure. For example, a ph_lmcs_over_virtual_boundaries_flag may be included in the picture parameter set syntax structure together with other syntax elements for virtual boundaries. For example, the ph_lmcs_over_virtual_boundaries_flag may be conditioned by a LMCS enabled flag, e.g., ph_lmcs_enabled_flag, as shown below in Table E. 
     
       
         
          TABLE E
           
               
               
             
               
                 Picture header structure syntax 
                   
               
             
            
               
                 picture_header_structure( ) { 
                 Descriptor 
               
               
                  gdr_or_irap_pic_flag 
                 u(1) 
               
               
                  if( gdr_or_irap_pic_flag ) 
                   
               
               
                   gdr_pic_flag 
                 u(1) 
               
               
                 ... 
                   
               
               
                  if( sps_lmcs_enabled_flag ) { 
                   
               
               
                   ph_lmcs_enabled_flag 
                 u(1) 
               
               
                   if( ph_lmcs_enabled_flag ) { 
                   
               
               
                   ph_lmcs_aps_id 
                 u(2) 
               
               
                   if( ChromaArrayType != 0 ) 
                   
               
               
                    ph_chroma_residual_scale_flag 
                 u(1) 
               
               
                  } 
                   
               
               
                 } 
                   
               
               
                 ... 
                   
               
               
                  if( sps_virtual_boundaries_enabled_flag &amp;&amp; ! sps_virtual_boundaries_present_flag ) { 
                   
               
               
                   ph_virtual_boundaries_present_flag 
                 u(1) 
               
               
                   if( ph_virtual_boundaries_present_flag ) { 
                   
               
               
                    if (ph_lmcs_enabled_flag) { 
                   
               
               
                    ph_lmcs_over_virtual_boundaries_flag 
                 u(1) 
               
               
                   ph_num_ver_virtual_boundaries 
                 u(2) 
               
               
                   for( i = 0; i &lt; ph_num_ver_virtual_boundaries; i++ ) 
                   
               
               
                    ph_virtual_boundaries_pos_x[ i ] 
                 u(13) 
               
               
                   ph_num_hor_virtual_boundaries 
                 u(2) 
               
               
                   for( i = 0; i &lt; ph_num_hor_virtual_boundaries; i++ ) 
                   
               
               
                    ph_virtual_boundaries_pos_y[ i ] 
                 u(13) 
               
               
                  } 
                   
               
               
                 } 
                   
               
            
           
         
       
     
     In another embodiment, any of the above-described embodiments involving decoding process changes can be applied for indicated virtual boundaries. An encoder may indicate that any of the above-described embodiments applies as if WithinGdrPeriodFlag were equal to 1 for the pictures within a GDR period (e.g., among pictures for which WithinGdrPeriodFlag is or would be equal to 1) and applies as if WithinGdrPeriodFlag were equal to 0 in pictures outside any GDR period (e.g., among pictures for which WithinGdrPeriodFlag is or would be equal to 0). However, an encoder may alternatively or additionally use the indications for purposes other than GDR; as one example, for projected 360° video with discontinuous projection faces, such as cube faces of a cubemap, bounded by virtual boundaries. An encoder indicates in a syntax structure, such as in a picture parameter set, and a decoder decodes from a syntax structure, such as from a picture parameter set, one or more syntax elements, such as a flag, controlling how any of the above-described embodiments involving decoding process changes applies. The syntax element(s), such as the flag, may collectively control all virtual boundaries within the scope of the syntax structure, or there may be separate syntax element(s) controlling one or more of the virtual boundaries separately. For example, any of the above-described embodiments may apply by setting WithinGdrPeriodFlag equal to the value of the flag in the syntax structure. For example, a ph_lmcs_over_virtual_boundaries_flag may be included in the picture parameter set syntax structure together with other syntax elements for virtual boundaries. For example, syntax elements for virtual boundary may be presented before syntax elements for LMCS, and the ph_lmcs_over_virtual_boundaries_flag may be conditioned by a virtual boundary present flag, e.g., ph_virtual_boundaries_present_flag, as shown below in Table F. 
     
       
         
          TABLE F
           
               
               
             
               
                 Picture header structure syntax 
                   
               
             
            
               
                 picture_header_structure( ) { 
                 Descriptor 
               
               
                  gdr_or_irap_pic_flag 
                 u(1) 
               
               
                  if( gdr_or_irap_pic_flag ) 
                   
               
               
                   gdr_pic_flag 
                 u(1) 
               
               
                 ... 
                   
               
               
                  if( sps_virtual_boundaries_enabled_flag &amp;&amp; !sps_virtual_boundaries_present_flag ) { 
                   
               
               
                   ph_virtual_boundaries_present_flag 
                 u(1) 
               
               
                   if( ph_virtual_boundaries_present_flag ) { 
                   
               
               
                    ph_num_ver_virtual_boundaries 
                 u(2) 
               
               
                    for( I = 0; i &lt; ph_num_ver_virtual_boundaries; i++ ) 
                   
               
               
                     ph_virtual_boundaries_pos_x[ i ] 
                 u(13) 
               
               
                    ph_num_hor_virtual_boundaries 
                 u(2) 
               
               
                    for( i = 0; i &lt; ph_num_hor_virtual_boundaries; i++ ) 
                   
               
               
                     ph_virtual_boundaries_pos_y[ i ] 
                 u(13) 
               
               
                   } 
                   
               
               
                  } 
                   
               
               
                 ... 
                   
               
               
                  if( sps_lmcs_enabled_flag) { 
                   
               
               
                   ph_lmcs_enabled_flag 
                 u(1) 
               
               
                   if( ph_lmcs_enabled_flag ) { 
                   
               
               
                    if( ph_virtual_boundaries_present_flag ) 
                   
               
               
                     ph_lmcs_over_virtual_boundaries_flag 
                 u(1) 
               
               
                   ph_lmcs_aps_id 
                 u(2) 
               
               
                   if( ChromaArrayType != 0) 
                   
               
               
                    ph_chroma_residual_scale_flag 
                 u(1) 
               
               
                  } 
                   
               
               
                 } 
                   
               
            
           
         
       
     
     In another example, a ph_lmcs_over_virtual_boundaries_flag may be included in the picture parameter set syntax structure together with other syntax elements for virtual boundaries conditioned by the ph_chroma_residual_scale_flag being equal to 1 as shown below in Table G. 
     
       
         
          TABLE G
           
               
               
             
               
                 Picture header structure syntax 
                   
               
             
            
               
                 picture _header_structure( ) { 
                 Descriptor 
               
               
                  gdr_or_irap_pic_flag 
                 u(1) 
               
               
                  if( gdr_or_irap_pic_flag ) 
                   
               
               
                   gdr_pic_flag 
                 u(1) 
               
               
                 ... 
                   
               
               
                  if( sps_virtual_boundaries_enabled_flag &amp;&amp; !sps_virtual_boundaries_present_flag) { 
                   
               
               
                   ph_virtual_boundaries_present_flag 
                 u(1) 
               
               
                   if( ph_virtual_boundaries_present_flag) { 
                   
               
               
                    ph_num_ver_virtual_boundaries 
                 u(2) 
               
               
                    for( i = 0; i &lt; ph_num_ver_virtual_boundaries; i++ ) 
                   
               
               
                     ph_virtual_boundaries_pos_x[ i ] 
                 u(13) 
               
               
                    ph_num_hor_virtual_boundaries 
                 u(2) 
               
               
                    for( i = 0; i &lt; ph_num_hor_virtual_boundaries; i++ ) 
                   
               
               
                     ph_virtual_boundaries_pos_y[ i ] 
                 u(13) 
               
               
                   } 
                   
               
               
                  } 
                   
               
               
                 ... 
                   
               
               
                  if( sps_lmcs_enabled_flag) { 
                   
               
               
                   ph_lmcs_enabled_flag 
                 u(1) 
               
               
                   if( ph_lmcs_enabled_flag ) { 
                   
               
               
                    ph_lmcs_aps_id 
                 u(2) 
               
               
                    if( ChromaArrayType != 0 ) { 
                   
               
               
                     ph _chroma_residual_scale_flag 
                 u(1) 
               
               
                     if( ph_chroma_residual_scale_flag &amp;&amp; ph_virtual_boundaries_present_flag) 
                   
               
               
                      ph_lmcs_over_virtual_boundaries_flag 
                 u(1) 
               
               
                   } 
                   
               
               
                  } 
                   
               
               
                 } 
                   
               
            
           
         
       
     
     As described above, a method, apparatus, and computer program product are disclosed for facilitating LMCS processes in accordance with pictures of a GDR period and providing solutions to compatibility issues with LMCS and GDR. Benefits of this design include reduced transmission time of pictures and decreased encoder to decoder delay. 
       FIGS.  4 A,  5 A,  6 A, and  7 A  illustrate flowcharts depicting methods according to a certain example embodiments. It will be understood that each block of the flowcharts and combination of blocks in the flowcharts may be implemented by various means, such as hardware, firmware, processor, circuitry, and/or other communication devices associated with execution of software including one or more computer program instructions. For example, one or more of the procedures described above may be embodied by computer program instructions. In this regard, the computer program instructions which embody the procedures described above may be stored by a memory device  34  of an apparatus employing an embodiment of the present disclosure and executed by a processor  32 . As will be appreciated, any such computer program instructions may be loaded onto a computer or other programmable apparatus (for example, hardware) to produce a machine, such that the resulting computer or other programmable apparatus implements the functions specified in the flowchart blocks. These computer program instructions may also be stored in a computer-readable memory that may direct a computer or other programmable apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture the execution of which implements the function specified in the flowchart blocks. The computer program instructions may also be loaded onto a computer or other programmable apparatus to cause a series of operations to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide operations for implementing the functions specified in the flowchart blocks. 
     Accordingly, blocks of the flowcharts support combinations of means for performing the specified functions and combinations of operations for performing the specified functions for performing the specified functions. It will also be understood that one or more blocks of the flowcharts, and combinations of blocks in the flowcharts, can be implemented by special purpose hardware-based computer systems which perform the specified functions, or combinations of special purpose hardware and computer instructions. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. 
     Moreover, although the foregoing descriptions and the associated drawings describe example embodiments in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.