FR3064784A1 - METHOD AND DEVICE FOR ACQUIRING AND RETRIEVING DIGITAL IMAGES WITH EXTENDED DYNAMICS - Google Patents

Abstract

The invention relates to a method for acquiring digital images with an extended dynamic range and an associated digital image rendering method, as well as corresponding devices. The method for acquiring digital images comprises steps of: -determining (30) a plurality of different integration times, and for each of the determined integration times, obtaining (32) a digital image initial acquisition with said integration time, each initial digital image having a corresponding signal level, for each initial digital image, application (34) of a clipping function with an associated clipping threshold to obtain a level of linear signal according to the illumination, to obtain a processed digital image, and -combination (36) of the processed digital images to obtain a final image of extended dynamic range, said combination comprising a summation of the processed digital image signals, final image having a piecewise linear signal level as a function of illumination.

Description

® FRENCH REPUBLIC

NATIONAL INSTITUTE OF INDUSTRIAL PROPERTY © Publication number: 3,064,784 (to be used only for reproduction orders)

©) National registration number: 17 52699

COURBEVOIE © Int Cl ⁸ : G 06 K 9/20 (2017.01), G 06 T 5/00

A1 PATENT APPLICATION

©) Date of filing: 30.03.17. (© Priority: © Applicant (s): NATIONAL CENTER FOR SPATIAL STUDIES — FR. @ Inventor (s): TOULEMONT ARTHUR. ©) Date of public availability of the request: 05.10.18 Bulletin 18/40. ©) List of documents cited in the preliminary search report: See the end of this booklet (© References to other related national documents: ® Holder (s): NATIONAL CENTER FOR SPATIAL STUDIES. ©) Extension request (s): © Agent (s): CABINET GERMAIN ET MAUREAU.

RENDERING DIGITAL IMAGES WITH A ++ / METHOD AND DEVICE FOR EXTENDED ACQUISITION AND DYNAMICS.

FR 3 064 784 - A1 (b /) The invention relates to a method for acquiring digital images with a wide dynamic range and to a method for restoring digital images, as well as corresponding devices.

The digital image acquisition method comprises steps of: determining (30) a plurality of different integration times,

and for each of the determined integration times, obtaining (32) an initial digital image acquired with said integration time, each initial digital image having a corresponding signal level,

for each initial digital image, application (34) of a clipping function with an associated clipping threshold to obtain a level of linear signal as a function of the illumination, to obtain a processed digital image, and

-combination (36) of the digital images processed to obtain a final image of wide dynamic range, said combination comprising a summation of the digital image signals processed, the final image having a level of linear signal in pieces as a function of the illumination .

Method and device for acquiring and restoring digital images with extended dynamics

The present invention relates to a method for acquiring digital images with extended dynamics. It also relates to an associated device, a method and a device for rendering associated digital images.

It is in the field of wide dynamic range image processing, known as "High Dynamic Range" or HDR in English.

In some applications, the dynamic acquisition of conventional digital images is not sufficient for a sufficiently precise representation of the physical data represented.

Indeed, a digital image is conventionally obtained by an image acquisition device provided with sensors composed of thousands of pixels, each pixel being capable of transforming photons received during an integration time into an electrical signal, said electrical signal being transformed into a digital image sample value. The numerical values in the dynamic range of representation are therefore representative of the quantity of light received and integrated by the pixels. Low numerical values correspond to a small quantity of photons, therefore light, in other words to dark areas, while high values correspond to light areas.

In addition to the useful image signal, several noises "pollute" this image. When the useful signal is weak, the noise becomes significant and the useful signal can be embedded in the noise. The SNR (from English: Signal to Noise Ratio) corresponds to the ratio of the useful signal level to the noise level.

There are techniques for processing several digital images making it possible to obtain a digital image whose dynamics are increased, but such techniques generally require significant computing power.

Such image processing techniques are not applicable in certain applications, for example in the capture of images embedded on autonomous mobile devices having a limited computing power, for example drones or vehicles designed to move on the surface of stars, also called "rovers" in English.

In particular, a problem arises in the case of spatial applications, in the context of the acquisition of images by autonomous robots, equipped with communication systems. In such applications, the on-board computing power is low, and the communication bandwidth available for transmitting data to a remote receiver system may also be low.

It would therefore be useful to develop a method allowing the acquisition of digital images with a wide dynamic range, and a good signal-to-noise ratio in the range of the representation range and requiring only low computational means.

To this end, the invention proposes a digital image acquisition method implemented in a system comprising a digital image acquisition device, said digital image acquisition device being provided with sensors, each sensor being able to transform an illuminance detected during an integration time into an electrical signal, said electrical signal being transformed into a radiometric value of digital image sample. The process includes the following steps:

-determination of a plurality of different integration times,

-and for each of the determined integration times, obtaining an initial digital image acquired with said integration time, each initial digital image having a corresponding signal level,

for each initial digital image, application of a clipping function with an associated clipping threshold to obtain a level of linear signal as a function of the illumination, to obtain a processed digital image, and

-combination of the digital images processed to obtain a final image of wide dynamic range, said combination comprising a summation of the digital image signals processed, the final image having a linear signal level by pieces as a function of the illumination.

Advantageously, the method of acquiring digital images of the invention is simple and requires little computational resources, and makes it possible to obtain a final digital image of great dynamics.

The method of acquiring digital images according to the invention may have one or more of the characteristics below, taken independently or in any technically acceptable combination.

The clipping step consists in comparing, for each digital image acquired, the value of each sample with the predetermined clipping threshold value, and when said sample value is greater than the clipping threshold value, replacing said sample value with said clipping threshold value.

For each of the acquired images, the same clipping threshold value is applied.

The step of determining the integration times comprises a determination of a minimum integration time and / or a maximum integration time, and the calculation of the other integration times as a function of the maximum integration time. and a number of images to be processed.

The integration time determination step comprises the determination of the integration times as a function of a quality objective of the final digital image and of a number of images to be processed.

The objective of digital image quality is a minimum signal to noise ratio objective and the determination of the integration times implements the minimum signal to noise ratio, the number of images processed and a level of saturation of a captor.

According to another aspect, the invention relates to a computer program comprising software instructions which, when implemented by a programmable device, implement a method of acquiring digital images as briefly described below. above.

According to another aspect, the invention relates to an information recording medium suitable for storing a computer program comprising software instructions which, when implemented by a programmable device, implement a method of acquisition of digital images as briefly described above.

According to another aspect, the invention relates to a digital image acquisition device implemented in a system comprising a digital image acquisition device, said digital image acquisition device being provided with sensors, each sensor being able to transform an illuminance detected during an integration time into an electrical signal, said electrical signal being transformed into a radiometric value of digital image sample. The digital image acquisition device comprises a programmable device comprising at least one processor suitable for implementing calculations, comprising:

-a module for determining a plurality of different integration times,

a control module for acquiring a corresponding initial digital image for each of the determined integration times, each initial digital image having a corresponding signal level,

-for each digital image acquired, a clipping function application module with an associated clipping threshold to obtain a linear signal level as a function of the illumination, to obtain a processed digital image,

a module combining the digital images processed to obtain a final image of wide dynamic range, said combination comprising a summation of the digital image signals processed, the final image having a level of linear signal in pieces as a function of the illumination.

According to another aspect, the invention relates to a method for restoring digital images from a final digital image of wide dynamic range obtained by an acquisition method as briefly described above. The restitution process includes steps of:

obtaining a plurality of different integration times and clipping thresholds used to obtain the final digital image of extended dynamics, corresponding to a number N of initial digital images;

- calculation of N final thresholds from the integration times and the clipping thresholds;

-obtaining at least one restored image having a linear signal level with the illumination from the final digital image and the calculated final thresholds.

The method of restoring digital images according to the invention may have one or more of the characteristics below, taken independently or according to any technically acceptable combination.

The step of obtaining at least one restored image comprises the calculation of a single image having a linear signal level.

The step of obtaining at least one restored image comprises the calculation of N restored images corresponding to the N initial digital images.

According to another aspect, the invention relates to a computer program comprising software instructions which, when implemented by a programmable device, implement a method for restoring digital images as briefly described above. .

According to another aspect, the invention relates to an information recording medium suitable for storing a computer program comprising software instructions which, when implemented by a programmable device, implement a restitution method digital images as briefly described above.

According to another aspect, the invention relates to a device for restoring digital images from a final digital image of wide dynamic range obtained by acquisition device as briefly described above. The device comprises a programmable device comprising at least one processor suitable for implementing modules of:

obtaining a plurality of different integration times and clipping thresholds used to obtain the final digital image of extended dynamics, corresponding to a number N of initial digital images;

- calculation of N final thresholds from the integration times and the clipping thresholds;

-obtaining at least one restored image having a linear signal level with the illumination from the final digital image and the calculated final thresholds.

Other characteristics and advantages of the invention will emerge from the description which is given below, for information and in no way limitative, with reference to the appended figures, among which:

FIG. 1 is a diagram representing the functional blocks of a programmable device capable of implementing the invention;

FIG. 2 is a block diagram of the main steps of a process for acquiring extended dynamic range images according to an embodiment of the invention;

FIG. 3 illustrates the signal levels corresponding to images acquired as a function of the amount of light integrated by the sensors of the image acquisition device, with different integration times;

FIG. 4 schematically illustrates the signal level Σ of the final image;

FIG. 5 is a block diagram of the main steps for restoring image signals acquired from a digital image with extended dynamics according to an embodiment of the invention.

FIG. 1 schematically illustrates a system 1 for acquiring / restoring digital images capable of implementing the method of acquiring digital images with extended dynamics according to the invention.

The system 1 comprises an image acquisition system 2, which can for example be carried on board an autonomous mobile platform, for example a robot.

The image acquisition system 2 comprises an image acquisition device 4, provided with sensors, each sensor being able to transform photons received during an integration time into an electrical signal, said electrical signal being transformed into a digital sample image digital value.

Preferably, CMOS type digital output sensors are used.

The image acquisition device 4 is connected to a programmable device 6, able to implement the image acquisition method according to the invention, and in particular to control the acquisition of images by the device. image acquisition 4.

In one embodiment, the programmable device 6 is produced in the form of programmable logic components, such as an FPGA (from the English FieldProgrammable Gâte Array), or also in the form of dedicated integrated circuits, of the ASIC type (of English Application-Specific Integrated Circuit).

The programmable device 6 is connected to a memory type storage device making it possible to store digital images.

The image acquisition system 2 includes, optionally, a communication module 8 capable of communicating remotely with a rendering system 10, also comprising a communication module 12, according to a dedicated communication protocol. For example, satellite communication is implemented.

Thus, the image acquisition system 2 is able to transmit digital data, in particular digital images, to the reproduction system 10.

In one embodiment, the rendering system 10 is a programmable device capable of implementing the invention, for example a computer. It comprises a central processing unit 16, or CPU, capable of executing computer program instructions when the programmable device is powered up. The programmable device also comprises means for storing information 16, for example registers or memories, capable of storing executable code instructions allowing the implementation of programs comprising code instructions capable of implementing the method according to the invention.

Optionally, the programmable device 10 includes a screen 18 and a means 20 for entering operator commands, for example a keyboard, optionally an additional pointing device 22, such as a mouse, making it possible to select graphic elements displayed on the screen. screen 18.

The various functional blocks 12 to 22 of the programmable device 10 described above are connected via a communication bus 24.

FIG. 2 is a block diagram of the main steps of a process for acquiring digital images with extended dynamics according to an embodiment of the invention.

During a first step 30, a number N of digital image acquisitions IM is determined, to be carried out by the image acquisition device 2, and an integration time T, associated with each image acquisition IM ,.

The number N is an integer strictly greater than 1.

In one embodiment, the number N is predetermined, for example provided by an operator and stored, as well as the associated integration times.

As a variant, N is determined as a function of the storage capacities in the image acquisition system 2.

For example, N = 4 image acquisitions made with four integration times T _b T ₂ , T ₃ and T ₄ . In one embodiment, T _{1 =} 1ms, T ₂ = 2ms, T ₃ = 3ms, T ₄ = 4ms.

In one embodiment, the integration time T ₄ is the longest, and the other integration times are fixed by a calculation rule with respect to T ₄ , by

1 3 example: T = -xT ,, T ₂ = -χΤ, ·, Ί \ = -χΊ \.

4 2 4

Of course, this embodiment is given by way of example.

It is possible to apply other calculation rules according to the longest integration time and / or according to the shortest integration time.

For example, a constant time increment is applied to the shortest integration time.

According to a variant, each integration time T, is determined separately.

According to another variant, the number N and the integration times T are calculated as a function of environmental capture conditions or of a fixed quality objective, for example in terms of minimum signal-to-noise ratio to be obtained.

Preferably, the integration times are chosen so that the amount of movement in the scene observed between the shortest integration time h and the longest integration time T _N is negligible.

This is for example the case if the image capture device is fixed and if the brightness of the scene observed is stable for the entire duration of the image taking, that the image acquisitions are carried out in series by a single sensor. or in parallel by several sensors.

Step 30 is followed by a step 32 of acquiring each digital image IM, by adjusting the integration time of the sensors of the image acquisition device to the integration time T, previously determined.

A digital image is made up of a two-dimensional matrix of image samples, each image sample having an associated digital value called the signal (corresponding to the sum of the useful signal and the noises). The useful signal corresponds to a radiometry value. In a dynamic range of representation, the radiometry value is representative of the illumination, or in other words of the quantity of light, received and integrated by the sensor or sensors associated with said sample.

Low radiometry values correspond to dark areas, and high radiometry values correspond to bright areas.

For example, each digital image sample value is represented on P bits, P being for example between 8 and 10, the dynamic range of corresponding representation being the range of integer values between 0 and 2 ^P -1.

As a variant, a digital image is composed of several such two-dimensional matrices of sample images, each matrix corresponding to a color, for example red, green, blue.

The shortest integration time, T _b makes it possible to acquire a digital image having a good representation of the light areas.

The longest integration time T _N makes it possible to acquire a digital image having a good representation of the dark areas.

For each initial digital image IM ,, step 32 is followed by a clipping step 34, making it possible to use the corresponding sensor in its linear area. The linear area of the sensor is the area in which the useful signal level is proportional to the number of photons.

Step 34 implements a clipping function applied to each initial image IM ,, and consisting in replacing all the values of the signal of IM, greater than or equal to a clipping threshold SeuilJ by the value SeuilJ:

If IM _t (k, r)> Threshold_i, IM Jk, l) = Threshold _i (Eq 1)

Otherwise, IMJk, T) = IMJJ) for 1 <k <L, 1 <l <C, L, C being the respective dimensions, in number of rows and number of columns, of the image matrix IM ,.

At the end of step 34, for each index i, a digital image IM ’,, is obtained, called the processed image.

In one embodiment, the clipping threshold is constant and equal to a predetermined Sat threshold value for all the digital images acquired.

As a threshold value Sat, for example, the value up to which the signal is linear is chosen as a function of the quantity of photons received by the pixel.

Preferably, this Sat value is close to the saturation of the sensor, so as to use almost the entire linear region of the sensor.

Advantageously, this embodiment comprises simple computational operations, easily achievable by an FPGA. In practice, only the values of IMj (k, l) greater than the ThresholdJ clipping threshold are modified.

FIG. 3 schematically illustrates the signal level of each image after the clipping step 34 as a function of the quantity of light integrated by the sensor or sensors of the image acquisition device, for a set of four times of integration T _1; T ₂ , T3, T4 with T1 <T ₂ <T3 <T4.

In FIG. 3, 4 graphs have been illustrated, each presenting the illumination on the abscissa (denoted "light"), and on the ordinate the signal level, which is a numerical value.

The signal level Si corresponds to the clipped image IMj, the signal level S ₂ corresponds to the clipped image IM ' ₂ and so on. The integration time corresponding to each image acquisition, noted T _int , is also specified for each graph.

As illustrated in FIG. 3, the longer the integration time, the lower the lighting value required to saturate the pixel. The saturation stages L _satJ are inversely proportional to the integration times T _{N. i + 1} .

The signal levels S, are linear between 0 and L _sa , j, then constant, thanks to the selection of the clipping threshold values SeuilJDeturn to FIG. 2, following the clipping step 34, a step 36 of summing the clipped digital images IM ′ ,, and a final image of extended representation range, denoted IM _F , is obtained:

NOT

IM _P (k, l) = ^ IM _n (k, l) for each (k, l), l <k <L, l <l <C (Eq 2) z '= l

When each image IM ′ is coded on P bits, the final image IM _F of extended representation range is coded on P ′ = P + log ₂ (/ V) bits.

For example, if N = 4, the signal values of IM _F are represented on P + 2 bits. Thus, for example, we pass from values represented on 10 bits to values represented on 12 bits.

The final digital image thus obtained IM _F is then stored or transmitted to a rendering device during a storage or transmission step 38.

In one embodiment, the final digital image IM _F is transmitted by wireless communication means, for example by radio communications or by satellite communications, to a remote reception device, which receives and stores or processes the image. final digital dynamic range IM _F.

FIG. 4 illustrates the signal level Σ of the final image IM _F thus obtained by summation, as a function of the quantity of integrated light.

The schematic representation of FIG. 4 includes a graph presenting on the abscissa the quantity of integrated light (denoted “light”), and on the ordinate the signal level, which is a numerical value.

The signal level Σ corresponds to the sum of the respective signal levels of the clipped images:

NOT

I (%) = J> ( ^X ) (Eq 3) z '= l

The signal level Σ is piecewise linear, as a function of the corresponding saturation levels L _satJ .

A final threshold value Σ, corresponds to each level of saturation L _satJ

Advantageously, each final threshold value Σ can be calculated, as a function of the clipping threshold values SeuilJ in the following manner:

NOT

E _N = ^ Threshold_p (Eq4) p = l

And then :

2V-1 γ

Σ _Ν _ _γ ⁼ Σ Threshold _p + - x Threshold _ 1

P = l C „r. -, F x Threshold _i + T _? x Threshold _2 + ... + T _N _x Threshold _r

L _r = SSeuil _p - \ - (Eq 5) (Eq 6)

The formulas are simplified in the particular cases where all the ThresholdJ clipping thresholds are equal to a predetermined Sat value:

E _N = NxSat (Eq 7) (τ + + T>

Σ _,. = _{R +} ^j i \ _{xSa {} (Eq8)

V P N-r + λ. )

The final thresholds Σ, are used either to restore each of the N initial images, or to create a single image of extended dynamics.

The signal-to-noise ratio of the final digital image IM _F , denoted here SNR (IM _F ), can be calculated theoretically as a function of the illumination received. This makes it possible to determine integration times T, making it possible to ensure a quality objective for the final image, for example to ensure that the SNR (IM _F ) of the final digital image is greater than a signal to minimum noise, SNR _min , for a final digital image obtained from N initial images.

The SNR of the final image IM _F is calculated as a function of the N digital images processed:

NOT

IM _F (k, l) = XlM'jk, l) (Eq 9) z '= l

The signal of the processed digital images can be expressed as a function of the response of the sensor R _sensor , of the illumination received by the pixel of the sensor Z) [w] and of the integration time 7j [s]:

^N Γ 1 z = l (Eq 10)

Considering that the majority noise of current sensors is photonic noise (photonic noise is equal to the square root of the signal in electrons), at the first order, we can therefore write the SNR as follows:

^N Γ 1

Σ Ksnr (im _f ) = (Eq 11)

We can deduce :

SNR (IM _f ) = ^ R _sensor -E (k, l) (Eq 12)

To ensure a minimum SNR signal to noise ratio (IM _F ), according to one embodiment, the shortest integration time, noted T _v , is fixed.

By considering the level of electron saturation, denoted Sat ^e , and the desired minimum SNR level, denoted SNR _min , we obtain the successive clipping times by the following equations:

SNR „σε _ σ: /

4s, at

(Eq 13)

For example, Sat ^e = 10,000 electrons. The value of SNRmin is fixed according to the intended application.

FIG. 5 is a block diagram of the main restitution steps, in one embodiment, from a final image of extended dynamic range obtained by the method described above.

The restitution method comprises a first step 40 of receiving or reading in memory the final digital image IM _F.

Step 40 is followed by a step 42 for obtaining the parameters used: the number N of acquisitions, the associated integration times, the ThresholdJ clipping thresholds used.

Step 42 is followed by a step 44 of determining the final thresholds Σ, corresponding to each level of saturation L _satJ of the signal level Σ, using the calculation formulas given by (Eq 5) to (Eq 8).

Step 44 is followed by a step 46 for obtaining a linearized final image.

In one embodiment, we also take into consideration the darkness radiometry levels, which are specific to the sensors used and which depend on the integration time.

We note V _obs the level of darkness corresponding to the acquisition of the image IM, with the integration time Ti.

The IM signal _F (k, l) of the pixels of the final image is piecewise linear with the illumination, or, in other words, the IM signal _P (k, l) can be expressed as an affine function of the amount of illumination. To make the interpretation of the images easier, it may be interesting to linearize the final image. We denote by IM ^l F (k, l) the signal of the linearized pixel (k, l).

For example, for 4 integration times Γ ₁ <Γ ₂ <Γ ₃ <Γ ₄ with clipping thresholds equal to a Sat value, the linearized signal IM ^l 7 (k, l) is calculated as follows.

For IM _P (k, l) <L _l we apply:

IM _p ⁿ (k, l) = IM _p (k, l) - (V _obs _, + V _obs _ ₂ + V _obs _ ₃ + V _obs _ ₃ ) (Eq 14)

For Ej <IM _F (k, l) <£ ₂

IM ^h ; (k, l) = (l + _T J _r \ _T ) (IM _F (k, D- (V _obs _, + v _oto _ ₂ + V _obs _ ₃ ) - Sat) (Eq 15) For Σ ₂ < IM _F {k, l) <Σ ₃

IM ^h ; (k, l) = (l + (IM _F (k, l) - (V _obs _, + V _obs _ ₂ ) - 2 Sat) (Eq 16)

For Σ ₃ <IM _F {k, Z) <Σ ₄

IM ^h ; (k, l) = f 1 + ^{Γ2 +} ^ ^{3 + Γ} Α · (IM _F (k, l) - V _obs _, -3 · Sat) (Eq 17) v 7 y

In a simplified embodiment of step 46, the darkness levels are neglected.

In this case, the previous relationships are simplified:

For IM _F (k, l) <L _l we apply:

IM _F (k, l) = IM _F (k, l) (Eq18)

For Ej <IM _P (k, l) <Z ₂

IM ^l ,; (k, l) =

T -lT J-T

For E ₂ <IM _F (k, l) <L ₃ (IM _F (k, l) -Sat) (Eq19)

IM ^l ,; (k, l) = + Zk ± Z ±. ç _IMp (k, l) -2 · Sat) 1 \ + T ₂ J (Eq 20)

For Σ ₃ <IM _F (k, l) <Σ ₄

IM ^h ; (k, l) = i + + ^Γ 3 + ^Γ 41 _Ζ) _ ₃ , _{Sa {)} (Eq21)

V)

In FIG. 4, the signal level Σ ′ ¹ is shown in dotted lines corresponding to the final image of linearized extended dynamic range.

As an alternative or in addition, step 44 is followed by step 48 of restitution of the N images acquired, in their initial dynamic range, from the signal IM _F (k, l) of the pixels of the final image.

In the embodiment without taking into account the darkness levels, the following formulas are applied to determine the signal value of each pixel of each of the final restored images IM _XP , IM _2F , IM _3F , ΙΜ _{Λ Ρ} :

For IM _F (k, l) <E _ï : IM _lF (k, l) = --- IM _F (k, l) (T _{ï +} T ₂ + T ₃ + T ₄ ) (Eq 22) IM _2F (k, l) = ^Tl -IM _P (k, l) (T _{ï +} T ₂ + T ₃ + T ₄ ) (Eq 23) IM _3F (k, l) = -IM _P (k, l) (T _{ï +} T ₂ + T ₃ + T ₄ ) (Eq 24) IM, _F (k, l) = --- IM _F (k, l) (T _{ï +} T ₂ + T ₃ + T ₄ ) (Eq 25) For Σ! <IM _F (k, l) <E ₂ IM, _F (k, l) = ^Γι - (IM _F (k, l) Sat) (T _x + T ₂ + T ₃ ) (Eq 26) IM _2F (k, l) = ^Tl - (IM _F (k, l) Sat) (T _x + T ₂ + T ₃ ) (Eq 27) IM _3F (k, l) = - (IM _F (k, l) Sat) (T _x + T ₂ + T ₃ ) (Eq 28) For Σ ₂ <IM _F (k, l) <Σ ₃ IM _lF (k, l) = ^Τχ - (IM _F (k, l) 2 Sat) (7d + T ₂ ) (Eq 29) IM _2F (k, l) = - (IM _F (k, l) 2-Sat) (Τγ + T ₂ ) (Eq 30) For Σ ₃ <IM _F (k, l) <Σ ₄

IM, _F (k, l) = IM _F (k, l) -3-Sat (Eq31)

As a variant, it is of course possible to take account of the levels of darkness, in a manner similar to what has been explained above with reference to step 46.

Obviously, the formulas were detailed in the embodiment with 5 N = 4 acquired images, but the principle applies in a similar manner to a number N of acquired images, with any N.

In addition, on the same principle, it is possible to determine the formulas to be applied to restore a final linearized image or to restore N final images corresponding to the images acquired when using threshold values Seuil_i different.

Claims (10)

1. - Method for acquiring digital images implemented in a system comprising a device for acquiring digital images, said device for acquiring digital images being provided with sensors, each sensor being capable of transforming an illumination detected during an integration time into an electrical signal, said electrical signal being transformed into a value for radiometry of digital image sample, characterized in that it comprises the following steps: -determination (30) of a plurality of different integration times (T _1; T ₂ , T ₃ , T ₄ ), and for each of the integration times (T _1; T ₂ , T ₃ , T ₄ ) determined, obtaining (32) an initial digital image acquired with said integration time, each initial digital image having a level of corresponding signal, -for each initial digital image, application (34) of a clipping function with an associated clipping threshold (Seuil_1, Seuil_2, Seuil_3, Seuil_4) to obtain a signal level (S ₁₅ S ₂ , S ₃ , S ₄ ) linear as a function of the illumination, in order to obtain a processed digital image, and -combination (36) of the digital images processed to obtain a final image of wide dynamic range, said combination comprising a summation of the digital image signals processed, the final image having a level of signal (Σ) linear by pieces as a function of the illumination. 2, - A method of acquiring digital images according to claim 1, characterized in that the clipping step (34) consists in comparing, for each acquired digital image, the value of each sample with the threshold value clipping (Seuil_1, Seuil_2, Seuil_3, Seuil_4), and when said sample value is greater than the clipping threshold value (Seuil_1, Seuil_2, Seuil_3, Seuil_4), to replace said sample value by said value clipping threshold. 3. - A method of acquiring digital images according to claim 2, characterized in that for each of the acquired images, the same clipping threshold value is applied. 4, - Method for acquiring digital images according to one of claims 1 to 3, characterized in that the step of determining (30) the integration times comprises a determination of a minimum integration time and / or a maximum integration time, and the calculation of the other integration times as a function of the maximum integration time and of a number of images to be processed. 5. - A method of acquiring digital images according to one of claims 1 to 3, characterized in that the step of determining integration time comprises determining the integration times as a function of an objective of quality of the final digital image and of a number of images to be processed. 6. - Digital image acquisition device implemented in a system comprising a digital image acquisition device, said digital image acquisition device being provided with sensors, each sensor being able to transform an illumination detected during an integration time into an electrical signal, said electrical signal being transformed into a radiometric value of digital image sample, characterized in that it comprises a programmable device comprising at least one processor adapted to be implemented calculations, including: -a module for determining a plurality of different integration times, a control module for acquiring a corresponding initial digital image for each of the determined integration times, each initial digital image having a corresponding signal level, -for each digital image acquired, a clipping function application module with an associated clipping threshold to obtain a linear signal level as a function of the illumination, to obtain a processed digital image, a module combining the digital images processed to obtain a final image of wide dynamic range, said combination comprising a summation of the digital image signals processed, the final image having a level of linear signal in pieces as a function of the illumination. 7. - Method for restoring digital images from a final digital image of wide dynamic range obtained by an acquisition method according to one of claims 1 to 5, characterized in that it comprises steps of : obtaining (42) a plurality of different integration times and clipping thresholds used to obtain the final digital image of extended dynamics, corresponding to a number N of initial digital images; -calculating (44) of N final thresholds (Σ _υ ..., Σ ₄ ) from the integration times and the clipping thresholds; -obtaining (46, 48) at least one restored image having a linear signal level with the illumination from the final digital image and the calculated final thresholds. 8. A method of restoring digital images according to claim 7, characterized in that the step of obtaining at least one restored image (46) comprises the calculation of a single image of extended dynamic range having a linear signal level. 9. The method for restoring digital images according to claim 7, characterized in that the step of obtaining at least one restored image (48) comprises the calculation of N restored images corresponding to the N initial digital images. 10.- Device for restoring digital images from a digital image 15 final of dynamic range extended obtained by acquisition device according to claim 6, characterized in that it comprises a programmable device comprising at least one processor suitable for implementing modules of: -obtaining a plurality of different integration times and clipping thresholds used to obtain the final digital image of extended dynamics, corresponding to a 20 number N of initial digital images; - calculation of N final thresholds from the integration times and the clipping thresholds; -obtaining at least one restored image having a linear signal level with the illumination from the final digital image and the calculated final thresholds.