Abstract:
The invention relates to a method wherein at least one output signal of a movement sensor ( 105; 505 ) is taken into account for the potential division of an integrating period (T; T″) of an image sensor ( 101; 501 ) into a plurality of sub-periods (Ti; Ti′).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a National Stage of PCT International Application Serial Number PCT/FR2013/053114, filed Dec. 17, 2013, which claims priority under 35 U.S.C. §119 of French Patent Application Serial Number 12/62133, filed Dec. 17, 2012, the disclosures of which are incorporated by reference herein. 
     BACKGROUND 
     The present disclosure generally relates to image acquisition methods and devices. It more generally relates to so-called image stabilization methods and devices, that is, aiming at avoiding or limiting the presence of visible artifacts in the image, which may result from unwanted motions of the acquisition device during an image capture. 
     DISCUSSION OF RELATED ART 
     Various image stabilization techniques have been provided. Such techniques however all have their specific disadvantages. 
     SUMMARY 
     Thus, an embodiment provides a method wherein an integration period of an image sensor is divided into a plurality of sub-periods having their durations selected by taking into account at least one output signal of a motion sensor. 
     According to an embodiment, at the end of each integration sub-period, an intermediate image acquired by the image sensor is read, and the image sensor is reset. 
     According to an embodiment, said intermediate images are combined to form a final image. 
     According to an embodiment, at least one output signal of the motion sensor is taken into account to perform the combination of intermediate images. 
     According to an embodiment, the combination does not take into account intermediate images having a signal-to-noise ratio lower than a threshold. 
     According to an embodiment, the combination does not take into account intermediate images acquired during an integration sub-period shorter than a threshold. 
     According to an embodiment, an image quality index is calculated by taking into account said at least one output signal of the motion sensor. 
     According to an embodiment, the quality index is taken into account to divide or not the integration period into a plurality of sub-periods. 
     According to an embodiment, the image sensor and the motion sensor belong to a same image acquisition device. 
     According to an embodiment, the motion sensor is configured to deliver signals representative of motions of the image acquisition device. 
     According to an embodiment, the combination takes into account the brightness level in intermediate images to reconstruct a final image with a wide dynamic range. 
     Another embodiment provides an image acquisition device comprising an image sensor, a motion sensor, and a circuit capable of dividing an integration period of the image sensor into a plurality of integration sub-periods having their durations selected by taking into account at least one output signal of the motion sensor. 
     According to an embodiment, the motion sensor is configured to deliver signals representative of motions of the image acquisition device. 
     According to an embodiment, the device further comprises an optical compensation device and a circuit capable of controlling the optical compensation device by taking into account the at least one output signal of the motion sensor. 
     According to an embodiment, the device comprises no optical compensation device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         FIG. 1  schematically illustrates in the form of blocks an embodiment of an image acquisition device; 
         FIGS. 2A, 2B, and 3  illustrate the operation of an embodiment of an image acquisition method; 
         FIG. 4  schematically illustrates in the form of blocks an alternative embodiment of an image acquisition device; and 
         FIG. 5  is a simplified perspective view illustrating an embodiment in integrated form of an image acquisition device. 
     
    
    
     For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further,  FIGS. 2A, 2B, 3, and 5  are not to scale. Further, only those elements which are useful to the understanding of the present invention have been shown and will be described. 
     DETAILED DESCRIPTION 
     An image acquisition device, for example, a digital camera, conventionally comprises an image sensor placed behind an optical system, the assembly being assembled in a protection package. To avoid or limit the presence of visible artifacts in the image in case of an unwanted motion of the acquisition device during an image capture (for example, due to the shaking of the user&#39;s hand), the acquisition device may comprise an image stabilizer comprising a device for measuring the package motions, or package motion sensor, and a device for optically compensating for such motions. As an example, the motion measurement device may comprise one or a plurality of motion sensors, for example of gyroscope, gyrometer, accelerometer type, etc. and be configured to deliver signals representative of package motions. The optical compensation device may comprise actuation elements configured to displace the image sensor or all or part of the optical system as a response to a control signal. The optical compensation device may be controlled by taking into account the output signals of the motion measurement device so that, during image capture or image acquisition phases, the image projected on the sensor is as independent as possible from the motions of the acquisition device. 
     A problem is that when the motions of the acquisition device have strong amplitudes, and/or when the focal distance of the optical system is large, the optical compensation device may reach a stop position without succeeding in totally compensating for the measured motions. Further, the response time of the optical compensation system may be too slow to compensate for certain fast motions of the acquisition device, or the compensation system may not be sufficiently accurate to exactly compensate for the measured motions. Thus, even in the presence of an image stabilizer, artifacts, and particularly fuzziness, may be present in the output image, especially when the sensor integration period (or integration time) is long. 
     It would be desirable to be able to stabilize an image whatever the amplitude, the direction, and the speed of the jerks of the image acquisition device, the focal distance of the optical system, and the sensor integration time (or time of integration of the image on the sensor). 
     According to an aspect, it is provided, in an image acquisition device comprising an image sensor and a device for measuring motions of the acquisition device, to divide an integration period of the sensor into one or a plurality of integration sub-periods having their durations selected by taking into account output signals of the motion measurement device. More particularly, when, during an image acquisition phase, motions of the acquisition device capable of significantly impacting the image rendering are detected, it is provided to interrupt the sensor integration, to read an intermediate image or frame already integrated on the sensor, and then to reset the sensor at once to start a new integration sub-period, and so on until the sum of the integration sub-periods is equal to the targeted integration period. The intermediate images may be combined or accumulated by taking into account the date output by the motion measurement device, to restore a final image of greater clearness (and of equivalent brightness level) than the image which would have been obtained if the sensor integration had been performed in a single operation. 
       FIG. 1  schematically illustrates in the form of blocks an embodiment of an image acquisition device  100 . Device  100  comprises an image sensor  101  (IMG), which may be assembled in a protection package (not shown), for example, behind an optical system (not shown). An image output of sensor  101  is connected to a memory  103  (MEM) of device  100 , where image data acquired by the sensor may be stored, for example, to be submitted to a digital processing and/or while waiting to be recorded in another storage support (not shown). Device  100  further comprises an image stabilization system. In this example, the stabilization system comprises an image stabilizer of the above-mentioned type, that is, comprising a device  105  (MS) capable of measuring motions of device  100  (independently from possible motions of all or part of the scene seen by the sensor) and a device  107  (MC) of for optically compensating for such motions. In this example, the stabilization system comprises a calculation and control circuit  109  (UC), for example, a microcontroller, configured to receive output signals from device  105 , and to accordingly control device  107 , so that the image projected by sensor  101  is as independent as possible from the motions of device  100 . In this example, the stabilization system further comprises a memory area  113  (PSF), which may be separate from memory  103  or included in memory  103 , into which circuit  109  can store data relative to the motions of device  100 . Circuit  109  is further capable of delivering control signals to image sensor  101 , and of reading from and writing into memory  103 . 
     Examples of operating modes of device  100  will now be described in relation with  FIGS. 1, 2A, 2B, and 3 . 
       FIG. 2A  is a timing diagram schematically showing the time variation, during an image acquisition phase, of equivalent position P x  of device  100 , after compensation of the motions of device  100  by device  107 . In other words, curve P x  of  FIG. 2A  does not show all the displacements of device  100  during the integration phase, but shows the portion of these displacements which is not compensated for by device  107 , for example, due to their too large amplitude or because they are too fast to be compensated for. Curve P x  may be obtained by comparing the output signals of motion measurement device  105  with the control signals delivered to compensation device  107 , while possibly taking into account the time response of compensation circuit  107 , or by means of sensors of the displacement of the compensation device itself. It should be noted that for simplification, it has been considered herein that image acquisition device  100  only moves in translation, and in a single direction of the sensor image plane. The described operating modes are however compatible with more complex motions of device  100 , provided for these motions to be measurable by device  105 . 
     Before a phase of image acquisition, a targeted integration period T of the sensor is selected, for example, automatically, by taking into account the ambient brightness conditions, or by manual parameterizing by the user. 
     At a time t0 of beginning of the image acquisition phase, the integration of sensor  101  starts. Starting from time t0 and until the end of the image acquisition phase, device  105  permanently measures the motions of device  100 , and transmits motion data to circuit  109  which, as a response, controls optical compensation device  107  so that the image projected on sensor  101  is as independent as possible from the motions of device  100 . Simultaneously, circuit  109  determines the equivalent residual motions or displacements of device  100 , that is, the motions of device  100  which are not compensated for by device  107  (signal P x ). 
     When circuit  109  detects that the residual displacements of circuit  100  are capable of causing a significant degradation of the rendering of the final image, it makes the interruption of sensor  101  stop, and an intermediate image is read and recorded into memory  103 . This marks the end of a first integration sub-period τ 1  of the sensor. The sensor is then immediately reset and a second integration sub-period τ 2  starts, and so on until the sum of the integration sub-periods is equal to the targeted integration period T. In the shown example, period T is divided into four successive sub-periods τ 1 , τ 2 , τ 3 , and τ 4 , that is, four intermediate images are read during the image acquisition phase. 
     During the image acquisition phase, data relative to the residual displacements of device  100  may be recorded in memory area  113 . 
     The intermediate images are combined to reconstruct a final image clearer than the image which would have been obtained if the sensor integration had been performed in a single operation. As an example, to achieve this result, data relative to the residual displacements of device  100  determined by circuit  109  may be taken into account during the intermediate image combination. The intermediate images may for example by offset with respect to one another before being added, to at least partly compensate for these residual displacements. As a variation, other methods of estimating residual displacements and recombining the intermediate images may be used, for example, a method using convolution techniques to have blocks of pixels representative of a same portion of the scene to be acquired coincide. 
     The reconstruction of the final image can be integrally performed after the reading of the last intermediate image. However, to minimize the quantity of memory necessary to store the intermediate images, a partial reconstruction may be performed after each intermediate reading. As an example, in the case of  FIG. 2A , a first intermediate image is read at the end of integration sub-period τ 1 , and is recorded into memory  103 . At the end of integration sub-period τ 2 , a second intermediate image is read and is directly combined with the first intermediate image, taking into account the residual displacements of device  100  during sub-period τ 2 . At the end of integration sub-period τ 3 , a third intermediate image is read and is directly combined with the partially reconstructed image contained in memory  103 , taking into account the residual displacements of device  100  during sub-period τ 3 . At the end of integration sub-period τ 4 , a fourth intermediate image is read and is directly combined with the partially reconstructed image contained in memory  103 , taking into account the residual displacements of device  100  during sub-period τ 4 . This enables to only have to store a single intermediate image during the acquisition period, independently from the number of integration sub-periods into which period T is divided. 
     To determine by what extent the residual motions of device  100  are capable of affecting the rendering of the final image, and to decide whether the sensor integration should be interrupted or carried on, circuit  109  may calculate, based on the residual displacement data, the point spread function or matrix of device  100 , that is, the deformation caused by the residual deformations of device  100 , of a scene selected so as to, in the absence of residual displacements, only illuminate a single pixel of sensor  101 . The point spread function may also be used to reconstruct the final image. Indeed, by comparing the states of the point spread function at the end of two successive integration sub-periods, the residual displacements of device  100  during the second sub-period can be determined, and the pixel offsets to be provided during the combination to compensate for these displacements can be deduced therefrom. 
     In a preferred embodiment illustrated in  FIG. 2B , circuit  109  calculates, by taking into account the residual displacement data of device  100 , for example, based on the spread point function, a quality index JND of the image being acquired. This index may be used as a criterion by circuit  109 , to decide whether the sensor integration should be interrupted or whether it should be carried on. 
       FIG. 2B  shows the time variation, during the image acquisition phase of  FIG. 2A , of quality index JND calculated by circuit  109 . At time t0 of beginning of the image acquisition phase, index JND is set to a reference value for example, zero. All along the image acquisition phase, circuit  109  recalculates quality index JND by taking into account the residual motions of device  100 . When index JND reaches a low threshold JND min  (lower than the reference level set at time t0), the sensor integration is interrupted, an intermediate image is read, and a new integration sub-period starts. Before the beginning of the new integration period, index JND is reset to its initial value (zero in this example). 
     Threshold JND min  defines a required quality level set point in each intermediate image. For a given motion sequence during the integration phase, the higher threshold JND min , the larger the number of integration sub-periods will be to correspond to this set point, and conversely. The quality of the final image obtained by combination of the intermediate images depends on set point JND min . 
     In a preferred embodiment, quality index JND is a perceptual quality index calculated based on the point spread function by the method described in article “Perceptual Image Quality Assessment Metric That Handles Arbitrary Motion Blur” of Fabien Gavant et al. (Proc. SPIE 8293, Image Quality and System Performance IX, 829314 (Jan. 24, 2012)). According to this method, for a given point spread matrix, the coordinates of the center of gravity of the matrix will be calculated, after which each coefficient of the matrix is weighted by its distance to the center of gravity, and the weighted coefficients are added to obtain a standard deviation E. The quality index is then calculated according to formula JND=−a*1n(E+1)+b, where a and b are adjustment coefficients. 
     Any other quality index taking into account residual motions of the image acquisition device may however be used. 
     It should be noted that in the embodiments described in relation with  FIGS. 1, 2A, and 2B , in the case of a strong jerk of device  100  during an image acquisition phase, certain integration sub-periods may be very short. The intermediate images acquired during such integration sub-periods may accordingly be relatively noisy, which may adversely affect the quality of the final image obtained by recombination of the intermediate images. In a preferred embodiment illustrated in  FIG. 3 , to further improve the quality of the final image, it may be provided not to take into account, in the construction of the final image, intermediate images having a signal-to-noise ratio lower than a threshold, and/or having an integration time shorter than a threshold. 
       FIG. 3  shows the acquisition of an image by a method of the type described in relation with  FIGS. 1, 2A, and 2B . In this example, an integration period T′ is divided into nine successive integration sub-periods respectively bearing references τ 1 ′ to τ 9 ′. As shown in the drawings, sub-periods τ 3 ′, τ 4 ′, τ 5 ′, and τ 8 ′ are much shorter than the others, which means that during these sub-periods, motions of device  100  have caused a fast degradation of the quality of the image being acquired. The corresponding intermediate images (in hatchings in  FIG. 3 ) are accordingly relatively noisy. To avoid degrading the quality of the final image, it may be provided not to take into account frames τ 3 ′, τ 4 ′, τ 5 ′, and τ 8 ′ in the reconstruction of the final image. To obtain a final image having a brightness level equivalent to the brightness level of the image which would have been obtained if no frame had been suppressed, it may be provided to multiply all the pixel values of the final image by a coefficient or gain proportional to the integration time which has not been taken into account in the construction of the final image (τ 3 ′+τ 4 ′+τ 5 ′+τ 8 ′ in this example). As a variation, to compensate for the brightness loss caused by the suppression of noisy frames, the sensor integration may be extended until the sum of the integration sub-periods effectively taken into account in the construction of the final image is equal or close to integration period T′. 
       FIG. 4  schematically illustrates in the form of blocks an example of an alternative embodiment of an image acquisition device  400 . In this example, device  400  comprises the same elements as device  100 , except for optical compensation device  107 . In other words, acquisition device  400  comprises no image stabilizer, but only a device  105  for measuring the motions of the acquisition device. 
     The embodiments described in relation with  FIGS. 1, 2A, 2B, and 3  are compatible with device  400 , with the difference that while, in device  100 , the equivalent residual motions of device  100  are taken into account after optical compensation by device  107 , in device  400 , the motions effectively measured by device  105  are directly taken into account. 
     An advantage of device  400  is that it comprises no optical compensation device, which decreases its cost, its weight, and its bulk. 
       FIG. 5  very schematically illustrates an embodiment in integrated form of an image acquisition device  500  of the type described in relation with  FIGS. 1 to 4 . In this example, device  500  is formed according to a semiconductor chip stack technology, or 3D technology. An image sensor  501  comprising an array of photodiodes  502  is formed in a first stacking level. Photodiodes  502  for example occupy the entire surface area of the stack to capture as much light as possible. A memory  503  capable of containing at least one image acquired by sensor  501  is formed in a second stacking level, under sensor  501 . Under memory  503 , a control circuit  509  is formed in a third stacking level. Circuit  509  is particularly capable of performing combinations of intermediate images during phases of reconstruction of a final image. Device  500  for example comprises a motion measurement device  505 , for example comprising a gyroscope. Device  505  may be integrated in one of the above-mentioned stacking levels. As an example, device  505  may be made in MEMS technology. Device  500  may further comprise an optical compensation device (not shown), for example comprising a liquid lens having an electrically-controllable form. An image stabilization can thus be performed by controlling the lens according to the motion information measured by device  505 , while keeping a high integration level. 
     An advantage of device  500  of  FIG. 5  is its low bulk and its low weight. 
     An advantage of the embodiments described in relation with  FIGS. 1 to 4  is that they enable to obtain a clear image whatever the amplitude and the speed of the motions of the image acquisition device, the focal distance of the optical system, and the sensor integration time. 
     Further, in the described embodiments, the segmentation of the sensor integration period only occurs when motions capable of affecting the quality of the image being acquired are detected. In particular, if no significant motion is detected during an image acquisition phase, the sensor integration period will not be divided, and the final image will be obtained directly, with no intermediate image combination step (that is, in this case, the integration period will be divided into a single integration sub-period). This enables not to introduce noise into the final image by needlessly segmenting the integration period when the acquisition device does not move. 
     Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. 
     In particular, the described embodiments are not limited to the specific examples of image acquisition devices described in relation with  FIGS. 1, 4, and 5 . More generally, the image stabilization method described in relation with  FIGS. 1 to 5  may be implemented in any image acquisition device comprising at least one image sensor and one device for measuring the motions of the acquisition device. 
     Further, so-called high dynamic range image acquisition processes may comprise successive acquisitions of a plurality of images of a same scene with different integration times, and the reconstruction, from these images, of a final image having a homogeneous brightness level, with a wide dynamic range. One may for example select, from each image area, the frames having the best adapted brightness level. In the brightest areas of the scene, frames having a short exposure time may be preferred and in the darkest areas of the scene, frames having a long exposure time may be preferred. Such methods particularly enable to limit overexposure or underexposure phenomena when the scene to be acquired has a high contrast. It may be provided to combine the stabilization method described in relation with  FIGS. 1 to 5  with a high-dynamic-range image acquisition method. As an example, the segmentation of the integration time generated by the stabilization method may be used to reconstruct a final high-dynamic-range image and, if necessary, to add segmentation specifically dedicated to obtaining such a high-dynamic-range image.