Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T5/00—Image enhancement or restoration
  • G06T5/50—Image enhancement or restoration using two or more images, e.g. averaging or subtraction
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T5/00—Image enhancement or restoration
  • G06T5/90—Dynamic range modification of images or parts thereof
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/70—Circuitry for compensating brightness variation in the scene
  • H04N23/73—Circuitry for compensating brightness variation in the scene by influencing the exposure time
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
  • H04N23/70—Circuitry for compensating brightness variation in the scene
  • H04N23/743—Bracketing, i.e. taking a series of images with varying exposure conditions
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T2207/00—Indexing scheme for image analysis or image enhancement
  • G06T2207/10—Image acquisition modality
  • G06T2207/10141—Special mode during image acquisition
  • G06T2207/10144—Varying exposure
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T2207/00—Indexing scheme for image analysis or image enhancement
  • G06T2207/20—Special algorithmic details
  • G06T2207/20172—Image enhancement details
  • G06T2207/20208—High dynamic range [HDR] image processing
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T2207/00—Indexing scheme for image analysis or image enhancement
  • G06T2207/20—Special algorithmic details
  • G06T2207/20212—Image combination
  • G06T2207/20221—Image fusion; Image merging

Definitions

• the present invention relates to a method for imaging a scene in space. It also relates to a system implementing the imaging method according to the invention.
• the visual inspection of satellites in orbit corresponds to an increasingly strong expectation of satellite operators.
• This inspection is in practice carried out from a spacecraft, in particular a dedicated satellite which embeds a system for taking pictures or acquiring images which are stored locally and then transmitted via a terrestrial reception station to a processing site. to be analyzed there, by a user or by automatic means of analysis, to detect anomalies.
• a photographic sensor or imager can take a photo over a dynamic range of 8 to 10EV, while the shooting conditions represent a dynamic range of 20EV.
• Exposure bracketing [1] which consists of taking a series of successive photographs in a single trigger, the exposure varying automatically between the shots.
• the exposure variation is obtained by changing the exposure time and/or the aperture. This feature is used in cases where the determination of exposure values is difficult (situation of strong backlight for example, or when multiple reflections distort the exposure measurement by the cell or the exposure meter), or simply for security purposes.
• the exposure calculation currently performed on digital cameras is most often done automatically, by taking an average of the luminance of a certain amount of pixels of the whole image or of a user-defined area so that the main subject of the photo is exposed correctly [2] [3]
• the object of the present invention is to overcome these drawbacks by proposing a new method for imaging a scene in space which provides, on a computer screen of a user on the ground, images having a dynamic range wide enough to allow easy detection of anomalies.
• This objective is achieved with a method for imaging a scene in space from a spacecraft, comprising an acquisition of a sequence of N images of said scene, with a partial overlap of the dynamic range of said images and with exposure times adjusted respectively to pass from a first darkest image to increasingly brighter images, so as to cover a dynamic range greater than that of each of said acquired images.
• this imaging method comprises: a first step for determining a reference exposure time for the first image, the darkest, implementing a first criterion according to which the first image can comprise white pixels of maximum brightness, and a second criterion according to which the first image cannot have white areas of maximum brightness greater than or equal to 2x2 pixels, a step for acquiring an image of a scene of interest with a determined exposure time, an analysis of said image to determine whether it satisfies said first and second criteria, and if not, a new image acquisition with a modified exposure time, followed by a new analysis, said new acquisition steps and of new analysis being repeated until said first and second criteria are satisfied, a last step to acquire the Nl images.
• this imaging method according to the invention is intended for a sequence of images, i.e. allowing overexposed images and underexposed images on one of the images but not all of them.
• Each image in the sequence of images beyond the first can be at least 16 times brighter (+4EV) than the previous one.
• the sequence of images can be taken in the shortest possible time to avoid pixel shifts between images taken from the same scene.
• the imaging method according to the invention may further comprise a step for determining the exposure time modified by successive approximations, implementing a rapid calculation method such as a method by dichotomy, consisting in repeating partitions of an interval in two parts then to select the sub-interval in which there is a zero of the function, with the aim of reducing the determination time.
• a rapid calculation method such as a method by dichotomy, consisting in repeating partitions of an interval in two parts then to select the sub-interval in which there is a zero of the function, with the aim of reducing the determination time.
• the analysis step may only be performed for 1 pixel out of N, N being greater than or equal to 2.
• the imaging method according to the invention can be implemented to image a satellite in orbit.
• a system for imaging a scene in space from a spacecraft implementing the imaging method according to the invention, this system comprising means for taking of views provided for acquiring a sequence of N images of said scene, with a partial overlap of the dynamic range of said images, and means for adjusting the exposure time of each image, respectively for acquiring images of a first image darker to increasingly brighter images, so as to cover a dynamic range greater than that of each of said acquired images.
• the imaging system according to the invention can be embedded in a visual inspection satellite communicating with a terrestrial reception station. DESCRIPTION OF FIGURES
• Figure 1 illustrates an example of a dynamic range implemented for shots according to the invention
• Figure 3 illustrates the exposure times for a sequence of shots (drawings not to scale) produced with the method according to the invention.
• Exposure Value for "Exposure Value” 1EV corresponds to a doubling of the brightness.
• An exemplary embodiment of an on-board imaging system comprises, by way of non-limiting example, optical imaging equipment, a control and processing unit, an image storage unit and a radio communication unit with a ground receiving station.
• This on-board imaging system is for example installed in a visual inspection satellite designed to inspect one or more satellites in Earth orbit.
• the control and processing unit is programmed to control the shooting equipment so that it takes a sequence of N photographic images in a row, in the shortest possible period of time so as not to have any lag pixels between photos.
• This consists of taking N photos, from the darkest to the lightest, which makes it possible to cover a much greater dynamic range. For example, if with reference to Figure 1 we acquire four images in a row, using a dynamic range of 8EV for each image to limit noise, even if the photo has a higher dynamic range, each image being 16 times clearer (+4EV) than the previous one. We then have a covered dynamic range of 20EV.
• the control and processing unit is also programmed to determine a reference exposure time for the first image (Photo 1), which then makes it possible to calculate the exposure time for Photos 2, 3 and 4.
• the SHDR imaging method according to the invention implements an algorithm which determines the optimal exposure time for each image of the sequence.
• the very dark image must make it possible to distinguish all the details in the light areas.
• the very clear image must make it possible to distinguish all the details in the dark areas.
• the other intermediate images serve as a transition for an SHDR display or for a classic treatment of HDR type photos (High Dynamic Range: wide dynamic range).
• the algorithm is dedicated to determining the reference exposure time, i.e. the one corresponding to the darkest photo. For this, the following criteria are applied:
• the algorithm controls the acquisition of an image of a scene of interest such as a satellite, the analysis of this image, and if it does not meet the criteria, a new image acquisition by changing the time of exposure.
• the new exposure time is found by dichotomy, so as to reduce the number of photos to be taken to find the right exposure time.
• For the analysis to be fast we only analyze 1 pixel out of 4, with reference to figure 2. With this simplification, the analysis time is divided by 4, we are sure that a block of 2x2 saturated pixels will be detected, and we authorize white pixels which could correspond to stars.
• Tl reference exposure time
• the images are acquired (Photos 1, 2, 3 and 4) by multiplying the exposure time by 16 each time (which corresponds to the overlap of 4EV), with reference to figure 3.

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• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Theoretical Computer Science (AREA)
• Studio Devices (AREA)
• Closed-Circuit Television Systems (AREA)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

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• 2020
  • 2020-07-29 FR FR2008033A patent/FR3113165B1/fr active Active
• 2021
  • 2021-07-28 EP EP21762748.8A patent/EP4189636A1/de active Pending
  • 2021-07-28 WO PCT/FR2021/051406 patent/WO2022023668A1/fr unknown

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