Event Video Sequence

Abstract

To facilitate power saving in an event-triggered camera.SOLUTION: In a monitoring camera 100, pre-event image frames are stored in raw format up to a time in which an event occurs. The power saving is achieved by deferring image processing for a pre-event video sequence. A post-event video sequence reduces the total power required by performing processing at a lower clock frequency.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to capturing and processing event-triggered video sequences. [Background technology]

[0002] Video cameras, such as surveillance or monitoring cameras, are currently used in many different applications, both indoors and outdoors. Larger image sensors and other technological advances have greatly improved the image quality of video from such cameras, but at the same time, the video stream size, or in other words, the bit rate, has also increased. Because both storage capacity and network bandwidth are limited resources, a common solution is to store or stream only the video that is triggered or initiated by an event, rather than all video.

[0003] The term event is used in its broadest sense herein, and thus an event can be anything that may justify the need for video documentation, and the term can encompass anything from a burglar alarm sounding when a door is forced open to a visit by a care professional in the patient's home.

[0004] Often it is of interest not only to have video available from the time an event starts, but also to record video showing the monitored area or scene for a period immediately prior to the start of the event. This is commonly solved by the use of a so-called pre-event buffer, in which video of a recent time interval is continuously recorded. When the event starts, the video recorded in the pre-event buffer is retrieved and included in a video clip showing the event. U.S. Patent No. 8,676,027 by the applicant of the present application discloses a method relating to such a pre-event buffer in a camera.

[0005] Surveillance cameras can be mounted in various locations and often have one cable connection to provide power and another network access, or both by one connection in the case of power over ethernet, PoE. However, recently, body worn cameras, often called body worn cameras, BWCs, have seen an increase in popularity. Body worn cameras are used, for example, by police officers or security guards to capture video and other data during patrols and incidents. Such cameras are typically battery powered. This means that the power available for body worn cameras is much more limited than for cameras mounted in fixed locations. Thus, measures and countermeasures that are perfectly effective when used in cabled cameras, for example cameras mounted on buildings, can be problematic for BWCs, because much greater attention to power consumption is required. One challenge is in the area of ​​event-triggered recording, where the use of a pre-event buffer requires the camera to constantly record video. This can quickly drain the battery, and therefore improvements in this area are important. Summary of the Invention

[0006] It is an object of the present invention to facilitate power conservation in cameras associated with event-triggered video.

[0007] According to a first aspect, a method for recording an event video sequence is presented, the event video sequence comprising: - a pre-event video sequence including a video showing the monitored scene for a predetermined period of time before the event is detected; and - a post-event video sequence containing video showing the monitored scene from the time of the event detection; Including, The method is: - capturing image frames successively; - continuously storing newly captured image frames in a pre-event buffer by appending the newly captured image frames to a set of pre-event image frames corresponding to a predetermined time period; - receiving a first signal indicating that an event is detected; - upon receiving the first signal, - interrupting the storage of newly captured image frames in the pre-event buffer; feeding the newly captured image frame to an image processing unit; performing image processing of the newly captured image frame in an image processing unit; storing the processed image frames in a memory as a post-event video sequence; - receiving a second signal indicating that the event has ended; - Upon receiving a second signal, - interrupting the feeding of newly captured image frames to the image processing unit; - feeding a set of image frames from a pre-event buffer to an image processing unit; performing image processing of the set of image frames in an image processing unit; storing the processed set of image frames in a memory as a pre-event video sequence; Includes.

[0008] By storing the set of image frames in the pre-event buffer in the pre-event buffer without prior processing in the image processing unit, a substantial reduction in power consumed by the camera during periods when no event occurs can be achieved while still allowing the pre-event video sequence to be produced when needed, providing valuable insight into operations occurring in the period leading up to the time the first signal is received. By prioritizing the processing of the post-event video sequence over the processing of the set of pre-event image frames, it is possible to stream the post-event portion of the event video sequence from the camera to be viewed by a user in real time or near real time. The pre-event video sequence, showing the scene for a limited period leading up to the first signal, can then be processed and viewed immediately after the second signal is received and the event ends.

[0009] In battery-powered cameras, postponing image processing of a set of images forming a pre-event video sequence until the time when it is actually needed prevents unnecessary draining of battery power. Thus, the battery will last longer and the camera may be used for a longer period without needing to be recharged. The method will also be useful in cameras where the total power draw needs to be kept within certain limits for other reasons, for example when the camera is powered from a Power over Ethernet PoE connection, and needs to stay within a given power class. A possible disadvantage is that a larger pre-event buffer is needed, since image frames take up more space in unprocessed (raw) format than in processed, especially encoded, format, but in situations where power consumption reduction is very important, the method will be an attractive option.

[0010] Another effect of deferring the processing and subsequent storage of the image in memory is that there will be fewer read / write operations performed to the memory, which in turn will extend the life of the memory.

[0011] Additionally, deferring processing means that the processing of image frames from the pre-event buffer may not need to be performed at full frame rate, i.e., the processing of each image frame can be allowed to take longer. This can allow more advanced image processing and encoding algorithms to be utilized without adding more processing power. Allowing the processing to occur at a rate slower than the full frame rate also means that the processing can be performed at a lower clock frequency, which reduces the total power required.

[0012] The method may further comprise the step of continuously discarding the oldest image frame from the pre-event buffer as a newly captured image frame is stored, or in other words a circular buffer (FIFO (: first-in-first-out)) memory may be used for the pre-event buffer, which provides an efficient option for storing the unprocessed set of image frames that constitute a possible pre-event video sequence.

[0013] The method may further include feeding a subset of image data of the newly captured image to the image processing unit prior to receiving the first signal. This subset of image data may be used, for example, for streaming to a remote user to give an overall view of the scene during an entire visit to the scene by the person holding the camera. The subset of image data may additionally or alternatively be stored in memory for later use.

[0014] The subset of image data can be generated by spatially subsampling the newly captured image, whereby an image frame having a reduced resolution compared to the newly captured image is fed to the image processing unit, and also or alternatively, the subset of image data can be generated by temporally subsampling the newly captured image frame, whereby a stream of image frames having a reduced frame rate compared to the frame rate of the newly captured image frames is fed to the image processing unit. Both alternatives provide a means of creating a subset having only a very limited amount of image data, while still allowing an overall appearance of the scene, that can be processed in the image processing without incurring major power usage.

[0015] The step of storing the newly captured image frames in the pre-event buffer may include compressing the image frames. In this way, a more efficient storage of the unprocessed set of image frames is achieved, allowing a smaller pre-event buffer to be used for the same amount of image frames stored in the pre-event buffer. The compression used in this step will be a power-efficient compression, such as run-length encoding or jpg encoding, which will not increase the power consumed by the camera while substantially reducing the size of the image frames.

[0016] The method may further include resuming continuous storage of newly captured image frames in the pre-event buffer after feeding the set of image frames from the pre-event buffer to the image processing unit, such that a new set of image frames becomes available which may then form the basis of a new pre-event video sequence.

[0017] The step of performing image processing may include one or more of the following: defective pixel correction, white balancing, demosaicing, matrixing, gamma correction, sharpening, noise filtering, scaling, and encoding.

[0018] Deferring image encoding until the time when the event occurs means that it will be possible to start both the pre-event video sequence and the post-event video sequence with an intra-coded frame (I-frame), which will then make it much more convenient to decode and view the event video sequence. In prior art solutions where the pre-event video sequence is encoded continuously, there is little opportunity to control the encoding to ensure that an I-frame will have been encoded at the start of the event video sequence, and when viewing a prior art event video sequence, there will be a need to go back to the start of the encoded group of pictures (GOP), which can sometimes be up to a minute when decoding and viewing the event video. In the present method, since it is possible to start encoding the pre-event video sequence with an I-frame, it will be possible to start decoding and viewing just at the start of the event video sequence.

[0019] The first signal may be received from an image analysis unit arranged to detect an event in the monitored scene based on the captured image frames. Examples of processes used by such an image analysis unit are video motion detection, object identification, foreground-background segmentation, and other methods for determining based on the video images that some type of event of interest has occurred in the scene.

[0020] Additionally or alternatively, the first signal can be received from an external event detection unit arranged to detect an event in the monitored scene. The term external should be interpreted as being outside the camera or working independently of the camera or being separated from the camera, such as being physically or logically separated from the camera. Such an external event detection unit can include a sensor for detecting that a weapon has been unholstered, or a motion sensor for detecting, for example, that a person holding the camera has fallen, been pushed, or started running. The external event detection unit can also include some kind of location measurement sensor, such as for receiving a GPS signal, to indicate that the camera has entered an area where the camera needs to record video. Other examples of events that can be detected by the external event detection unit are changes in GPS connectivity or connectivity to a communication network, such as connectivity to a wifi network or a cellular network.

[0021] The external event detection unit may also include a sensor capable of detecting motion or objects in the scene, such as a PIR sensor, radar, or lidar. In some cases, the external event detection unit may also be another camera. The other camera may be of the same type as the camera itself, or may have a completely different configuration. As an example, if the camera using the method presented herein is a body-worn camera, the other camera may be another similar body-worn camera, or the other camera may be a monitoring camera mounted for monitoring the scene, such as a thermal or visual camera. This camera may transmit a first signal to any body-worn cameras in the vicinity.

[0022] As another option, audio or sounds from the scene, for example, the sound of a gunshot or a loud scream, can be analyzed to find indications that an event is occurring that warrants video recording.

[0023] The second signal may be generated in response to the first signal no longer being received. This may be the case, for example, when a first signal is received from a motion sensor indicating that a person wearing the camera is running, and the sensor indicates that the person has stopped running.

[0024] Additionally or alternatively, the second signal can be generated in response to a timer reaching a predetermined value, the timer being started when the first signal is received. This can be useful in situations where it can be reliably assumed that any event that needs to be recorded on video will have a duration shorter than the predetermined value. The second signal can be generated by an external event detection unit based on some kind of data, or lack of such data, from any of the sensors used to generate the first signal.

[0025] The method may further include streaming the processed image frames of the post-event video sequence to a recipient upon receiving a first signal indicating that an event is detected. In this manner, a third party, such as a security operator, may be immediately notified that an event is occurring within the scene and may take appropriate action, such as sending backup to personnel within the scene. The video may be streamed to a remote server, for example, via a cellular network or other type of wireless network. The security operator may view the streamed images by a video management system connected to the remote server.

[0026] According to a second aspect of the present invention there is provided a monitoring camera configured to record an event video sequence, comprising: The event video sequence is - a pre-event video sequence including a video showing the monitored scene for a predetermined period of time before the event is detected; and - a post-event video sequence containing video showing the monitored scene from the time of the event detection; Including, The camera is an image capture unit configured to successively capture image frames; - a pre-event buffer configured to continuously store newly captured image frames by appending the newly captured image frames to a set of pre-event image frames corresponding to a predetermined time period; - an event unit configured to receive a first signal indicating that an event is detected and a second signal indicating that the event has ended; an image processing unit configured to process image frames; - a memory configured to store the processed image frames; Equipped with Upon receiving the first signal, - the camera is configured to interrupt storing a newly captured image frame in a pre-event buffer and to feed the newly captured image to an image processing unit; - the image processing unit is configured to perform image processing of the newly captured image frames and store the processed images in a memory as a post-event video sequence; Upon receiving the second signal, the camera is configured to interrupt the feeding of newly captured image frames to the image processing unit; the camera is configured to feed a set of image frames from a pre-event buffer to an image processing unit; - an image processing unit configured to perform image processing of the set of image frames; The camera is configured to store the processed set of image frames as a pre-event video sequence in a memory.

[0027] The above mentioned features of the method according to the first aspect also apply to this second aspect, where applicable, and to avoid undue repetition reference is made thereto.

[0028] Further scope of applicability of the present invention will become apparent from the detailed description set forth hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

[0029] Thus, it is understood that the present invention is not limited to the specific component parts of the described device or the specific actions of the described method, since such devices and methods may vary. It is also understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. It should be noted that, as used in this specification and the appended claims, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of an element, unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" can include several devices and the like. Furthermore, the terms "comprising", "including", "containing", and similar terms do not exclude other elements or steps.

[0030] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which presently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and to fully convey the scope of the invention to those skilled in the art. [Brief description of the drawings]

[0031] [Figure 1] FIG. [Diagram 2] FIG. 2 is a diagram showing a situation in which the camera of FIG. 1 is used. [Diagram 3] 1 is a flow chart illustrating a method according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] 1 shows a camera, such as a wearable or body-worn camera, BWC, 100. The camera 100 comprises an optical component 102 and an image capture unit 104, as is commonly known per se.

[0033] 1, the camera 100 also includes an image processing unit 106 and a memory 108 for storing processed images. The memory 108 may be provided in the form of an SD card or a flash memory. In the context of this application, the term image processing may encompass a wide variety of operations, some examples being defective pixel correction, white balancing, demosaicing, matrixing, gamma correction, sharpening, noise filtering, scaling, and encoding.

[0034] The camera also includes a pre-event buffer 112 configured to store the set of pre-event image frames received from the image capture unit 104. The pre-event buffer may be provided in the form of a memory adapted for temporary storage, such as a RAM, DRAM, or SRAM memory.

[0035] Typically, the pre-event buffer 112 is configured as a circular buffer or a first-in-first-out FIFO buffer, where a newly captured image fed from the image capture unit 104 to the pre-event buffer 112 will replace the oldest image already stored in the pre-event buffer 112. However, other configurations of the pre-event buffer 112 would be possible. Further refinements could also be made in the selection of which images to discard from the pre-event buffer. As an example, in a situation where HDR / WDR video is captured, it could be decided to discard all long exposure or all short exposure images. When creating a pre-event video sequence, for example, if a longer period with a lower frame rate would be considered useful, it could also be possible to discard all second images or all third images, etc.

[0036] The pre-event buffer 112 is disposed upstream of the image processing unit 106. Thus, the set of pre-event image frames stored in the pre-event buffer 112 has not been processed by the image processing unit 106, meaning that the set is typically larger than it would be after being processed by the image processing unit 106, as is the case in prior art solutions for producing a pre-event video sequence. However, as the inventors have recognized, in situations where conserving power is of great importance, adding more storage space to the pre-event buffer 112 to allow for storage of a set of raw or near-raw image frames from the image capture unit 104 is better than spending power unnecessarily processing image data that may later be discarded due to the lack of an event requiring the pre-event video sequence.

[0037] Optionally, a simple compression in compression unit 114, such as, for example, lossless run-length encoding using dedicated hardware blocks, or a form of jpeg compression using, for example, 4x4 or 8x8 blocks, which does not consume excessive amounts of power and can still reduce the size of the captured image frame by approximately 10 times, can be performed on the image frames before storing them in the pre-event buffer 112. The compression method selected is usually of a type that avoids time dependencies between images. A smaller pre-event buffer can be used to store the same amount of images.

[0038] Additionally or alternatively, limited image processing can be performed using processing steps that consume very little power before storing the image frame in the pre-event buffer 112. This is shown as moving such steps from the image processing unit 106 into a separate initial image processing unit 116, and may allow the pre-event image frame to be processed in the initial image processing unit 116 before being stored in the pre-event buffer 112. It may be noted that showing this as a separate unit is for illustrative purposes only, and that the image processing unit 106 could also be shown having an output that feeds the image to the pre-event buffer 112 after performing only the processing steps illustrated below.

[0039] Examples of steps that may be performed by the initial image processing unit 116 prior to storage in the pre-event buffer 112 are defective pixel correction and demosaicing, the latter also called color interpolation or reconstruction, i.e. the process of converting an image from Bayer values ​​to RGB values. The reason for performing these steps within the initial image processing unit 116 is that they consume a limited amount of power and because they are performed at the beginning of image processing.

[0040] Additionally, the Bayer to RGB conversion results in increased smoothness, which in turn makes the image more easily compressible, thereby allowing the optional compression unit 114 to increase the reduction in image size, which in turn allows for the use of a smaller pre-event buffer 112 to store a set of pre-event image frames, or allows for the storage of multiple pre-event image frames in the pre-event buffer 112, if so desired.

[0041] However, it is important to note that in order to consume as little power as possible for processing images that will presumably become uninteresting and be discarded, the pre-event image frames are stored in a largely unprocessed or raw format in the pre-event buffer 112. It should be noted that while the camera 100 is continuously capturing video of the scene, the images are not fed to the image processing unit 106 and memory 108. The captured image frames are typically only temporarily stored in the pre-event buffer 112, as explained above.

[0042] Thus, image frames are not sent to image processing unit 106 until an event occurs that requires video recording by camera 100. However, as discussed in more detail below in this text, there may be cases where a subset of image data from image capture unit 104 is delivered to image processing unit 106 during periods when no event is occurring.

[0043] 2, a typical situation is shown in which camera 100 is in use. Camera 100 is used to monitor a scene 200 and is worn by a person 202. Various activities may occur in scene 200, and at some point, person 202 may determine that an event is occurring that requires video recording to begin.

[0044] The person 202 will then activate a switch or press a button 204 provided on the camera 100 to enable video recording to begin. When the button 204 is pressed, a first signal 120 is sent to an event unit 118 provided within the camera 100. Pressing the button 204 is one of several examples where an external event detection unit signals to the camera 100, or more precisely, to the event unit 118, that an event requiring video recording has occurred, started, or is occurring.

[0045] Other examples of such external event detection units and detected events that may be the cause of the first signal 120 include pressure or motion sensors that typically signal that a weapon has been removed from a holster in a situation where the camera 100 is being held by a police officer, or sensors with accelerometers or gyros that signal that a certain type of motion is occurring that indicates that the person holding the camera 100 is moving in a certain way. One example of this is motion indicating that a fall has occurred, for example, that the camera (and person) has moved from a vertical position (standing) to a horizontal position (lying down). Another example is the sensor detecting motion indicating that the person holding the camera is being pushed or shoved.

[0046] Another example of an external event detection unit that can be used to provide the first signal 120 to the event unit 118 is a motion detector that detects motion in the scene. The motion detector that detects motion in the scene can be a PIR sensor, a depth of field sensor, a laser, a radar, or another type of device that can sense motion in the scene. A more advanced analysis of the scene can also be performed, which can provide the possibility to ignore motion below a threshold, repetitive motion, or motion from certain scene areas that are known to contain, for example, swaying trees. All of these are examples that can cause an external event detection unit to send the first signal 120 to the event unit 118.

[0047] Another example of an external event detection unit generating the first signal would be an audio analysis unit configured to capture and analyze sounds or audio in the scene, such as a gunshot or a scream indicating an event is occurring, and then transmit the first signal 120 in response.

[0048] Another example of an external event detection unit is a positioning unit that uses a GPS signal or some other positioning signal to determine that the camera has reached a location or area where video needs to be recorded. In response to such a determination, the external event detection unit transmits a first signal 120 to the event unit 118. A changed connection state to a gps network or another type of positioning network can also cause the first signal 120 to be transmitted.

[0049] Another example of an event that may be detected by the external event detection unit and cause the first signal 120 to be sent to the event unit 118 is a change in connection to a communication network, such as a connection to a wifi network or a mobile phone network.

[0050] Yet another example of how the first signal can be achieved is to use the image analysis unit 122 to analyze the captured image frames to find specific content within the scene, such as motion with certain characteristics or objects that meet certain pre-defined criteria.

[0051] When the event unit 118 receives the first signal 120, the image capture unit 104 will stop feeding image frames to the pre-event buffer 112 and newly captured image frames will instead be fed from the image capture unit to the image processing unit 106 which performs image processing on the image frames. The processed image frames from the image processing unit 106 are stored in memory 108 as a post-event video sequence.

[0052] At some point after the first signal is received by the event unit 118, a second signal 124 will be received by the event unit 118. The second signal 124 indicates that the event requiring video recording has stopped, or in some cases is likely to have stopped. The second signal can be generated by the person 202 pressing the button 204 again to indicate that the video recording may stop. A second button can also be provided on the camera 100 to generate the second signal.

[0053] Another option for generating the second signal 124 is from a timer, which starts when the first signal 120 is received and generates the second signal 124 after a predefined time has elapsed or after an input from a sensor included in the external event detection unit or after loss of that input. For example, the second signal 124 can be generated by a sensor indicating that the weapon is reinserted into the holster or a sensor indicating that movement has stopped. Additionally, just as the first signal is generated, image frames can be analyzed by the image analysis unit 122 to determine that the event that triggered the video recording has now stopped, triggering the generation of the second signal.

[0054] Other types of inputs can also be used to generate the second signal, such as from a sensor indicating that the person holding the camera has left a certain area or position, that a sound has stopped, or that movement in the scene has stopped or an object has left the scene.

[0055] As discussed above, there is often a need to understand what has happened even before the first signal 120 is generated, i.e., a pre-event video sequence is needed. Thus, upon receipt of the second signal 124, the feeding of image frames from the image capture unit 104 to the image processing unit 106 will be stopped, and instead, the set of image frames that were waiting in the pre-event buffer 112 while the image processing unit 106 was receiving the image frames from the image capture unit 104 will now be fed to the image processing unit 106. There, the set of images will be processed, and then the processed set of images will be added to the memory 108 as a pre-event video sequence to the post-event video sequence previously stored in the memory 108. Furthermore, the image capture unit 104 will resume feeding the newly captured images to the pre-event buffer 112, which is now ready to start receiving images again.

[0056] It may be noted that the set of image frames from the pre-event buffer may be processed at a rate slower than the full frame rate, i.e., more time may be available to process each image frame than is allowed when processing live video. Thus, more advanced image processing may be used, or the image processing unit 106 may be configured to use less power to process each image frame, for example, by being configured to run at a lower clock frequency. Processing the set of image frames at a rate slower than the full frame rate, advantageously combined with using a larger pre-event buffer, allows for storage of possible new pre-event image frames while the set of image frames is being processed.

[0057] 3, a method 300 of recording an event video sequence is shown. At step 302, image frames are captured by image capture unit 104. At optional step 304, the image frames are compressed before being stored in pre-event buffer 112 at step 306. At some point, a first signal 120 indicating that an event will occur is received at step 308. Image capture unit 104 then stops sending image frames to pre-event buffer 112 for storage, and the image frames are instead sent to image processing unit 106 for processing and to memory 108 for storage to produce a post-event video sequence at step 310.

[0058] As an optional measure in this step, the processed images can also be streamed to a remote recipient in the form of a server providing a logged-in operator with immediate access to the video captured by the camera 100 after receiving the first signal 120, and the operator determines whether any action is required, such as sending help to the person 202 holding the camera 100.

[0059] A second signal 124 is received indicating the end of the event at step 312. A pre-event video sequence is then created at step 314 by feeding the set of image frames stored in the pre-event buffer 112 to the image processing unit 106 for processing, and by feeding the set of processed images to the memory 108 for storage as the pre-event video sequence. If the image frames were compressed in connection with being stored in the pre-event buffer 112, appropriate decompression is performed prior to processing in the image processing unit 106. The post-event video sequence and the pre-event video sequence together form an event video sequence at step 316, and the event video sequence is stored in the memory 108.

[0060] It should be noted that step 302 occurs continuously during operation of the camera and is only presented as a first step for illustration purposes. Similarly, upon receipt of a second signal, when the contents of the pre-event buffer 112 are fed to the image processing unit, the image capture unit 104 will once again feed the image frames to the pre-event buffer 112 which, after optional compression of the images, will store the image frames as a new set of image frames in preparation for the next event occurring and a new pre-event video sequence being created.

[0061] As mentioned above, in a variation of the solution presented in this text, a subset of image data can be fed from the image capture unit 104 to the image processing unit 106 during periods when image frames are fed to the pre-event buffer 112, i.e., periods when no event has been indicated to occur. This subset of image data can be used, for example, for streaming from the camera 100. The subset of image data can be provided by spatially subsampling the captured image frames to provide lower resolution image frames, or by temporally subsampling the captured image frames to provide a reduced frame rate, or both. The processed subset of image data can also be stored in memory 108 and used in creating the event video sequence.

[0062] The storage of the processed subset of image data can be done in a manner similar to how images are stored in the pre-event buffer 112, i.e., old images can be discarded after a certain period of time or after a certain amount of new images are added to the memory. The camera can be set up to feed only image data from the image capture unit 104 to the pre-event buffer 112 that was not included in the temporally or spatially subsampled image data used for streaming. Once the pre-event video sequence has been created, the image processing unit 106 will then combine the set of image frames from the pre-event buffer 112 with the previously processed subset of image data to provide the complete pre-event video sequence.

[0063] It may be noted that the different units presented in the above text, e.g., the image processing unit 106, the compression unit 114, the initial image processing unit 116, the event unit 118, and the image analysis unit 122, can each be implemented in hardware, software, or any combination thereof. One or more of the units may be provided in the form of dedicated circuitry configured to perform the functions of the respective unit, or as a set of software program instructions executed on a standard processing unit and implemented to perform the functions of the respective unit.

[0064] Finally, it should be stated that those skilled in the art can understand and effect other variations to the disclosed embodiments when practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0065] An example of such a variation is that although the invention has been described in relation to a body-worn camera, it could also be used in cameras mounted on buildings or in vehicles, for example. If the camera has a limited amount of power available, or there are other reasons for wanting to conserve power in relation to event video recording in situations where it is possible to provide a more generous amount of memory space in the pre-event buffer, the solutions described herein can be utilized to enable lower power consumption during periods when no events are occurring.

[0066] In summary, a method is presented for providing an event-triggered video sequence including a pre-event sequence and a post-event sequence. The pre-event image frames are stored in a raw format until the time the event occurs. By deferring image processing of the pre-event video sequence, power savings are achieved.

Claims

1. A method (300) for recording an event video sequence, the event video sequence comprising: a pre-event video sequence showing the monitored scene (200) for a predetermined period of time before the event detection, and a post-event video sequence showing said monitored scene from the time of event detection; Including, The method comprises: - capturing (302) successive image frames; - storing (304) a newly captured image frame in a pre-event buffer (112) consecutively by appending the newly captured image frame to a set of pre-event image frames corresponding to the predetermined period, wherein the newly captured image frames stored in the pre-event buffer (112) have not yet been subjected to image processing in an image processing unit (106) consisting of one or more of the following: defective pixel correction, white balancing, demosaicing, matrixing, gamma correction, sharpening, noise filtering, scaling and encoding; receiving (308) a first signal (120) indicating that an event is detected; upon receiving said first signal, interrupting the storage of newly captured image frames in the pre-event buffer; feeding the newly captured image frame to said image processing unit (106); - performing image processing of said newly captured image frame in said image processing unit; storing (310) the processed image frames as the post-event video sequence in memory (108); receiving (312) a second signal (124) indicating that said event has ended; upon receiving said second signal, interrupting the feeding of newly captured image frames to the image processing unit; - feeding said set of image frames from said pre-event buffer to said image processing unit; performing image processing of said set of image frames in said image processing unit; o storing (314) the processed set of image frames in the memory as the pre-event video sequence; Including, A method (300), wherein said step of performing image processing comprises one or more of the following: defective pixel correction, white balancing, demosaicing, matrixing, gamma correction, sharpening, noise filtering, scaling, and encoding.

2. The method of claim 1 , further comprising the step of continuously discarding the oldest image frame from the pre-event buffer as a newly captured image frame is stored.

3. The method of claim 1 , further comprising: prior to receiving the first signal, sending a subset of image data of the newly captured image to the image processing unit.

4. 4. The method of claim 3, wherein the subset of image data is generated by spatially subsampling the newly captured image, whereby an image frame having a reduced resolution compared to the newly captured image is delivered to the image processing unit.

5. 5. The method of claim 3 or 4, wherein the subset of image data is generated by temporally subsampling the newly captured image frames, whereby a stream of image frames having a reduced frame rate compared to a frame rate of the newly captured images is fed to the image processing unit.

6. 6. The method of claim 1, further comprising resuming continuous storage of newly captured image frames in the pre-event buffer after delivering the set of image frames from the pre-event buffer to the image processing unit.

7. 7. The method according to claim 1, wherein the first signal is received from an image analysis unit (122) arranged to detect an event in the monitored scene based on the captured image frames.

8. The method according to claim 1 , wherein the first signal is received from an external event detection unit arranged to detect an event in the monitored scene.

9. 9. The method of claim 1, wherein the second signal is generated in response to the first signal being no longer received.

10. 10. A method according to claim 1, wherein the second signal is generated in response to a timer reaching a predetermined value, the timer being started when the first signal is received.

11. 11. The method of claim 1, further comprising the step of streaming the processed image frames of the post-event video sequence to a recipient upon receiving the first signal indicating that an event is detected.

12. A monitoring camera (100) configured to record an event video sequence, comprising: The event video sequence includes: a pre-event video sequence comprising a video showing the monitored scene (200) for a predetermined period of time before the event detection; and a post-event video sequence comprising video showing said monitored scene from the time of event detection; Including, The monitoring camera includes: an image capture unit (104) configured to capture image frames successively; an event unit (118) configured to receive a first signal indicating that an event is detected and a second signal indicating that said event has finished; an image processing unit (106) configured to process image frames; a pre-event buffer (112) arranged upstream of said image processing unit and configured to successively store newly captured image frames by appending said newly captured image frames to a set of pre-event image frames corresponding to said predetermined period, wherein said newly captured image frames stored in said pre-event buffer have not yet undergone image processing in an image processing unit consisting of one or more of the following: defective pixel correction, white balancing, demosaicing, matrixing, gamma correction, sharpening, noise filtering, scaling and encoding; a memory (108) adapted to store the processed image frames; Equipped with Upon receiving the first signal, the monitoring camera is adapted to interrupt the storage of newly captured image frames in the pre-event buffer and to feed newly captured images to the image processing unit; - said image processing unit is configured to perform image processing of said newly captured image frames and to store said processed images as said post-event video sequence in said memory; Upon receiving the second signal, the monitoring camera is adapted to interrupt the feeding of newly captured image frames to the image processing unit; the monitoring camera is configured to deliver the set of image frames from the pre-event buffer to the image processing unit; - said image processing unit is configured to perform image processing of said set of image frames; - said monitoring camera is configured to store said processed set of image frames as said pre-event video sequence in said memory; The image processing of the monitoring camera (100) includes one or more of the following: defective pixel correction, white balancing, demosaicing, matrixing, gamma correction, sharpening, noise filtering, scaling, and encoding.

Images