Patent Description:
Intra-frame and inter-frame image processing algorithms can be used for a wide variety of applications.

Intra-frame image processing involves extracting information within a single frame, and can for example be used for applications such as object detection, for image histogram analysis and in methods of scene classification.

Inter-frame image processing involves extracting information from a sequence of two or more frames, and can for example be used for applications such as motion detection, object tracking, and in methods of temporal filtering.

In certain operating modes of an image processor, it may be desirable to employ both intra-frame and inter-frame image processing on captured images. However, there is a technical difficulty in applying both intra-frame and inter-frame image processing algorithms while remaining robust in the presence of changing scene conditions.

Document <CIT> disclosing an imaging method and device in which the exposure time varies. There is thus a need in the art for an improved method and device for performing both intra-frame and inter-frame image processing.

It is an aim of embodiments of the present description to at least partially address one or more problems in the prior art.

According to one aspect, there is provided a method of performing intra-frame and inter-frame image processing according to claim <NUM>.

According to a further aspect, there is provided an image capture device according to claim <NUM>.

The foregoing and other features and advantages will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which:.

The term "intra-frame image" is used herein to designate a captured frame of a video sequence to which intra-frame image processing is applied.

The term "intra-frame image processing" or "intra-frame processing" is used herein to designate the processing of a frame of a video sequence to extract information only from within the given frame. Examples of applications of intra-frame image processing include:.

The term "inter-frame image" is used herein to designate a captured frame of a video sequence to which inter-frame image processing is applied.

The term "inter-frame image processing" or "inter-frame processing" is used herein to designate image processing of video in order to extract information from a plurality of frames, generally in sequence. Examples of applications of inter-frame image processing include:.

The term "image capture parameter" is used to designate any of a broad range of parameters than may be set when an image is to be captured by an image sensor. These for example include:.

<FIG> schematically illustrates an example of an image capture device <NUM> comprising an image sensor (IMAGE SENSOR) <NUM>, and an image processing circuit (IMAGE PROCESSING CIRCUIT) <NUM>. The image processing circuit <NUM> receives image data on output lines <NUM> of the image sensor <NUM>, and provides output data (DATA) on output lines <NUM> of the image capture device. The output data on the lines <NUM> for example comprises information detected based on the captured images. The image processing circuit <NUM> is integrated in a same integrated circuit as the image sensor <NUM>. This provides the advantage that certain image processing can be performed on-chip based on the image data captured by the image sensor <NUM>, and the number of output pins of the image capture device <NUM> can thus be reduced.

The image sensor <NUM> captures images based on image capture parameters (CTRL PARAMETERS) provided on input lines <NUM> of the image capture device <NUM>.

The present inventors have found that, in the case that it is desired to perform both intra-frame and inter-frame image processing on the images captured by the image sensor <NUM> using the image processing circuit <NUM>, there is a technical difficulty in providing appropriate image capture parameters suited to both types of imaging processing.

Indeed, in the case of intra-frame image processing, the image quality of each captured image should be high. In particular, constraints during intra-frame image processing can be relaxed by providing images with suitably adapted focus and exposure time, such that the images are sharp, and properly exposed providing an acceptable dynamic range. Indeed, processing such as contour or edge detection is considerably harder if edges in the image are blurred, or if there is insufficient contrast. Obtaining appropriate focusing and exposure for example implies continuously adapting the image capture parameters in order to adjust the exposure time, focus, dynamic range, contrast, granularity, etc., based on the scene.

On the contrary, in the case of inter-frame image processing, it is desirable to detect changing features in the scene from one image to the next. Therefore, the image capture parameters for setting the exposure time, focus, dynamic range, contrast, granularity, etc., should remain relatively constant from one image to the next in order to allow accurate detection of changes in the scene. Indeed, in the particular context of motion detection, which is generally based on the detection of changes of image features, an abrupt modification of the image capture parameters can lead to a false positive detection. In that case, what is detected by the motion detection algorithm is thus an artefact due to the modification of image capture parameters from one frame to another.

While one solution could be to attempt to find a compromise between the image capture parameters adapted to inter-frame and intra-frame image processing by limiting the variations in the image capture parameters between successive images, this would still imply added complexity to the image processing algorithms implemented by the image processing circuit <NUM>. Indeed, greater complexity would be required to provide both robust intra-frame and inter-frame image processing of images captured using suboptimal image capture parameters. Such added complexity is undesirable, particularly in the case of an on-chip image processing circuit such as the circuit <NUM> of <FIG>, as it implies higher chip area, power consumption and/or processing time.

<FIG> represents a method of image capture according to an embodiment of the present disclosure. An image sensor is for example controlled in order to alternate between capturing images IA suitable for intra-frame image processing and images IB suitable for inter-frame image processing. In other words, intra-frame and inter-frame image capture is for example interleaved. The images IA are captured based on a set of image capture parameters PA, the values of which for example adapt rapidly to changing conditions in the image scene. The parameters PA are for example updated between the capture of each image IA. The images IB are captured based on a set of image capture parameters PB, the values of which remain relatively constant.

In some embodiments, the image capture parameters PB remain constant for certain periods, such as for a period <NUM> represented in <FIG> spanning the capture of at least two images IB. After this period, the image capture parameters PB are for example updated to reflect the new image scene conditions ready for the capture of a subsequent image IB <NUM>. The parameters PB are then for example held constant for a further period.

Alternatively, the image capture parameters PB may be permitted to vary by less than a significant amount between the capture of successive images IB. A significant amount for example corresponds to a variation that would trigger the false positive detection of an event during the inter-frame image processing operation in the absence of a change in the image scene. In some embodiments, a periodic recalibration of the image capture parameters PB is also performed in order to reflect larger changes in the image scene conditions, as described above.

While in the example of <FIG> the image sensor alternates between capturing IA and IB images for each captured image, more generally the image sensor could alternate between intra-frame and inter-frame image capture operations, each intra-frame image capture operation corresponding to the capture of one or more images suitable for intra-frame image processing, and each inter-frame image capture operation corresponding to the capture of one or more images suitable for inter-frame image processing. However, in some embodiments, each intra-frame and inter-frame image capture operation is limited to the capture of <NUM> images or frames, and preferably to the capture of <NUM> images or frames.

<FIG> schematically illustrates an image capture device <NUM> according to an example embodiment of the present disclosure. The device <NUM> comprises an image sensor (IMAGE SENSOR) <NUM>, which is for example a CMOS sensor, and an image processing circuit <NUM> comprising a portion <NUM> configured to perform intra-frame image processing, and a portion <NUM> configured to perform inter-frame image processing. The portion <NUM> for example provides intra-frame processing output data OUTA on output lines of the device <NUM>, and the portion <NUM> for example provides inter-frame processing output data OUTB on output lines of the device <NUM>. The image processing circuit <NUM> is for example implemented by dedicated hardware.

The image processing circuit <NUM> is for example integrated in a same integrated circuit as the image sensor <NUM>, although in alternative embodiments they could be implemented by separate chips. More generally, image capture device <NUM> is for example a full custom CMOS System on Chip, including all previously described elements of the device <NUM>, where the image sensor <NUM> represents the array of pixels on the focal plane.

The image capture device <NUM> for example further comprises demultiplexers <NUM> and <NUM> respectively controlled by control signals S1 and S2 generated by a control circuit (CTRL) <NUM>. The demultiplexer <NUM> provides image capture parameters P to the image sensor <NUM> in order to control the image capture operations, these parameters being selected from a set PA suitable for capturing images for intra-frame image processing, and a set PB suitable for capturing images for inter-frame image processing. The demultiplexer <NUM> distributes the images (IMAGE) captured by the image sensor <NUM> to the portions <NUM> and <NUM> of the image processing circuit <NUM>.

The set PA of image capture parameters is for example provided by a parameter update circuit (PARAMETER UPDATE) <NUM>, based for example on the result of intra-frame image processing performed by an algorithm (ALGORITHM A) <NUM> of the portion <NUM> of the image processing circuit <NUM>, and/or on the result of inter-frame image processing performed by an algorithm (ALGORITHM B) <NUM> of the portion <NUM> of the image processing circuit <NUM>.

The set PB of image capture parameters is for example provided by the parameter update circuit <NUM> via a memory (MEM) <NUM>, which for example controls the rate at which the parameters PB are updated. The memory <NUM> is for example under control of the control circuit <NUM>, which for example provides a refresh signal R indicating when the parameters PB are to be refreshed.

Operation of the image capture device <NUM> will now be described in more detail with reference to <FIG>.

<FIG> is a timing diagram showing examples of the signals R, PA, PB, S1, P, S2 and IMAGE in the circuit <NUM> of <FIG>.

The signal S1 for example alternates between high and low levels, and in the example of <FIG>, the high level causes the set of parameters PA to be provided to the image sensor <NUM>, and the low level causes the set of parameters PB to be provided to the image sensor <NUM>.

The set of parameters PA is for example refreshed before the capture of each intra-frame image IA, for example during the low period of the signal S1 so that the set of parameters PA remains constant while the signal S1 is high. In the example of <FIG>, the set of parameters PA transitions from a set of values PA1 to sets of values PA2, PA3, PA4, PA5, PA6, etc..

The set of parameters PB is for example refreshed by the refresh signal R at a time t1 to assume a set of values PB1, and then again at a time t2 to assume a set of values PB2. The times t1 and t2 are for example chosen to fall during high periods of the signal S1 so that the set of parameters PB remains constant during the low periods of the signal S1. In the example of <FIG>, the set of parameters PB remains at the set of values PB1 for four inter-frame images, before transitioning to the set of values PB2 following the high pulse of the refresh signal R at the time t2. Of course, in alternative embodiments, depending on the inter-frame image processing to be applied, the set of parameters PB could remain constant for fewer images or a greater number of images.

The signal S2 for example alternates between high and low levels. Each high level of the signal S2 is for example synchronized with the application of the set of parameters PA and causes the output image from the image sensor <NUM> to be treated as an intra-frame processing image, which is directed to the portion <NUM> of the image processing circuit <NUM> for intra-frame image processing. Each low level of the signal S2 is for example synchronized with the application of the set of parameters PB and causes the output image from the image sensor <NUM> to be treated as an inter-frame processing image, which is directed to the portion <NUM> of the image processing circuit <NUM> for inter-frame image processing.

The image processing circuit <NUM> may operate in any of several different operating modes, and in at least some of these operating modes both intra-frame and inter-frame image processing is for example performed. For example, both the intra-frame and inter-frame image processing is performed during one mode of operation of the circuit <NUM>, and a transition to another mode of operation occurs when a condition is met, the condition being based on the intra-frame image processing and/or on the inter-frame image processing.

One particular example of the operating modes of the image processing circuit <NUM> will now be described in more detail with reference to <FIG>.

<FIG> is a state diagram representing an example of the operation of the image processing circuit <NUM> according to an example embodiment in which object and motion detection is performed.

According to the example of <FIG>, the image capture device <NUM> is an always-on smart CMOS image sensor having multiple awakening levels, modes or states denoted as M1, M2 and M3 in <FIG>. For example, the device <NUM> is in a fixed position, and is woken by a detected movement within its field of view.

The image processing circuit <NUM> for example starts in a first state or mode M1, corresponding to a deep sleep mode. In this mode, intra-frame images IA and inter-frame images IB are alternately captured by the image sensor <NUM>, and the image processing circuit <NUM> performs both intra-frame and inter-frame image processing.

During the mode M1, the intra-frame images IA are for example employed to track the average luminosity and then to perform, by retroaction, auto-exposure adjustment for the capture of subsequent intra-frame images IA. The inter-frame images IB are for example employed to detect motion with a fixed exposure time.

For example, the average luminosity in the intra-frame images IA is calculated by the algorithm <NUM> in the portion <NUM> of the image processing circuit <NUM>, and is used by the parameter update circuit <NUM> to update the set of parameters PA between the capture of each intra-frame image. The exposure time of the intra-frame images can thus be controlled automatically based on the calculated average luminosity.

The inter-frame images IB are for example employed to detect frame differences due to scene changes, e.g. the movement of objects between images. In some embodiments, the exposure time of the inter-frame images is also periodically refreshed based on the detected luminosity calculation based on the intra-frame images IA.

In the mode M1, motion is detected when a condition Q<NUM>→<NUM> is met, which corresponds for example to the amount of change between successive inter-frame images exceeding a certain threshold. When this condition is met, the circuit <NUM> for example transitions to the mode M2. Alternatively, if the condition Q<NUM>→<NUM> is not met, the circuit <NUM> remains in the mode M1 (Q<NUM>→<NUM>).

In the mode M2, the inter-frame images IB are for example employed to confirm the motion detection, for example by detecting further motion, and to identify a region of interest (ROI) based on the location of the detected movement in the images. Furthermore, the auto-exposure based on the intra-frame images IA as described above for the mode M1 is for example also applied in the mode M2, but only to the identified region of interest.

The motion is confirmed when a condition Q<NUM>→<NUM> is met, and the circuit <NUM> then transitions to the mode M3. Alternatively, if not enough motion is detected, the circuit <NUM> transitions back to the mode M1 (Q<NUM>→<NUM>).

In the mode M3, the intra-frame images IA are for example employed for the detection of objects of interest within the region of interest. Furthermore, the inter-frame images IB continue to be employed for motion detection. The circuit <NUM> for example remains in the mode M3 if an object of interest is detected in the region of interest or if motion continues to be detected (Q<NUM>→<NUM>). Alternatively, if there is not enough motion detected based on the inter-frame images IB and no object of interest is detected, the circuit <NUM> for example returns to the mode M2 (Q<NUM>→<NUM>).

An advantage of the embodiments described herein is that inter-frame and intra-frame image processing performed by an image processing circuit can be made more robust by alternating between the capture of intra-frame and inter-frame images, and adjusting image capture parameters for the intra-frame and inter-frame images independently from each other.

Having thus described at least one illustrative embodiment, various alterations, modifications and improvements will readily occur to those skilled in the art. For example, it will be apparent that the principles described herein could be applied to the capture of any types of images from an image scene. For example, the image sensor may be sensitive to visible light or infrared light, or any combination of wavelengths, and may comprise any type, or combination of types, of photosites.

Claim 1:
A method of performing intra-frame and inter-frame image processing comprising:
alternating first and second image capture operations, wherein the first image capture operation comprises capturing one or more first images (IA) using an image sensor (<NUM>) based on a first set of image capture parameters (PA) and the second image capture operation comprises capturing one or more second images (IB) using the image sensor (<NUM>) based on a second set of image capture parameters (PB), the values of the parameters of the first set being modified between each of said first image capture operations based on changing scene conditions detected by image processing of one or more of the first images (IA) captured during at least one of the first image capture operations, and the values of the parameters of the second set remaining constant between each of the second image capture operations or varying by less than a significant amount between the capture of successive images;
performing intra-frame image processing on each first image (IA) captured during the first image capture operation; and
performing inter-frame image processing on each second image (IB) captured during the second image capture operation.