PATENT DOCUMENT

Publication Number: US-12212776-B2
Application Number: US-202217692095-A
Country: US
Kind Code: B2

Title: Systems and methods for low resolution motion estimation searches

Abstract:
A video encoding system encodes source image data corresponding with an image includes a low resolution pipeline that receives the source image data corresponding with a first coding block in the image. The low resolution pipeline includes a low resolution motion estimation block programmed to generate a first downscaled coding block by downscaling resolution of the source image data corresponding with the first coding block. The first downscaled coding block comprises a first downscaled prediction block corresponding with a first prediction block in the first coding block. The low resolution pipeline may also perform several low resolution motion estimation searches to generate motion vector candidates. The video encoding system also includes a main pipeline that receives the source image data and determines encoding parameters to be used to encode the first coding block based at least partially on the motion vector candidates.

Claims:
What is claimed is: 
     
       1. A video encoding system configured to encode source image data corresponding with an image, comprising:
 a low resolution pipeline configured to receive the source image data corresponding with a first coding block in the image, wherein the low resolution pipeline comprises a low resolution motion estimation block programmed to:
 generate a first downscaled coding block by downscaling resolution of the source image data corresponding with the first coding block, wherein the first downscaled coding block comprises a first downscaled prediction block corresponding with a first prediction block in the first coding block; 
 perform a first plurality of low resolution motion estimation searches based on the first downscaled prediction block to determine a first plurality of downscaled reference samples and a first plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled reference samples; and 
 perform, based on at least a portion of the first plurality of motion vector candidates, a second plurality of low resolution motion estimation searches based on the first downscaled prediction block to determine a second plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled references samples, wherein the low resolution motion estimation block is programmed to perform the second plurality of low resolution motion estimation searches in a diagonal ordering with respect to relative locations of downscaled prediction blocks; and 
 
 a main pipeline configured to receive the source image data corresponding with the first coding block and to determine first encoding parameters to be used to encode the first coding block based at least partially on motion vector candidates in the first plurality of motion vector candidates and the second plurality of motion vector candidates. 
 
     
     
       2. The video encoding system of  claim 1 , wherein the low resolution motion estimation block is programmed to determine a first motion vector candidate of the second plurality of motion vector candidates associated with the first downscaled prediction block based on a portion of the first plurality of motion vector candidates corresponding to a set of the downscaled prediction blocks that border the first downscaled prediction block. 
     
     
       3. The video encoding system of  claim 2 , wherein the set of the downscaled prediction blocks comprises three or four downscaled prediction blocks. 
     
     
       4. The video encoding system of  claim 2 , wherein the set of the downscaled prediction blocks comprises eight downscaled prediction blocks. 
     
     
       5. The video encoding system of  claim 2 , wherein a second motion vector of the second plurality of motion vector candidates comprises a zero vector. 
     
     
       6. The video encoding system of  claim 2 , wherein the low resolution motion estimation block is programmed to perform a third plurality of low resolution motion estimation searches based on the first downscaled prediction block to determine a third plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled references samples. 
     
     
       7. The video encoding system of  claim 2 , wherein the first motion vector candidate comprises a motion vector of the first plurality of motion vector candidates having a horizontal component that has been adjusted based on a first pseudorandom value, vertical component that has been adjusted based on a second pseudorandom value, or both. 
     
     
       8. The video encoding system of  claim 1 , wherein the diagonal ordering comprises starting with a top-left downscaled prediction block of the downscaled prediction blocks and ending with a bottom-right downscaled prediction block of the downscaled prediction blocks. 
     
     
       9. The video encoding system of  claim 1 , wherein the diagonal ordering comprises starting with a bottom-right downscaled prediction block of the downscaled prediction blocks and ending with a top-left downscaled prediction block of the downscaled prediction blocks. 
     
     
       10. A method, comprising:
 receiving, via a low resolution pipeline, source image data corresponding to a first coding block in an image; 
 generating, via the low resolution pipeline, a first downscaled coding block by downscaling resolution of the source image data corresponding with the first coding block, wherein the first downscaled coding block comprises a first downscaled prediction block corresponding with a first prediction block in the first coding block; 
 performing, via the low resolution pipeline, a first plurality of low resolution motion estimation searches based on the first downscaled prediction block to determine a first plurality of downscaled reference samples and a first plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled reference samples; and 
 performing, via the low resolution pipeline and based on at least a portion of the first plurality of motion vector candidates, a second plurality of low resolution motion estimation searches based on the first downscaled prediction block to determine a second plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled references samples, wherein a first motion vector candidate of the second plurality of motion vector candidates associated with the first downscaled prediction block comprises a motion vector of the first plurality of motion vector candidates having at least one component adjusted based on a pseudorandom value. 
 
     
     
       11. The method of  claim 10 , comprising:
 receiving, via a main pipeline, the source image data corresponding with the first coding block; and 
 determining, via the main pipeline, first encoding parameters to be used to encode the first coding block based at least partially on motion vector candidates in the first plurality of motion vector candidates and the second plurality of motion vector candidates. 
 
     
     
       12. The method of  claim 11 , comprising, after performing the second plurality of low resolution motion estimation searches, performing, via the low resolution pipeline, a third plurality of low resolution motion estimation searches to determine a third plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled references samples. 
     
     
       13. The method of  claim 12 , comprising, after performing the third plurality of low resolution motion estimation searches, performing, via the low resolution pipeline, a fourth plurality of low resolution motion estimation searches to determine a fourth plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled references samples. 
     
     
       14. The method of  claim 13 , comprising:
 after performing the fourth plurality of low resolution motion estimation searches, performing, via the low resolution pipeline, a fifth plurality of low resolution motion estimation searches to determine a fifth plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled references samples; and 
 determining, via the main pipeline, the first encoding parameters based at least partially on a motion vector candidate in the fifth plurality of motion vector candidates. 
 
     
     
       15. An electronic device, comprising:
 an image data source configured to generate source image data corresponding with an image frame; 
 a low resolution pipeline configured to receive the source image data corresponding with a plurality of coding blocks in the image frame, wherein the low resolution pipeline comprises a low resolution motion estimation block programmed to:
 generate a plurality of downscaled coding blocks by downscaling resolution of the source image data, wherein the plurality of downscaled coding blocks comprises a plurality of prediction blocks; 
 perform a first plurality of low resolution motion estimation searches based on the plurality of prediction blocks to determine a first plurality of downscaled reference samples and a first plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled reference samples; 
 perform, based on at least a portion of the first plurality of motion vector candidates, a second plurality of low resolution motion estimation searches based on the plurality of prediction blocks to determine a second plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled references samples; and 
 in response to performing a search of the second plurality of low resolution motion estimation searches associated with a prediction block bordering a search window, compensate for the prediction block bordering the search window; and 
 
 a main pipeline configured to receive the source image data and determine first encoding parameters to be used to encode at least a portion of the plurality of coding blocks based on least partially on one or more of the first plurality of motion vector candidates. 
 
     
     
       16. The electronic device of  claim 15 , wherein the low resolution motion estimation block is programmed to perform, based on the second plurality of motion vector candidates, a third plurality of low resolution motion estimation searches to determine a third plurality of motion vector candidates each indicative of location of a corresponding one of the first plurality of downscaled reference samples. 
     
     
       17. The electronic device of  claim 16 , wherein the low resolution motion estimation block is programmed to determine a portion of the third plurality of motion vector candidates associated with a prediction block of the plurality of prediction blocks based on a portion of the second plurality of motion vector candidates corresponding to a portion of the plurality of prediction blocks that borders the prediction block. 
     
     
       18. The electronic device of  claim 17 , wherein the portion of the plurality of prediction blocks comprises three, four, or eight prediction blocks. 
     
     
       19. The electronic device of  claim 17 , wherein an amount of prediction blocks included in the portion of the plurality of prediction blocks is dependent upon a location of the prediction block within the search window. 
     
     
       20. The method of  claim 10 , wherein the at least one component comprises a horizontal component, a vertical component, or both.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 63/248,199, entitled “Systems and Methods for Low Resolution Motion Estimation Searches,” filed on Sep. 24, 2021, which is incorporated by reference herein in its entirety for all purposes. 
    
    
     BACKGROUND 
     The present disclosure generally relates to image processing, and, more particularly, to video encoding. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Electronic devices often use one or more electronic displays to present visual representations of information, for example, as text, still images, and/or video based on corresponding image data. Since image data may be received from another electronic device and/or stored in the electronic device, the image data may be encoded (e.g., compressed) to reduce size (e.g., number of bits) and, thus, resources (e.g., transmission bandwidth and/or memory addresses) used to transmit and/or store image data. To display image frames, the electronic device may decode encoded image data and instruct the electronic display to adjust luminance of its display pixels based on the decoded image data. 
     To facilitate encoding, prediction techniques may be used to indicate the image data by referencing other image data. For example, since successively displayed images (e.g., image frames) may be generally similar, inter (e.g., inter-frame) prediction techniques may be used to indicate image data (e.g., a prediction unit) corresponding with a first image frame by referencing image data (e.g., a reference sample) corresponding with a second image frame, which may be displayed before or after the first image frame. To facilitate identifying the reference sample, a motion vector may indicate position of a reference sample in the second image frame relative to position of a prediction unit in the first image frame. In other words, instead of directly compressing the image data, the image data may be encoded based at least in part on a motion vector used to indicate desired value of the image data. 
     In some instances, image data may be captured for real-time or near real-time display and/or transmission. For example, when an image sensor (e.g., digital camera) captures image data, an electronic display may shortly thereafter display image frames based on the captured image data. Additionally or alternatively, an electronic device may shortly thereafter transmit the image frames to another electronic device and/or a network. As such, the ability to display and/or transmit in real-time or near real-time may be based at least in part on efficiency with which the image data is encoded. However, for certain types of image data, such as image data associated with relatively higher resolution images, determining encoding parameters to utilize to encode image data may be computationally complex, for example, due to the amount of image data being analyzed. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     The present disclosure generally relates to processing techniques that may be utilized when performing image processing. For example, the techniques described herein may be utilized as part of a process for encoding source image data. In particular, the techniques described herein relate to scaling source image data prior to performing encoding operations such as determining encoding parameters. For instance, full-resolution image data and low-resolution image data may be derived from source image data. The full-resolution image data may be encoded based on encoding parameters that can be determined based on the low-resolution image data and the full-resolution image data. By utilizing scaled image data, memory bandwidth may be reduced. Additionally, as discussed below, portions of a video encoding system may utilize the low-resolution image data to perform low-resolution motion estimation techniques without downscaling image data (e.g., full-resolution image data). As also discussed below, multiple searches may be performed using downscaled (e.g., low-resolution image data), which may improve the accuracy of the techniques utilized to encode the source image data. Accordingly, the techniques described herein may enable video encoding systems to encode image data more efficiently. 
     A video encoding system may determine encoding parameters and implement the encoding parameters to encode the full-resolution image data that is generated from source image data. In some embodiments, the full-resolution image data may be encoded using prediction techniques (e.g., inter prediction techniques) by referencing other image data. For example, inter prediction techniques may facilitate encoding the full-resolution image data by referencing image data used to display other image frames. 
     The video encoding system may determine a reference sample in a second (e.g., reference) image frame for full-resolution image data corresponding with a first image frame using an inter prediction mode. The inter prediction mode may include a motion vector that indicates position (e.g., spatial position) of the reference sample in the second image frame relative to position of the source image data in the first image frame. Additionally, the inter prediction mode may include a reference index that indicates display order (e.g., temporal position) of the second image frame relative to the first image frame. 
     A motion estimation (ME) block in the video encoding system may determine one or more candidate inter prediction modes. The motion estimation block may perform a motion estimation search to determine reference samples that are similar to the full-resolution image data. Once a reference sample is determined, the motion estimation block may determine a motion vector and reference index to indicate location (e.g., spatial position and temporal position) of the reference sample relative to the full-resolution image data. Generally, performing motion estimation searches may be computationally complex and, thus, time-consuming. However, a duration provided for the motion estimation block to perform its search may be limited, particularly to enable real-time or near real-time transmission or display as refresh rate and/or resolution increases. 
     Accordingly, the present disclosure provides techniques to improve operational efficiency of the video encoding system. In some embodiments, operational efficiency may be improved by including a low resolution pipeline in parallel with a main encoding pipeline, which determines encoding parameters used to encode the full-resolution image data. Additionally, in some embodiments, the low resolution pipeline and the main encoding pipeline may both be provided access via direct memory access (DMA) to the full-resolution image data and low-resolution image data (derived from the source image data and) stored in memory. 
     Thus, in such embodiments, the low resolution pipeline and the main encoding pipeline may operate using relatively independent operational timing, which may enable the low resolution pipeline to operate one or more image frames ahead of the main encoding pipeline. In this manner, the low resolution pipeline may determine information ahead of time for use in the main encoding pipeline. By running the low resolution pipeline at least one image frame ahead of the main encoding pipeline, information (e.g., statistics and/or low resolution inter prediction modes) determined by the low resolution pipeline may be used by the main encoding pipeline, for example, to determine motion-weight (e.g., lambda) tuning information used in rate-distortion calculations, frame-rate conversion, image stabilization, and/or the like. 
     For example, the low resolution pipeline may include a low resolution motion estimation (LRME) block that processes the low-resolution image data to determine low resolution inter prediction modes. The low resolution motion estimation block may perform a motion estimation search on the low-resolution image data, which may be derived from full-resolution samples of image data used as references in the motion estimation search, to determine a downscaled reference sample that is similar to the downscaled source image data. To indicate location of the downscaled reference sample, the low resolution motion estimation block may determine a low resolution inter prediction mode, which includes a motion vector and a reference index. 
     Since downscaled image data (the low-resolution image data) should be similar to the full-resolution image data, low resolution inter prediction modes may provide an indication where reference samples in full resolution are expected to be located. Accordingly, the motion estimation block in the main encoding pipeline may be initialized with the low resolution inter prediction modes as candidates. In this manner, the low resolution motion estimation block may facilitate reducing amount of image data searched by the motion estimation block and, thus, improving operational efficiency of the video encoding system. To improve processing efficiency, the low resolution motion estimation block may prune the low resolution inter prediction modes before they are evaluated as candidate inter prediction modes by the main encoding pipeline, for example, to consolidate similar low resolution inter prediction modes and, thus, to enable the number of candidate inter prediction modes evaluated by the main encoding pipeline to be reduced. 
     Additionally, when the low resolution motion estimation block is operating one or more image frame ahead of the main encoding pipeline, the low resolution motion estimation block may determine statistics based at least in part on luma of the source image data. In some embodiments, the statistics may be indicative of global motion across multiple image frames and, thus, used for image stabilization. For example, the low resolution motion estimation block may determine a histogram statistic used to determine a best motion vector and, thus, a global motion vector determined based at least in part on the best motion vector. Based on the global motion statistics, the motion estimation block, which may be implemented in the main encoding pipeline, may determine a global motion vector indicative of motion across multiple image frames. Additionally, based on the global motion vector, the motion estimation block may adjust the candidate inter prediction modes considered, for example, by adjusting (e.g., offsetting) their motion vectors based at least in part on the global motion vector. Furthermore a search area in image data may be adjusted based on the global motion vector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG.  1    is a block diagram of an electronic device, in accordance with an embodiment; 
         FIG.  2    is an example of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  3    is another example of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  4    is another example of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  5    is another example of the electronic device of  FIG.  1   , in accordance with an embodiment; 
         FIG.  6    is block diagram of a portion of the electronic device of  FIG.  1    including a video encoding system, in accordance with an embodiment; 
         FIG.  7    is block diagram of a portion of the video encoding system of  FIG.  6    including a low resolution motion estimation block and a motion estimation block along with the image sensor and image pre-processing circuitry of  FIG.  1   , in accordance with an embodiment; 
         FIG.  8    is a flow diagram of a process for determining a candidate low resolution inter prediction mode, in accordance with an embodiment; and 
         FIG.  9    is diagrammatic representation of an image  118  divided into coding blocks and prediction blocks, in accordance with an embodiment; and 
         FIG.  10    is a diagrammatic representation of available and unavailable blocks when utilizing spatial candidates, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but may nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     An electronic device may facilitate visually presenting information by instructing an electronic display to display one or more images (e.g., image frames) based on corresponding image data. In some embodiments, the image data may be generated by an image sensor (e.g., digital camera) and stored in the electronic device. Additionally, when the image data is generated external from the electronic display, the image data may be transmitted to the electronic device. To reduce resource usage, image data may be encoded (e.g., compressed) to reduce size (e.g., number of bits) which, for example, may reduce transmission bandwidth and/or memory address usage. 
     A video encoding system may determine encoding parameters and implement the encoding parameters to encode source image data. To facilitate encoding, source image data for an image may be divided into one or more coding units. As used herein, a “coding unit” is intended to describe a sample of source image data (e.g., pixel image data) corresponding to a group of display pixels, which is encoded using the same prediction technique. However, it should be noted that “coding unit” may also refer to a sample of image data that is generated from source image data. For instance, as described herein, source image data may be scaled to generate different sets of image data (e.g., scaled image data). The sets of scaled image data, as discussed below, may include full-resolution image data and low-resolution image data. In such a case, a “coding unit” may be a sample of the full-resolution image data generated from source image data. 
     Accordingly, the video encoding system may determine a prediction technique (e.g., intra prediction technique or inter prediction technique) to be implemented to predict a coding unit, for example, as one or more prediction samples. Prediction techniques may facilitate encoding by enabling the source image data to be indicated via reference to other image data. For example, since an image frame may change gradually, the video encoding system may utilize intra prediction techniques to produce a prediction sample based on image data used to display the same image. Additionally, since successively displayed images may change gradually, the video encoding system may utilize inter prediction techniques to produce a prediction sample based on image data used to display other images. 
     Although conceptually similar, each prediction technique may include one or more prediction modes that utilize different encoding schemes. In other words, implementing different prediction modes may result in different prediction samples. For example, utilizing a first intra prediction mode (e.g., vertical prediction mode), the video encoding system may produce a prediction sample with each column set equal to image data for a pixel directly above the column. On the other hand, utilizing a second intra prediction mode (e.g., DC prediction mode), the video encoding system may produce a prediction sample set equal to an average of adjacent pixel image data. Additionally, utilizing a first inter prediction mode (e.g., first reference index and first motion vector), the video encoding system may produce a prediction sample based on a reference sample at a first position within a first image frame. Utilizing a second inter prediction mode (e.g., second reference index and second motion vector), however, the video encoding system may produce a prediction sample based on a reference sample at a second position within a second image frame. 
     Although using the same prediction technique, a coding unit may be predicted using one or more different prediction modes. As using herein, a “prediction unit” is intended to describe a sample within a coding unit that utilizes the same prediction mode. In some embodiments, a coding unit may include a single prediction unit. In other embodiments, the coding unit may be divided into multiple prediction units, which each uses a different prediction mode. 
     Accordingly, the video encoding system may evaluate candidate prediction modes (e.g., candidate inter prediction modes, candidate intra prediction modes, and/or a skip mode) to determine what prediction mode to use for each prediction unit in a coding unit. To facilitate, a motion estimation (ME) block in the video encoding system may determine one or more candidate inter prediction modes. In some embodiments, an inter prediction mode may include a reference index (e.g., temporal position), which indicates in which image a reference sample is located, and a motion vector (e.g., spatial position), which indicates the position of the reference sample relative to a prediction unit. 
     To determine a candidate inter prediction mode, the motion estimation block may search image data (e.g., reconstructed samples) used to display other image frames for reference samples that are similar to a prediction unit. Once a reference sample is determined, the motion estimation block may determine a motion vector and reference index to indicate location of the reference sample. 
     Generally, the quality of the match between prediction unit and reference sample may be dependent on search area (e.g., amount of image data). For example, increasing search area may improve likelihood of finding a closer match with the prediction unit. However, increasing search area may also increase computation complexity as well as increase memory bandwidth utilized to perform searches, which may cause increases in searching duration. In some embodiments, duration provided for the motion estimation block to perform its search may be limited, for example, to enable real-time or near real-time transmission and/or display. 
     Accordingly, as will be described in more detail below, the present disclosure provides techniques to improve operational efficiency of a video encoding system, for example, by enabling search area and/or candidate prediction modes evaluated by a main encoding pipeline to be adaptively (e.g., dynamically) adjusted based at least in part on processing performed by a low resolution pipeline. In some embodiments, operational efficiency may be improved by including a low resolution pipeline in parallel with the main encoding pipeline. Additionally, in some embodiments, the low resolution pipeline and the main encoding pipeline may both be provided access via direct memory access (DMA) to source image data stored in memory. 
     Thus, the low resolution pipeline and the main encoding pipeline may operate using relatively independent operational timing. In fact, the low resolution pipeline may operate one or more image frames ahead of the main encoding pipeline. In this manner, the low resolution pipeline may process image data ahead of time to determine information (e.g., low resolution inter prediction modes, luma histogram statistics, and/or sum of absolute difference statistics) to be used in the main encoding pipeline. 
     To facilitate determining the relevant information, the low resolution pipeline may include a low resolution motion estimation (LRME) block. In some embodiments, the low resolution motion estimation block may downscale source image data (e.g., a coding unit). For example, a low resolution motion estimation block may downscale a 32×32 coding unit to one-sixteenth resolution to generate an 8×8 downscaled coding unit. As also discussed herein, the low resolution motion estimation block may receive (e.g., via DMA access) scaled image data (e.g., downscaled image data) that is generated from source image data by other circuitry (e.g., image pre-processing circuitry) and stored in memory. In some cases, the resolution of the scaled image data may correspond to one-sixteenth of a resolution of other image data generated from source image data. For example, as discussed below, the image pre-processing circuitry may generate full-resolution image data and low-resolution image data from source image data. The low-resolution image data may have a resolution that is one-sixteenth of a resolution of the full-resolution image data. Accordingly, the low resolution motion estimation block may generate a downscaled coding unit without downscaling source image data. Rather, the low resolution motion estimation block may generate the downscaled coding unit using low-resolution image data generated by image pre-processing circuitry (e.g., by utilizing a portion of the downscaled source image data). By doing so, more resources (e.g., processing resources) of the low resolution motion estimation block may be utilized to perform motion estimation techniques. Furthermore, by generating full-resolution image data and low-resolution image data from source image data prior to performing motion estimation techniques and reading the low resolution image data (e.g., instead of the full-resolution image data) when performing low-resolution motion estimation techniques, the amount of memory bandwidth utilized to read image data may be reduced. 
     The low resolution motion estimation block may then search previously downscaled source image data to find (e.g., identify) a downscaled reference sample that is similar to a downscaled prediction unit within the downscaled coding unit. To indicate location of the downscaled reference sample, the low resolution motion estimation block may determine a low resolution inter prediction mode, which includes a motion vector and a reference index. More specifically, the motion vector may indicate spatial position of a reference sample in full resolution corresponding with the downscaled reference sample relative to a prediction unit in full resolution corresponding with the downscaled prediction unit. Additionally, the reference index may indicate display order (e.g., temporal position) of a reference image frame corresponding with the downscaled reference sample relative to an image frame corresponding with the downscaled prediction unit. 
     The low resolution motion estimation block may then enable the low resolution inter prediction mode to be accessed and used by the main encoding pipeline. In some embodiments, the low resolution motion estimation block may store the low resolution inter prediction mode in memory using direct memory access and the main encoding pipeline may retrieve the low resolution inter prediction mode using direct memory access. Additionally, the low resolution motion estimation block may store the downscaled source image data in memory for use in subsequent low resolution motion estimation searches. 
     In some embodiments, the motion estimation block in the main encoding pipeline may retrieve candidate inter prediction modes from memory. For each candidate inter prediction mode, the motion estimation block may perform a motion estimation search within a range of pixels (e.g., +/−3 pixel area) and/or sub-pixels (e.g., +/−0.5 pixel area) around its indicated reference sample in full resolution. Since downscaled image data should be similar to full resolution image data, low resolution inter prediction modes may provide an indication where closely matching reference samples are expected to be located. As such, the motion estimation block may utilize the low resolution inter prediction modes as candidates. In some embodiments, multiple passes of motion estimation searches (e.g., in the form of a recursive search) may be performed. In this manner, the low resolution motion estimation block may facilitate reducing amount of image data searched by the motion estimation block and, thus, searching duration, which may facilitate real-time or near real-time transmission and/or display of image data. 
     Additionally, when operating one or more image frames ahead of the main encoding pipeline, the low resolution motion estimation block may determine statistics used to improve operational efficiency of the main encoding pipeline. For example, the low resolution motion estimation block may determine luma histogram statistics that indicate number of pixels in downscaled image data at each luma value. Additionally or alternatively, the low resolution motion estimation block may determine a zero vector sum of absolute difference (SAD) statistics, which may indicate difference between a downscaled prediction unit and a downscaled reference sample indicated by a zero vector. In some embodiments, the statistics may be used to detect when a scene change is expected to occur. 
     As described above, inter prediction techniques are premised on successively displayed image frames being similar. Thus, effectiveness of inter prediction techniques across a scene change may be greatly reduced. As such, the main encoding pipeline may select a prediction mode from one or more candidate intra prediction modes and/or a skip mode. Thus, in some embodiments, the motion estimation block may be disabled, which may facilitate further reducing computational complexity, improving operational efficiency, and/or reducing power consumption of the main encoding pipeline and, thus, an electrical device in which it is implemented. 
     To help illustrate, an electronic device  10  (e.g., computing device) that may utilize an electronic display  12  to display image frames based on image data and/or an image sensor  13  to capture image data is described in  FIG.  1   . As will be described in more detail below, the electronic device  10  may be any suitable computing device, such as a handheld computing device, a tablet computing device, a notebook computer, and/or the like. Thus, it should be noted that  FIG.  1    is merely one example of a particular implementation and is intended to illustrate the types of components that may be present in the electronic device  10 . 
     The electronic device  10  includes the electronic display  12 , an image sensor  13 , one or more input structures  14  (e.g., input devices), one or more input/output (I/O) ports  16 , a processor core complex  18  having one or more processor(s) or processor cores, image pre-processing circuitry  19 , local memory  20 , a main memory storage device  22 , a network interface  24 , and a power source  26 . The various components described in  FIG.  1    may include hardware elements (e.g., circuitry), software elements (e.g., a tangible, non-transitory computer-readable medium storing instructions), or a combination of both hardware and software elements. It should be noted that the various depicted components may be combined into fewer components or separated into additional components. For example, the local memory  20  and the main memory storage device  22  may be included in a single component. 
     The processor core complex  18  image pre-processing circuitry  19  may execute instructions stored in local memory  20  and/or the main memory storage device  22  to perform certain image processing operations. For example, the processor core complex  18  may encode image data captured by the image sensor  13  and/or decode image data for display on the electronic display  12 . And, as discussed in greater detail below, the image pre-processing circuitry  19  may scale source image data (e.g., image data captured by the image sensor  13 ) to generate scaled image data that may be used to perform encoding operations. As such, the processor core complex  18  and image pre-processing circuitry  19  may include one or more general purpose microprocessors, one or more application specific processors (ASICs), one or more field programmable logic arrays (FPGAs), or any combination thereof. Additionally, in some embodiments, the image pre-processing circuitry  19  may be included in the processor core complex  18 . 
     The local memory  20  and/or the main memory storage device  22  may be tangible, non-transitory, computer-readable mediums that store instructions executable by and data to be processed by the processor core complex  18  and the image pre-processing circuitry  19 . For example, the local memory  20  may include random access memory (RAM) and the main memory storage device  22  may include read only memory (ROM), rewritable non-volatile memory such as flash memory, hard drives, optical discs, and the like. By way of example, a computer program product containing the instructions may include an operating system or an application program. 
     Using the network interface  24 , the electronic device  10  may communicatively couple to a network and/or other computing devices. For example, the network interface  24  may connect the electronic device  10  to a personal area network (PAN), such as a Bluetooth network, a local area network (LAN), such as an 802.11x Wi-Fi network, and/or a wide area network (WAN), such as a 4G or LTE cellular network. In this manner, the network interface  24  may enable the electronic device  10  to transmit encoded image data to a network and/or receive encoded image data from the network for display on the electronic display  12 . 
     The processor core complex  18  is operably coupled with I/O ports  16 , which may enable the electronic device  10  to interface with various other electronic devices. For example, a portable storage device may be connected to an I/O port  16 , thereby enabling the processor core complex  18  to communicate data with a portable storage device. In this manner, the I/O ports  16  may enable the electronic device  10  to output encoded image data to the portable storage device and/or receive encoded image data from the portable storage device. 
     The power source  26  may include any suitable source of energy, such as a rechargeable lithium polymer (Li-poly) battery and/or an alternating current (AC) power converter. Furthermore, as depicted, the processor core complex  18  is operably coupled with input structures  14 , which may enable a user to interact with the electronic device  10 . The input structures  14  may include buttons, keyboards, mice, trackpads, and/or the like. Additionally or alternatively, the electronic display  12  may include touch components that enable user inputs to the electronic device  10  by detecting occurrence and/or position of an object touching its screen (e.g., surface of the electronic display  12 ). 
     In addition to enabling user inputs, the electronic display  12  may present visual representations of information by display images (e.g., image frames), such as a graphical user interface (GUI) of an operating system, an application interface, a still image, or video content. As described above, the electronic display  12  may display an image based on corresponding image data. In some embodiments, the image data may be received from other electronic devices  10 , for example, via the network interface  24  and/or the I/O ports  16 . Additionally or alternatively, the image data may be generated by electronic device  10  using the image sensor  13 . In some embodiments, image sensor  13  may digitally capture visual representations of proximate physical features as image data. 
     As described above, the image data may be encoded (e.g., compressed), for example, by the electronic device  10  that generated the image data, to reduce number of memory addresses used to store and/or bandwidth used to transmit the image data. Once generated or received, the encoded image data may be stored in local memory  20 . Accordingly, to a display image corresponding with encoded image data, the processor core complex  18  or other image data processing circuitry may retrieve encoded image data from local memory  20 , decode the encoded image data, and instruct the electronic display  12  to display image frames based on the decoded image data. 
     The electronic device  10  may be any suitable electronic device. To help illustrate, one example of a handheld device  10 A is described in  FIG.  2   , which may be a portable phone, a media player, a personal data organizer, a handheld game platform, or any combination of such devices. For example, the handheld device  10 A may be a smart phone, such as any iPhone® model available from Apple Inc. The handheld device  10 A includes an enclosure  28 , which may protect interior components from physical damage and/or shields them from electromagnetic interference. The enclosure  28  may surround the electronic display  12 , which, in the depicted embodiment, displays a graphical user interface (GUI)  30  having an array of icons  32 . By way of example, when an icon  32  is selected either by an input structure  14  or a touch component of the electronic display  12 , an application program may launch. 
     Input structures  14  open through the enclosure  28 . As described above, the input structures  14  may enable user interaction with the handheld device  10 A. For example, the input structures  14  may activate or deactivate the handheld device  10 A, navigate a user interface to a home screen, navigate a user interface to a user-configurable application screen, activate a voice-recognition feature, provide volume control, and/or toggle between vibrate and ring modes. As depicted, I/O ports  16  also open through the enclosure  28 . In some embodiments, the I/O ports  16  may include an audio jack to connect to external devices. Furthermore, as depicted, the image sensor  13  opens through the enclosure  28 . In some embodiments, the image sensor  13  may include a digital camera that captures image data. 
     To further illustrate a suitable electronic device  10 , a tablet device  10 B is described in  FIG.  3   . For example, the tablet device  10 B may be any iPad® model available from Apple Inc. Additionally, in other embodiments, the electronic device  10  may take the form of a computer  10 C as described in  FIG.  4   . For example, the computer  10 C may be any Macbook® or iMac® model available from Apple Inc. Furthermore, in other embodiments, the electronic device  10  may take the form of a watch  10 D as described in  FIG.  5   . For example, the watch  10 D may be any Apple Watch® model available from Apple Inc. As depicted, the tablet device  10 B, the computer  10 C, and the watch  10 D may each also include an electronic display  12 , an image sensor  13 , input structures  14 , I/O ports  16 , an enclosure  28 , or any combination thereof. 
     As described above, source image data may be encoded (e.g., compressed) to reduce resource usage. Additionally, in some embodiments, the duration between generation of image data and display of a corresponding image based on the image data may be limited to enable real-time or near real-time display and/or transmission. For example, image data captured by the image sensor  13  may be displayed on the electronic display  12  with minimal delay to enable a user to determine physical features proximate the image sensor  13  in real-time or near real-time. Additionally, image data generated by the electronic device  10  (e.g., by the image sensor  13 ) may be transmitted (e.g., broadcast) to one or more other electronic devices  10  to enable a real-time or near real-time streaming. To enable real-time or near real-time transmission and/or display, duration available to encode image data may be limited—particularly as the resolution of images and/or refresh rates of electronic displays  12  increase. 
     An example of a portion of an electronic device  10 , which includes a video encoding system  34 , is shown in  FIG.  6   . The video encoding system  34  may be implemented via circuitry, for example, packaged as a system-on-chip (SoC). Additionally or alternatively, the video encoding system  34  may be included in the processor core complex  18 , a timing controller (TCON) in the electronic display  12 , one or more other processing units, other processing circuitry, or any combination thereof. 
     The video encoding system  34  may be communicatively coupled to a controller  40 . The controller  40  may generally control operation of the video encoding system  34 . Although depicted as a single controller  40 , in other embodiments, one or more separate controllers  40  may be used to control operation of the video encoding system  34 . Additionally, in some embodiments, the controller  40  may be implemented in the video encoding system  34 , for example, as a dedicated video encoding controller. 
     The controller  40  may include a controller processor  42  and controller memory  44 . In some embodiments, the controller processor  42  may execute instructions and/or process data stored in the controller memory  44  to control operation of the video encoding system  34 . In other embodiments, the controller processor  42  may be hardwired with instructions that control operation of the video encoding system  34 . Additionally, in some embodiments, the controller processor  42  may be included in the processor core complex  18  and/or separate processing circuitry (e.g., in the electronic display) and the controller memory  44  may be included in local memory  20 , main memory storage device  22 , and/or a separate, tangible, non-transitory computer-readable medium (e.g., in the electronic display). 
     The video encoding system  34  includes DMA circuitry  36 . In some embodiments, the DMA circuitry  36  may communicatively couple the video encoding system  34  to an image sensor, such as external memory that stores source image data, for example, generated by the image sensor  13  or received via the network interface  24  or the I/O ports  16 . 
     To facilitate generating encoded image data, the video encoding system  34  may include multiple parallel pipelines. For example, in the depicted embodiment, the video encoding system  34  includes a low resolution pipeline  46 , a main encoding pipeline  48 , and a transcode pipeline  50 . The main encoding pipeline  48  may encode source image data using prediction techniques (e.g., inter prediction techniques or intra prediction techniques), and the transcode pipeline  50  may subsequently entropy encode syntax elements that indicate encoding parameters (e.g., quantization coefficient, inter prediction mode, and/or intra prediction mode) used to prediction encode the image data. 
     To facilitate prediction encoding source image data, the main encoding pipeline  48  may perform various functions. To simplify discussion, the functions are divided between various blocks (e.g., circuitry or modules) in the main encoding pipeline  48 . In the depicted embodiment, the main encoding pipeline  48  includes a motion estimation block  52 , an inter prediction block  54 , an intra prediction block  56 , a mode decision block  58 , a reconstruction block  60 , and a filter block  64 . 
     The motion estimation block  52  is communicatively coupled to the DMA circuitry  36 . In this manner, the motion estimation block  52  may receive source image data via the DMA circuitry  36 , which may include a luma component (e.g., Y) and two chroma components (e.g., Cr and Cb). In some embodiments, the motion estimation block  52  may process one coding unit, including one luma coding block and two chroma coding blocks, at a time. As used herein a “luma coding block” is intended to describe the luma component of a coding unit and a “chroma coding block” is intended to describe a chroma component of a coding unit. 
     A luma coding block may be the same resolution as the coding unit. On the other hand, the chroma coding blocks may vary in resolution based on chroma sampling format. For example, using a 4:4:4 sampling format, the chroma coding blocks may be the same resolution as the coding unit. However, the chroma coding blocks may be half (e.g., half resolution in the horizontal direction) the resolution of the coding unit when a 4:2:2 sampling format is used and a quarter (e.g., half resolution in the horizontal direction and half resolution in the vertical direction) the resolution of the coding unit when a 4:2:0 sampling format is used. 
     As described above, a coding unit may include one or more prediction units, which may each be encoded using the same prediction technique, but different prediction modes. Each prediction unit may include one luma prediction block and two chroma prediction blocks. As used herein a “luma prediction block” is intended to describe the luma component of a prediction unit and a “chroma prediction block” is intended to describe a chroma component of the prediction unit. In some embodiments, the luma prediction block may be the same resolution as the prediction unit. On the other hand, similar to the chroma coding blocks, the chroma prediction blocks may vary in resolution based on chroma sampling format. 
     Based at least in part on the one or more luma prediction blocks, the motion estimation block  52  may determine candidate inter prediction modes that can be used to encode a prediction unit. An inter prediction mode may include a motion vector and a reference index to indicate location (e.g., spatial position and temporal position) of a reference sample relative to a prediction unit. More specifically, the reference index may indicate display order of a reference image frame corresponding with the reference sample relative to a current image frame corresponding with the prediction unit. Additionally, the motion vector may indicate position of the reference sample in the reference image frame relative to position of the prediction unit in the current image frame. 
     To determine a candidate inter prediction mode, the motion estimation block  52  may search reconstructed luma image data, which may be previously generated by the reconstruction block  60  and stored in internal memory  53  (e.g., reference memory) of the video encoding system  34 . For example, the motion estimation block  52  may determine a reference sample for a prediction unit by comparing its luma prediction block to the luma of reconstructed image data. In some embodiments, the motion estimation block  52  may determine how closely a prediction unit and a reference sample match based on a match metric. In some embodiments, the match metric may be the sum of absolute difference (SAD) between a luma prediction block of the prediction unit and luma of the reference sample. Additionally or alternatively, the match metric may be the sum of absolute transformed difference (SATD) between the luma prediction block and luma of the reference sample. When the match metric is above a match threshold, the motion estimation block  52  may determine that the reference sample and the prediction unit do not closely match. On the other hand, when the match metric is below the match threshold, the motion estimation block  52  may determine that the reference sample and the prediction unit are similar. 
     After a reference sample that sufficiently matches the prediction unit is determined, the motion estimation block  52  may determine location of the reference sample relative to the prediction unit. For example, the motion estimation block  52  may determine a reference index to indicate a reference image frame, which contains the reference sample, relative to a current image frame, which contains the prediction unit. Additionally, the motion estimation block  52  may determine a motion vector to indicate position of the reference sample in the reference frame relative to position of the prediction unit in the current frame. In some embodiments, the motion vector may be expressed as (mvX, mvY), where mvX is horizontal offset and mvY is a vertical offset between the prediction unit and the reference sample. The values of the horizontal and vertical offsets may also be referred to as x-components and y-components, respectively. 
     In this manner, the motion estimation block  52  may determine candidate inter prediction modes (e.g., reference index and motion vector) for one or more prediction units in the coding unit. The motion estimation block  52  may then input candidate inter prediction modes to the inter prediction block  54 . Based at least in part on the candidate inter prediction modes, the inter prediction block  54  may determine luma prediction samples (e.g., predictions of a prediction unit). 
     The inter prediction block  54  may determine a luma prediction sample by applying motion compensation to a reference sample indicated by a candidate inter prediction mode. For example, the inter prediction block  54  may apply motion compensation by determining luma of the reference sample at fractional (e.g., quarter or half) pixel positions. The inter prediction block  54  may then input the luma prediction sample and corresponding candidate inter prediction mode to the mode decision block  58  for consideration. In some embodiments, the inter prediction block  54  may sort the candidate inter prediction modes based on associated mode cost and input only a specific number to the mode decision block  58 . 
     The mode decision block  58  may also consider one or more candidate intra predictions modes and corresponding luma prediction samples output by the intra prediction block  56 . The main encoding pipeline  48  may be capable of implementing multiple (e.g., 13, 17, 25, 29, 35, 38, or 43) different intra prediction modes to generate luma prediction samples based on adjacent pixel image data. Thus, in some embodiments, the intra prediction block  56  may determine a candidate intra prediction mode and corresponding luma prediction sample for a prediction unit based at least in part on luma of reconstructed image data for adjacent (e.g., top, top right, left, or bottom left) pixels, which may be generated by the reconstruction block  60 . 
     For example, utilizing a vertical prediction mode, the intra prediction block  56  may set each column of a luma prediction sample equal to reconstructed luma of a pixel directly above the column. Additionally, utilizing a DC prediction mode, the intra prediction block  56  may set a luma prediction sample equal to an average of reconstructed luma of pixels adjacent the prediction sample. The intra prediction block  56  may then input candidate intra prediction modes and corresponding luma prediction samples to the mode decision block  58  for consideration. In some embodiments, the intra prediction block  56  may sort the candidate intra prediction modes based on associated mode cost and input only a specific number to the mode decision block  58 . 
     The mode decision block  58  may determine encoding parameters to be used to encode the source image data (e.g., a coding unit). In some embodiments, the encoding parameters for a coding unit may include prediction technique (e.g., intra prediction techniques or inter prediction techniques) for the coding unit, number of prediction units in the coding unit, size of the prediction units, prediction mode (e.g., intra prediction modes or inter prediction modes) for each of the prediction units, number of transform units in the coding unit, size of the transform units, whether to split the coding unit into smaller coding units, or any combination thereof. 
     To facilitate determining the encoding parameters, the mode decision block  58  may determine whether the image frame is an I-frame, a P-frame, or a B-frame. In I-frames, source image data is encoded only by referencing other image data used to display the same image frame. Accordingly, when the image frame is an I-frame, the mode decision block  58  may determine that each coding unit in the image frame may be prediction encoded using intra prediction techniques. 
     On the other hand, in a P-frame or B-frame, source image data may be encoded by referencing image data used to display the same image frame and/or a different image frames. More specifically, in a P-frame, source image data may be encoding by referencing image data associated with a previously coded or transmitted image frame. Additionally, in a B-frame, source image data may be encoded by referencing image data used to code two previous image frames. More specifically, with a B-frame, a prediction sample may be generated based on prediction samples from two previously coded frames; the two frames may be different from one another or the same as one another. Accordingly, when the image frame is a P-frame or a B-frame, the mode decision block  58  may determine that each coding unit in the image frame may be prediction encoded using either intra techniques or inter techniques. 
     Although using the same prediction technique, the configuration of luma prediction blocks in a coding unit may vary. For example, the coding unit may include a variable number of luma prediction blocks at variable locations within the coding unit, which each uses a different prediction mode. As used herein, a “prediction mode configuration” is intended to describe the number, size, location, and prediction mode of luma prediction blocks in a coding unit. Thus, the mode decision block  58  may determine a candidate inter prediction mode configuration using one or more of the candidate inter prediction modes received from the inter prediction block  54 . Additionally, the mode decision block  58  may determine a candidate intra prediction mode configuration using one or more of the candidate intra prediction modes received from the intra prediction block  56 . 
     Since a coding unit may utilize the same prediction technique, the mode decision block  58  may determine prediction technique for the coding unit by comparing rate-distortion metrics (e.g., costs) associated with the candidate prediction mode configurations and/or a skip mode. In some embodiments, the rate-distortion metric may be determined by summing a first product obtained by multiplying an estimated rate that indicates number of bits expected to be used to indicate encoding parameters and a first weighting factor for the estimated rate and a second product obtained by multiplying a distortion metric (e.g., sum of squared difference) resulting from the encoding parameters and a second weighting factor for the distortion metric. The first weighting factor may be a Lagrangian multiplier, and the first weighting factor may depend on a quantization parameter associated with image data being processed. 
     The distortion metric may indicate amount of distortion in decoded image data expected to be caused by implementing a prediction mode configuration. Accordingly, in some embodiments, the distortion metric may be a sum of squared difference (SSD) between a luma coding block (e.g., source image data) and reconstructed luma image data received from the reconstruction block  60 . Additionally or alternatively, the distortion metric may be a sum of absolute transformed difference (SATD) between the luma coding block and reconstructed luma image data received from the reconstruction block  60 . 
     In some embodiments, prediction residuals (e.g., differences between source image data and prediction sample) resulting in a coding unit may be transformed as one or more transform units. As used herein, a “transform unit” is intended to describe a sample within a coding unit that is transformed together. In some embodiments, a coding unit may include a single transform unit. In other embodiments, the coding unit may be divided into multiple transform units, which is each separately transformed. 
     Additionally, the estimated rate for an intra prediction mode configuration may include expected number of bits used to indicate intra prediction technique (e.g., coding unit overhead), expected number of bits used to indicate intra prediction mode, expected number of bits used to indicate a prediction residual (e.g., source image data—prediction sample), and expected number of bits used to indicate a transform unit split. On the other hand, the estimated rate for an inter prediction mode configuration may include expected number of bits used to indicate inter prediction technique, expected number of bits used to indicate a motion vector (e.g., motion vector difference), and expected number of bits used to indicate a transform unit split. Additionally, the estimated rate of the skip mode may include number of bits expected to be used to indicate the coding unit when prediction encoding is skipped. 
     In embodiments where the rate-distortion metric of equation (1) is used, the mode decision block  58  may select a prediction mode configuration or skip mode with the lowest associated rate-distortion metric for a coding unit. In this manner, the mode decision block  58  may determine encoding parameters for a coding unit, which may include prediction technique (e.g., intra prediction techniques or inter prediction techniques) for the coding unit, number of prediction units in the coding unit, size of the prediction units, prediction mode (e.g., intra prediction modes or inter prediction modes) for each of the prediction unit, number of transform units in the coding block, size of the transform units, whether to split the coding unit into smaller coding units, or any combination thereof. 
     To facilitate improving perceived image quality resulting from decoded image data, the main encoding pipeline  48  may then mirror decoding of encoded image data. To facilitate, the mode decision block  58  may output the encoding parameters and/or luma prediction samples to the reconstruction block  60 . Based on the encoding parameters and reconstructed image data associated with one or more adjacent blocks of image data, the reconstruction block  60  may reconstruct image data. 
     More specifically, the reconstruction block  60  may generate the luma component of reconstructed image data. In some embodiments, the reconstruction block  60  may generate reconstructed luma image data by subtracting the luma prediction sample from luma of the source image data to determine a luma prediction residual. The reconstruction block  60  may then divide the luma prediction residuals into luma transform blocks as determined by the mode decision block  58 , perform a forward transform and quantization on each of the luma transform blocks, and perform an inverse transform and quantization on each of the luma transform blocks to determine a reconstructed luma prediction residual. The reconstruction block  60  may then add the reconstructed luma prediction residual to the luma prediction sample to determine reconstructed luma image data. As described above, the reconstructed luma image data may then be fed back for use in other blocks in the main encoding pipeline  48 , for example, via storage in internal memory  53  of the main encoding pipeline  48 . Additionally, the reconstructed luma image data may be output to the filter block  62 . 
     The reconstruction block  60  may also generate both chroma components of reconstructed image data. In some embodiments, chroma reconstruction may be dependent on sampling format. For example, when luma and chroma are sampled at the same resolution (e.g., 4:4:4 sampling format), the reconstruction block  60  may utilize the same encoding parameters as used to reconstruct luma image data. In such embodiments, for each chroma component, the reconstruction block  60  may generate a chroma prediction sample by applying the prediction mode configuration determined by the mode decision block  58  to adjacent pixel image data. 
     The reconstruction block  60  may then subtract the chroma prediction sample from chroma of the source image data to determine a chroma prediction residual. Additionally, the reconstruction block  60  may divide the chroma prediction residual into chroma transform blocks as determined by the mode decision block  58 , perform a forward transform and quantization on each of the chroma transform blocks, and perform an inverse transform and quantization on each of the chroma transform blocks to determine a reconstructed chroma prediction residual. The chroma reconstruction block may then add the reconstructed chroma prediction residual to the chroma prediction sample to determine reconstructed chroma image data, which may be input to the filter block  64 . 
     However, in other embodiments, chroma sampling resolution may vary from luma sampling resolution, for example when a 4:2:2 or 4:2:0 sampling format is used. In such embodiments, encoding parameters determined by the mode decision block  58  may be scaled. For example, when the 4:2:2 sampling format is used, size of chroma prediction blocks may be scaled in half horizontally from the size of prediction units determined in the mode decision block  58 . Additionally, when the 4:2:0 sampling format is used, size of chroma prediction blocks may be scaled in half vertically and horizontally from the size of prediction units determined in the mode decision block  58 . In a similar manner, a motion vector determined by the mode decision block  58  may be scaled for use with chroma prediction blocks. 
     To improve quality of decoded image data, the filter block  62  may filter the reconstructed image data (e.g., reconstructed chroma image data and/or reconstructed luma image data). In some embodiments, the filter block  62  may perform deblocking and/or sample adaptive offset (SAO) functions. For example, the filter block  62  may perform deblocking on the reconstructed image data to reduce perceivability of blocking artifacts that may be introduced. Additionally, the filter block  62  may perform a sample adaptive offset function by adding offsets to portions of the reconstructed image data. 
     To enable decoding, encoding parameters used to generate encoded image data may be communicated to a decoding device. In some embodiments, the encoding parameters may include the encoding parameters determined by the mode decision block  58  (e.g., prediction unit configuration and/or transform unit configuration), encoding parameters used by the reconstruction block  60  (e.g., quantization coefficients), and encoding parameters used by the filter block  62 . To facilitate communication, the encoding parameters may be expressed as syntax elements. For example, a first syntax element may indicate a prediction mode (e.g., inter prediction mode or intra prediction mode), a second syntax element may indicate a quantization coefficient, a third syntax element may indicate configuration of prediction units, and a fourth syntax element may indicate configuration of transform units. 
     The transcode pipeline  50  may then convert a bin stream, which is representative of syntax elements generated by the main encoding pipeline  48 , to a bit stream with one or more syntax elements represented by a fractional number of bits. In some embodiments, the transcode pipeline  50  may compress bins from the bin stream into bits using arithmetic coding. To facilitate arithmetic coding, the transcode pipeline  50  may determine a context model for a bin, which indicates probability of the bin being a “1” or “0,” based on previous bins. Based on the probability of the bin, the transcode pipeline  50  may divide a range into two sub-ranges. The transcode pipeline  50  may then determine an encoded bit such that it falls within one of two sub-ranges to select the actual value of the bin. In this manner, multiple bins may be represented by a single bit, thereby improving encoding efficiency (e.g., reduction in size of source image data). After entropy encoding, the transcode pipeline  50 , may transmit the encoded image data to the output  38  for transmission, storage, and/or display. 
     Additionally, the video encoding system  34  may be communicatively coupled to an output. In this manner, the video encoding system  34  may output encoded (e.g., compressed) image data to such an output, for example, for storage and/or transmission. Thus, in some embodiments, the local memory  20 , the main memory storage device  22 , the network interface  24 , the I/O ports  16 , the controller memory  44 , or any combination thereof may serve as an output. 
     As described above, the duration provided for encoding image data may be limited, particularly to enable real-time or near real-time display and/or transmission. To improve operational efficiency (e.g., operating duration and/or power consumption) of the main encoding pipeline  48 , the low resolution pipeline  46  may include a scaler block  66  and a low resolution motion estimation (ME) block  68 . The scaler block  66  may receive image data and downscale the image data (e.g., a coding unit) to generate low-resolution image data. For example, the scaler block  66  may downscale a 32×32 coding unit to one-sixteenth resolution to generate an 8×8 downscaled coding unit. In other embodiments, such as embodiments in which pre-processing circuitry generates image data (e.g., low-resolution image data) from source image data, the low resolution pipeline may not include the scaler block  66 , or the scaler block  66  may not be utilized to downscale image data. 
     The low resolution motion estimation block  68  may improve operational efficiency by initializing the motion estimation block  52  with candidate inter prediction modes, which may facilitate reducing searches performed by the motion estimation block  52 . Additionally, the low resolution motion estimation block  68  may improve operational efficiency by generating global motion statistics that may be utilized by the motion estimation block  52  to determine a global motion vector. 
     To help illustrate, a portion  70  of the video encoding system  34 , which includes the low resolution motion estimation block  68  and the motion estimation block  52  coupled to external memory  72 , is shown in  FIG.  7    along with the image sensor  13 , and the image pre-processing circuitry  19 , and various types of image data (e.g., source image data  82 , full-resolution image data  84 , and low-resolution image data  86 ). In some embodiments, the external memory  72  may be a tangible, non-transitory, computer-readable medium accessible by the video encoding system  34 , for example, to store data and/or retrieve data, such as image data and/or statistics data. Accordingly, in some embodiments, the external memory  72  may be included in the controller memory  44 , the local memory  20 , or the main memory storage device  22 . In other embodiments, the external memory  72  may be a separate storage component dedicated to the video encoding system  34 . Furthermore, it should be noted that the image pre-processing circuitry  19  may be included in the video encoding system  34 . 
     The external memory  72  is communicatively coupled to the low resolution motion estimation block  68  and the motion estimation block  52  of the main encoding pipeline  48 . In some embodiments, the external memory  72  may provide direct memory access (DMA) that enables the low resolution motion estimation block  68  and the main encoding pipeline  48  to access the external memory  72  relative independently. Thus, in such embodiments, the low resolution motion estimation block  68  may process image frames in advance of the main encoding pipeline  48 , which may enable the low resolution motion estimation block  68  to determine information (e.g., low resolution candidate inter prediction modes and/or motion vector statistics) useful for the main encoding pipeline  48 , particularly the motion estimation block  52  and the mode decision block  58 . 
     For example, the low resolution motion estimation block  68  may analyze low resolution image data to determine one or more low resolution inter prediction mode candidates  74 , which may be analyzed as full resolution inter prediction mode candidates  80  by the motion estimation block  52 . To facilitate improving operational efficiency, in some embodiments, the low resolution motion estimation block  68  may prune the low resolution inter prediction mode candidates  74  before they are evaluated by the motion estimation block  52 , for example, to consolidate low resolution inter prediction mode candidates  74  that indicate similar motion vectors. 
     Additionally or alternatively, the low resolution motion estimation block  68  may determine global motion vector statistics  76  based at least in part on the low resolution inter prediction mode candidates  74 . In some embodiments, the global motion vector statistics  76  determined by the low resolution motion estimation block  68  may facilitate image stabilization. Additionally, in some embodiments, the low resolution motion estimation block  68  may determine similar portions of successively displayed images to determine trends in motion, for example, as a global motion vector. Based on the motion trends, successively displayed image frames may be stabilized. In this manner, the low resolution motion estimation block  68  may determine the global motion vector statistics  76  that are useful for improving operational efficiency of the main encoding pipeline  48  and, thus, may facilitate real-time or near real-time transmission and/or display of image data. 
     Furthermore, the low resolution inter prediction mode candidates  74  and global motion vector statistics  76  may be utilized by the motion estimation block  52  of the main encoding system  34  to determine a global motion vector  78  and full resolution inter prediction mode candidates  80 . In some embodiments, the global motion vector  78  may be indicative of motion trends across multiple image and, thus, may be used by the motion estimation block  52  to improve the evaluated full resolution inter prediction mode candidates  80 , for example, by offsetting a full resolution inter prediction mode candidate  80  to compensate for the motion trend. 
     Returning briefly to  FIG.  6   , the inter prediction block  54  may determine luma prediction samples by applying each of the full resolution inter prediction mode candidates  80 . Additionally, as described above, the mode decision block  58  may consider one or more candidate intra predictions modes and corresponding luma prediction samples to determine a candidate intra prediction mode and corresponding luma prediction sample for a prediction unit, which the reconstruction block  60  may use to generate reconstructed image data. 
     Continuing with the discussion of  FIG.  7   , to help describe an example of how image data may be encoded, the low resolution motion estimation block  68  and the motion estimation block  52  may perform several operations such as determining candidate low resolution inter prediction modes (e.g., via the low resolution motion estimation block  68 ), determining global motion vector statistics  76  based on the candidate low resolution inter prediction modes (e.g., via the low resolution motion estimation block  68 ), determining the global motion vector  78  (e.g., via the motion estimation block  52  based on the global motion vector statistics  76 ), and determining an inter prediction mode based on the global motion vector and the candidate low resolution inter prediction modes  80  (e.g., via the motion estimation block  52 ). Such operations may be implemented at least in part based on circuit connections formed (e.g., programmed) in the video encoding system  34 . Additionally or alternatively, these operations may be implemented at least in part by executing instructions stored in a tangible non-transitory computer-readable medium, such as the controller memory  44 , using processing circuitry, such as the controller processor  42 . Some image data encoding techniques are described in more detail in U.S. patent application Ser. No. 16/032,925, entitled “Global Motion Vector Video Encoding Systems and Methods,” which is hereby incorporated by reference in its entirety for all purposes. 
     Generally, the operations mentioned above could be performed utilized image data generated from the source image data  82 . For instance, as mentioned above, the low resolution motion estimation block  68  may generate downscaled image data from the source image data  82 . However, for certain types of source image data  82  (e.g., relatively higher resolution source image data), utilizing the low resolution motion estimation block  68  to scale source image data  82  may be burdensome (e.g., utilize high amounts of power and/or processing resources) and utilize relatively large amounts of the memory  72 . To enable more of the resources of the low resolution motion estimation block  68  to be utilized for encoding techniques (e.g., determining low resolution inter prediction mode candidates  74  and global motion vector statistics  76 ) and to reduce the amount of bandwidth of the memory  72  being utilized, the image pre-processing circuitry  19  may be utilized to generate image data (e.g., full-resolution image data  84  and low-resolution image data  86 ) from the source image data  82  that can be stored in the memory  72  and utilized by the low resolution motion estimation block  68  and the motion estimation block  52 . By doing so, the video encoding system  34  may be able to encode image data more quickly and efficiently. Some techniques for generating the full-resolution image data  84  and low-resolution image data  86  from the source image data  82  that can be stored in the memory  72  and utilized by the low resolution motion estimation block  68  and the motion estimation block  52  are described in more detail in U.S. patent application Ser. No. 17/020,750, entitled “Systems and Methods for Encoding Image Data,” which is hereby incorporated by reference in its entirety for all purposes. 
     As noted above, to encode image data, the low resolution motion estimation block  68  and the motion estimation block  52  may perform several operations such as determining candidate low resolution inter prediction modes (e.g., via the low resolution motion estimation block  68 ), determining global motion vector statistics  76  based on the candidate low resolution inter prediction modes (e.g., via the low resolution motion estimation block  68 ), determining the global motion vector  78  (e.g., via the motion estimation block  52  based on the global motion vector statistics  76 ), and determining an inter prediction mode based on the global motion vector and the low resolution inter prediction mode candidates  74  (e.g., via the motion estimation block  52 ). It should also be noted that the motion estimation block  52  may utilize low resolution inter prediction mode candidates  74  that the low resolution motion estimation block  68  may generate by performing a recursive search. 
     Before discussing the recursive search in the context of a process for determining low resolution inter prediction candidates (e.g., low resolution inter prediction mode candidates  74 ), it should be noted that the recursive search may be used in addition to another search (e.g., a first search). Alternatively, the searches may be considered a single search that includes multiple passes, with the recursive search corresponding to one or more passes that are performed after an initial pass of the search. In particular, the first pass may generate motion vectors or low resolution inter prediction mode candidates  74  that fit a particular criterion or criteria (e.g., minimized rate-distortion metrics), and the successive pass(es) corresponding to the recursive search may refine the output of the first pass to help generate a smoother motion field that may be more representative of true motion relative to the output of the first pass. Accordingly, by performing the recursive search discussed herein, the low resolution motion estimation block  68  may generate low resolution inter prediction mode candidates  74  that may be more accurate of motion in image data and improve the accuracy of the determination of full resolution inter prediction modes by the motion estimation block  52 . 
     Bearing this in mind,  FIG.  8    is a flow diagram of a process  100  for determining a candidate low resolution inter prediction mode. Generally, the process  100  includes determining a downscaled prediction block (process block  102 ), searching downscaled reference image data to identify a downscaled reference sample (process block  104 ), determining a low resolution motion vector based on location of the downscaled reference sample (process block  106 ), and determining a rate-match metric associated with the low resolution motion vector (process block  108 ), performing a recursive search (process block  114 ), determining a lower resolution motion vector based on the recursive search (process block  116 ), and determining a metric associated with the low resolution motion vector (process block  117 ). In some embodiments, the process  100  may be implemented at least in part based on circuit connections formed (e.g., programmed) in the video encoding system  34 . Additionally or alternatively, the process  100  may be implemented at least in part by executing instructions stored in a tangible non-transitory computer-readable medium, such as the controller memory  44 , using processing circuitry, such as the controller processor  42 . 
     Accordingly, in some embodiments, a controller  40  may instruct the low resolution motion estimation block  68  to determine a downscaled prediction block (process block  102 ). For example, the low resolution motion estimation block  68  may process a downscaled coding unit, such as a downscaled luma coding block. Additionally, as described above, a coding unit may include one or more prediction units, such as a luma prediction block. 
     To help illustrate, a diagrammatic representation of an image  118  divided into coding blocks and prediction blocks is shown in  FIG.  9   . In particular, the image  118  is divided into 2N×2N coding blocks  120 . For example, the 2N×2N coding blocks  120  may be 32×32 coding blocks. Additionally, as depicted, each 2N×2N coding block  120  is divided into one or more prediction blocks  122 . 
     In some embodiments, the prediction blocks  122  may be of various sizes or dimensions. For example, a first coding block  120 A may include a 2N×2N prediction block  122 A, a second coding block  120 B may include four N×N prediction blocks  122 B, a third coding block  120 C may include two 2N×N prediction blocks  120 C, and a fourth coding block  120 D may include two N×2N prediction blocks  120 D. In other words, when the 2N×2N coding blocks  120  are 32×32 coding blocks, the 2N×2N prediction block  122 A may be a 32×32 prediction block, the N×N prediction blocks  122 B may each be a 16×16 prediction block, the 2N×N prediction blocks  122 C may each be a 32×16 prediction block, and the N×2N prediction blocks  122 D may each be a 16×32 prediction block. 
     Additionally, as noted above, a low resolution motion estimation block  68  may downscale coding blocks and, thus, prediction blocks within the coding blocks. In some embodiments, the low resolution motion estimation block  68  may downscale (e.g., down sample or sub-sample) in a horizontal direction and/or a vertical direction. For example, when downscaled by a factor of four in both the horizontal direction and the vertical direction, a 32×32 (e.g., 2N×2N) coding block may result in an 8×8 downscaled coding block. Additionally, a 16×16 (e.g., N×N) prediction block may result in a 4×4 downscaled prediction block, a 32×16 (e.g., 2N×N) prediction block may result in an 8×4 downscaled prediction block, and a 16×32 (e.g., N×2N) prediction block may result in a 4×8 downscaled prediction block. In this manner, a low resolution motion estimation block  68  may determine one or more downscaled prediction blocks. 
     Returning to the process  100  of  FIG.  8   , the low resolution motion estimation block  68  may perform a first pass of a search on downscaled image data corresponding with a reference image to identify one or more downscaled reference samples, which may be used to predict the downscaled prediction block (process block  104 ). In some embodiments, the downscaled reference image data may be previously downscaled source image data, for example, corresponding to other image frames. In other words, the downscaled source image data corresponding with the downscaled prediction block may be searched when the low resolution motion estimation block  68  subsequently processes another image. 
     In any case, in some embodiments, the low resolution motion estimation block  68  may search the downscaled reference image data to determine one or more downscaled reference samples that are similar to luma of the downscaled prediction block. In some embodiments, the low resolution motion estimation block  68  may determine a degree of matching between a downscaled reference sample and the downscaled source image data corresponding with the downscaled prediction block. For example, the low resolution motion estimation block  68  may determine a match metric, such as sum of absolute difference (SAD) between luma of the downscaled prediction block and luma of the downscaled reference sample. 
     As described above, a coding unit may include one or more luma prediction blocks, which is each encoded using the same prediction technique. Additionally, as described above, a coding unit may utilize various prediction mode configurations (e.g., number, size, location, and/or prediction modes for the one or more luma prediction blocks). Thus, in such embodiments, the low resolution motion estimation block  68  may determine one or more downscaled reference samples for variously sized downscaled prediction blocks in a downscaled coding block. 
     After a downscaled reference sample is determined, the low resolution motion estimation block  68  may determine a motion vector (e.g., a low resolution motion vector) that indicates location of the downscaled reference sample relative to the downscaled prediction block (process block  106 ). As described above, a motion vector may indicate spatial position of a reference sample in the reference image frame relative to a prediction unit in the current image frame. Additionally, the reference sample may include blocks of image data that form a prediction block. Accordingly, in some embodiments, the low resolution motion estimation block  68  may determine a motion vector by determining a horizontal offset (e.g., mvX) and a vertical offset (e.g., mvY) between a prediction unit corresponding with the downscaled luma prediction block and a reference sample corresponding with a downscaled reference sample. In this manner, the low resolution motion estimation block  68  may determine one or more low resolution inter prediction mode (e.g., motion vector and reference index) candidates  74 . 
     Additionally, the low resolution motion estimation block  68  may determine a rate-match metric associated with one or more identified motion vectors (process block  108 ). In some embodiments, motion vector candidates may be sorted based on associated rate-match metrics (e.g., costs). In some embodiments, the rate-match metric may be determined as follows:
 
RM= C (rate)+ D (match)  (2)
 
where RM is the rate-match metric, rate is an estimated rate that indicates number of bits expected to be used to indicate a motion vector candidate (e.g., based at least in part on motion vector difference), match is a match metric (e.g., sum of absolute difference) associated with a reference sample identified by the motion vector candidate, C is a weighting factor for the estimated rate, and D is a weighting factor for the match metric. The weighting factor C may be a Lagrangian multiplier, and the weighting factor C may depend on a quantization parameter associated with image data being processed.
 
     The match metric may be indicative of matching degree between source image data and the reference sample identified by the motion vector candidate. As described above, in some embodiments, the match metric may be the sum of absolute difference (SAD) and/or the sum of absolute transformed difference (SATD) between a luma prediction block and luma of the reference sample and, thus, indicative of full resolution matching degree. Additionally or alternatively, the match metric may be the sum of absolute difference (SAD) and/or the sum of absolute transformed difference (SATD) between a downscaled luma prediction block and luma of a downscaled reference sample and, thus, indicative of downscaled matching degree. 
     Thus, in some embodiments, determining the one or more rate-match metrics may include determining one or more sum-of-absolute differences (process sub-block  110 ) and determining one or more expected bit rates of one or more motion vectors in the one or more inter prediction modes (process sub-block  112 ). For instance, the low resolution motion estimation block  68  may determine one or more sum-of-absolute differences between luma of downscaled source image data and luma of one or more downscaled prediction blocks  122  (process sub-block  110 ). Additionally, the low resolution motion estimation block  68  may determine estimated rate of one or more motion vectors in the one or more low resolution inter prediction modes (process sub-block  112 ). As described above, the estimated rate may include number of bits expected to be used to indicate the motion vector. Thus, the estimated rate may depend at least in part on how the motion vector is expected to be indicated. In some embodiments, the motion vector may be transmitted as a motion vector difference, which indicates change in horizontal offset and change in vertical offset from a previously transmitted motion vector. In such embodiments, the estimated rate of the motion vector may be the number of bits expected to be used to transmit the motion vector difference. 
     To help refine low resolution motion vectors (e.g., as determined at process block  106  based on the first pass of the search), the low resolution motion estimation block  68  may perform a recursive search (process block  108 ), which may be one or more additional passes of the search performed (e.g., after performing the first pass of the search at process block  104 ). For example, the recursive search may include one, two, three, four, or more passes of the search that are done in addition to the first pass performed at process block  104 . 
     Each pass of the recursive search may utilize the output of the preceding pass and forming motion vector candidates based on neighboring block results. In particular, each pass of the recursive search may progress in a top-to-bottom, left-to-right manner or a bottom-to-top, right-to-left manner. In the top-to-bottom, left-to-right manner, a pass may begin with a top-left block and progress down a leftmost column of blocks. After reaching the bottom block of the leftmost column, the search may continue from a top block of a second column that is adjacent (e.g., to the right) to the leftmost column in a similar manner. In the bottom-to-top, right-to-left manner, a pass may begin with a bottom-right block and progress up a rightmost column of blocks. After reaching the top block of the rightmost column, the search may continue from a bottom block of a second column that is adjacent (e.g., to the left) to the rightmost column in a similar manner. Each block may be an N×N block of downscaled image data. 
     For a given block being considered, the motion vectors from neighboring blocks (e.g., as considered during the first pass or the preceding pass) form a set of search vectors use for the block being considered. As discussed below, various types of candidates may be considered during the recursive search. In one embodiment, the recursive search includes four passes in which each pass utilizes one of the types of the candidates discussed below. Accordingly, the recursive search may utilize any combination of the searches that utilize any combination of the candidates discussed below. 
     A first type of candidate is called a “spatial candidate.” Spatial candidates are taken from a set of up to four possible neighbors, which are dependent upon the manner of traversal (e.g., top-to-bottom, left-to-right manner or bottom-to-top, right-to-left manner). For example, as illustrated in  FIG.  10   , in the top-to-bottom, left-to-right manner (as represented by block  140 A), for a current block  142 A being considered, a top-left block  144 A, top block  146 A, and left block  148 A will be available regardless of the position of the current block  142 A (e.g., relative to the search area). A bottom-left block  150 A will generally be available as well except in cases in which the current block  142 A is located on the bottom row of the search area. When searching in the left-most column of blocks, previous pass candidates (discussed below) may be used. Additionally, the unavailable neighboring blocks may include right block  152 A, bottom block  154 A, bottom-right block  156 A, and top right block  158 A. 
     When progressing in the bottom-to-top, right-to-left manner (as illustrated by block  140 B), with block  142 B being considered, right block  152 B, bottom block  154 B, and bottom-right block  156 B will be available, and top-right block  158 B may be available in most cases (e.g., except for when the block  142 B is in a top row or right-most column). When block  142 B is located in the right-most column, previous pass candidates may be used instead. As also illustrated, top-left block  144 B, top block  146 B, left block  148 B, and bottom-left block  150 B may be unavailable. In other modes of progressing through blocks, other techniques other than utilizing previous pass candidates may be used when a neighboring candidate is unavailable. For example, a different block may be used by offsetting a position indicating a neighboring candidate that is unavailable. If the offset still results in an invalid candidate, previous pass candidates may be utilized instead. 
     A second type of candidate is called a “previous pass candidate.” Previous pass candidates are taken from the previous pass (e.g., a previous pass of the recursive search). In embodiments in which pervious pass candidates utilize results from a previous pass of the recursive search, the first pass of the recursive search (e.g., a second pass of the overall search) may not utilize previous pass candidates. To account for instances in which a block currently being considered is located along a border of a search window, offset positions may be calculated so that a valid neighboring block exists for each possible position of a block currently under consideration. 
     A third type of candidate is called a “full search candidate.” Full search candidates include the output of the first pass, and in some cases motion vectors may be scaled (e.g., by a factor of two in both the horizontal and vertical direction). Because full search candidates are generated based on a previously conducted pass (i.e., the first pass), a valid neighboring candidate block exists in each neighboring position surrounding the block currently being evaluated. 
     A fourth type of candidate is called a “zero candidate,” which corresponds to a zero vector. In other words, the zero candidate corresponds to a (0, 0) motion vector. 
     When performing the passes of the recursive search, duplicate candidates may be removed from a list of candidates to be considered. Duplicate candidates may include candidates having a same horizontal portion of a motion vector, a same vertical portion of a motion vector, and a same metric value. Here, the metric value may be calculated by summing two submetrics, one in the horizontal direction and one in the vertical direction. The submetrics may be determined by determining the minimum absolute difference between the current block and the neighboring blocks for each horizontal and vertical motion vector. That is, a horizontal submetric may be determined by determining the minimum absolute difference between the current block and the neighboring block for each horizontal motion vector, and the vertical submetric may be determined by determining the minimum absolute difference between the current block and the neighboring block for each vertical motion vector. For such a determination, if the current frame being evaluated in on the edge of the frame, candidates that are outside of the frame may be excluded. In other words, only blocks that are available and inside of the frame may be utilized to determining the values of the submetrics. 
     In some cases, when accessing motion vectors, the motion vectors may be modified based on a random or pseudorandom number. For example, the horizontal value (e.g., an X value) and the vertical value (e.g., a Y value) of a motion vector may be modified based on a lookup table that defines an adjustment value (e.g., a value between −3 and 3, inclusive) for a position of a block within a search window. 
     Returning to  FIG.  8    and the discussion of the process  100 , the low resolution motion estimation block  68  may determine a motion vector (e.g., a low resolution motion vector) that indicates location of the downscaled reference sample relative to the downscaled prediction block (process block  116 ). As described above, a motion vector may indicate spatial position of a reference sample in the reference image frame relative to a prediction unit in the current image frame. Additionally, the reference sample may include blocks of image data that form a prediction block. Accordingly, in some embodiments, the low resolution motion estimation block  68  may determine a motion vector by determining a horizontal offset (e.g., mvX) and a vertical offset (e.g., mvY) between a prediction unit corresponding with the downscaled luma prediction block and a reference sample corresponding with a downscaled reference sample. In this manner, the low resolution motion estimation block  68  may determine one or more low resolution inter prediction mode (e.g., motion vector and reference index) candidates  74 . 
     Additionally, the low resolution motion estimation block  68  may determine a metric associated with one or more identified motion vectors (process block  117 ). In one embodiment, the metric may correspond to a SAD for a motion vector or the SAD added the metric value discussed above with respect to duplicate candidates (or a value generated based on the matric metric value discussed above). In some embodiments, motion vector candidates may be sorted based on associated the metric that is determined at process block  117 . 
     Utilizing the low resolution inter prediction candidates  74  and global motion vector statistics generated utilizing the low-resolution image data  86 , the motion estimation block  52  may encode the full-resolution image data  84  utilizing techniques discussed above. Accordingly, image data may be encoded by utilizing motion vectors generated from performing the recursive search. 
     Accordingly, the technical effects of the present disclosure include improving operational efficiency of a video encoding system used to encode (e.g., compress) source image data as well as improving the accuracy of motion estimation. Accordingly, video encoding systems utilizing the techniques described herein may have enhanced accuracy, efficiency, or both. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure. 
     It is well understood that the use of personally identifiable information should follow privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining the privacy of users. In particular, personally identifiable information data should be managed and handled so as to minimize risks of unintentional or unauthorized access or use, and the nature of authorized use should be clearly indicated to users.

Metadata:
Filing Date: 20220310
Publication Date: 20250128
Grant Date: 20250128
Priority Date: 20210924
Inventors: Chou, Jim C
RYGH, MARK P
PARK, JAE YOUNG
Assignee: APPLE INC
CPC Classifications: [{"code": "H04N19/31", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/43", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/176", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/139", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/52", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/147", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/159", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/105", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/132", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/132", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/176", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/159", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/52", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/139", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/105", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/56", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/147", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/43", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/56", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N19/53", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N19/147", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04N19/52", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/43", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/31", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/176", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/159", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/139", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/132", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/105", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N19/56", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 85719056