Method for coloring an infrared image

Abstract

The method of coloring an infrared image (11) comprises the steps of: acquiring an infrared image (11) obtained from an infrared sensor (10) comprising a set of base detectors, said infrared image (11) comprising, for each base detector, pixels whose values are encoded in a gray scale (NG) corresponding to values representative of the infrared radiation received by the base detector; and generating a colored image (13) by associating, by means of the palette (12), the respective pixel values encoded in gray scale (NG) with the pixel values encoded in the color space; the palette (12) is defined by a palette generation module (20) in CIE CAM02-UCS space using a parametric trajectory in which a parameter is a polar vector (Phi) of the parametric trajectory, the parametric trajectory being defined such that: the parametric trajectory projected into the hue and chroma planes (O, a *, b *) follows one or two ellipses; and the dynamic range of the perceived brightness (j *) for the grayscale values (NG) follows one or two straight lines.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic radiation detection, and more particularly to infrared radiation detection.

[0002] The invention more particularly relates to the problem of displaying infrared images, ie images resulting from the detection of infrared radiation. More particularly, the invention aims at colorizing infrared images in order to improve the interpretation of the infrared images by the human brain.

[0003] Therefore, the present invention can be implemented in many fields where infrared images are currently used, such as aerospace, security, defense, transportation, thermal imaging, industrial inspection, building inspection, leisure activities, health, etc. Background Art

[0004] In the health sector, infrared detectors have been widely developed since the COVID-19 pandemic. In fact, people suffering from this disease usually show an increase in their temperature, and infrared detectors make it possible to obtain images illustrating the temperature of objects present in the scene. Infrared detectors are therefore particularly used to identify the temperature of a person through enclosures, in order to detect whether a person has a high temperature and therefore a risk of being positive for COVID-19.

[0005] Infrared detectors generally make it possible to obtain an image restored in grayscale, wherein each pixel represents the amount of radiation captured by the elementary detector forming a pixel of the infrared image.

[0006] Now, this grayscale representation makes it difficult to visually identify the temperature difference between two objects in the scene that have close temperatures.

[0007] For example, in the case of detecting people with COVID 19, this grayscale representation is often not sufficient to enable an operator to quickly detect people with high temperatures in a crowd.

[0008] It is therefore desirable to improve the representation of the amount of radiation captured by the elementary detectors of the pixels forming the infrared image by coloring the representation.

[0009] This exploration is particularly motivated by the fact that the human eye is only able to perceive a limited number of shades of gray, between 120 and 180 shades for a trained Westerner, according to studies, while a person can perceive up to 2 million colors. Therefore, by properly choosing the colors, the perception of details in infrared images can be significantly improved.

[0010] For this purpose, it is known from documents WO 2016 / 179050, US 10,298,859 or WO 2014 / 200586 to transform a grayscale infrared image into a colored image by applying a color palette. The color palette makes it possible to associate a pixel value encoded in a color space for each pixel value encoded in grayscale.

[0011] To define the color space, the scene spectrum W(λ) is considered in units of power, where the wavelength λ is in micrometers. In the case of a real scene in visible light, W(λ) is the fraction reflected, diffused or transmitted from the scene illuminated by an external source of a given spectral length, preferably a white source, i.e., broadband in the visible range. In the case of a picture display, W(λ) is the superimposed linear combination of the emission spectra of a triplet of elementary sources (red, green and blue pixels).

[0012] The human eye uses rods and cones present in the retina to perceive brightness and color, respectively. Under low light conditions, usually in scotopic vision, rods provide the only spectral response. Under good light conditions, in photopic vision, cones participate in the perception of color and are classified according to three families: short, medium, and long. Cones have different spectral responses centered on short, medium, and long wavelengths.

[0013] Therefore, to the human eye, colors are represented as simple scalar triplets rather than the full scene spectrum. This argument can explain how colors can be faithfully rendered via a screen formed by three fundamental sources of red, green, and blue spectral lengths.

[0014] In sRGB, for the "Reg-Green-Blue" space, colors are represented by triplets of red, green, and blue, typically encoded at a depth of 3 times 8 bits (i.e. 24 bits).

[0015] To obtain a more precise representation of colors, other spaces have been defined over time. For example, the CIEXYZ space is a coordinate system for colorimetric space defined by the International Commission on Illumination (CIE) in 1931. It is derived from the sRGB space and assumes that the relationship between physical quantities and perception is linear. This approximation places the set of points representing colors in a three-dimensional vector space. To define this CIE XYZ space, colorimetric experiments are performed by asking people to adjust the proportions of the three experimental primaries to obtain a color patch identical to the one that has been evaluated. The sum of the "X", "Y" and "Z" components represents the absolute brightness, but not the actual perceived brightness.

[0016] The International Commission on Illumination (CIE) also defined the CIE LAB space based on the CIE XYZ space in 1976. It is a colorimetric space specifically used for characterizing surface colors.

[0017] Three quantities characterize color: the brightness L derived from the luminance of the surface, and two parameters a and b that express the difference of the color relative to the color of a gray surface of the same brightness. The existence of a gray, colorless, achromatic surface means an unambiguous indication of the composition of the light that illuminates the colored surface. This "illuminant" is usually standardized daylight.

[0018] The main disadvantage of the CIE LAB space is that it does not correctly model human perception of brightness and hue. In other words, a displacement of the basic Euclidean length in the CIE LAB space does not necessarily result in the same color perception distance, especially in terms of brightness and hue. This can be explained by the fact that the model does not take into account the principles of perceptual chromatic adaptation (such as local brightness and hue adaptation).

[0019] More recently, in 2002, the International Commission on Illumination (CIE) defined the CIE CAM02 space to calculate the mathematical correlation between the six technical dimensions related to color appearance: luminosity, luminance, colorization, chrominance, saturation, and hue.

[0020] The model is defined based on the X, Y and Z quantities of the CIE XYZ space by considering the visualization of a reference stimulus. This stimulus corresponds to a reference white point. The CIE CAM02 model also takes into account the context in which the stimulus is observed, information related to the brightness conditions of the environment of the stimulus and whether the observer of the stimulus takes the brightness conditions into account. The model makes it possible to predict the appearance of the color of a stimulus with these properties or to calculate the corresponding color under different viewing conditions.

[0021] There are other colorimetric spaces, such as the HSV space for Hue-Saturation-Value, or the HSL space for Hue-Saturation-Luminance.

[0022] For example, document WO 2014 / 200586 provides a method for generating a color palette based on interpolation in HSL space. Based on a series of predefined hue values, the color palette is constructed with constant brightness, fixed saturation and linear interpolation on hue. Depending on the type of scene, this method enables the use of an adaptive hue range, such as a red-orange-yellow palette for a desert scene, or a yellow-green-blue palette for a wood scene.

[0023] In document WO 2016 / 179050, a color palette is generated by interpolation in the sRGB space based on a series of base colors corresponding to interpolation nodes.

[0024] Document US10,298,859 discloses a solution for coloring in sRGB, CIE XYZ or CIE LAB space mainly by using a predefined palette for a specific application. In an embodiment, the palette is generated based on the minimum and maximum grayscale values ​​of the infrared image. Therefore, the palette is adapted to the dynamic range of the observed scene.

[0025] Thus, a scene with low thermal contrast is shown with a low tonal dynamic range, which is compensated by an adaptive luminance dynamic range. The document also provides for increasing the perceived contrast by using a luminance filter that exhibits oscillations.

[0026] Regardless of the method used, the goal is to obtain a color palette for a monochrome image by following the color sequence represented by the sRGB triplet while respecting multiple rules defined based on a specification. Such specifications often include both cognitive and communication criteria, which may be contradictory.

[0027] Communication codes involve the search for an aesthetic appearance, compliance with graphic charters, or consistency in the application of specific evoked ideas, such as the desired colors used to represent animals in hunting images.

[0028] The purpose of cognitive criteria is, for example, to facilitate the interpretation of images according to the type of scene, to improve the perception of image contrast, to increase the salience of the type of object of interest, or to facilitate the interpretation of images by visually impaired people (usually people with color blindness).

[0029] In all cases, cognitive norms vary from person to person.

[0030] Currently, existing solutions enable the design of color palettes that are linear in absolute brightness, but these palettes are not linear in perception with respect to the cognitive criteria of a particular person. As a result, certain details of a colored infrared image are not necessarily visible to a given person, while they may be visible to others.

[0031] The technical problem of the present invention is therefore to obtain a method for colorizing infrared images with a linear palette according to human perception, so that the human can simply modify the palette for a specific application and / or for personal perception. Summary of the invention

[0032] In order to cope with this technical problem, the present invention provides a color palette defined in the CIE CAM02-UCS space using a parameter trajectory. This unified CIE CAM02-UCS colorimetric space is defined in the article "Uniform color spaces based on CIECAM02 color appearance model" by R. Luo, G. Cui and C. Li, Color Research and Application, 2006. In this Cartesian space of axes j*, a* and b*, j* corresponds to perceived brightness, a* corresponds to the color axis from blue to yellow, and b* corresponds to the color axis from green to red.

[0033] Chroma is defined by:

[0034]

[0035] Hue is defined by:

[0036]

[0037] More specifically, the invention stems from the observation that, in order to obtain efficient linearity in perception, the parameters of this locus must be polar angle vectors. According to this same observation, the locus projected into the hue and chromaticity planes also needs to follow one or two ellipses, while the dynamic range of perceived brightness for grayscale values ​​needs to follow one or two straight lines.

[0038] With the present invention, the user can use the predefined mathematical definition of this parameter trajectory for different applications and obtain a color palette that is linear in perception with respect to general cognitive criteria.

[0039] Furthermore, the user may also adjust certain parameters of the parameter track in order to adapt the general cognitive criteria to his / her own perception.

[0040] Therefore, the present invention relates to a method for coloring an infrared image, the method comprising the following steps:

[0041] - acquiring an infrared image obtained from an infrared sensor comprising a set of elementary detectors, said infrared image comprising, for each elementary detector, a pixel whose value is coded in a grayscale corresponding to a value representative of the infrared radiation received by this elementary detector; and

[0042] - A rendered image is generated by associating each pixel value encoded in grayscale with a pixel value encoded in color space by means of a palette.

[0043] The invention is characterized in that the palette is defined in the CIECAM02-UCS space by a module for generating the palette using a parameter trajectory, in which the parameters are formed by the polar angle vector of the parameter trajectory, the parameter trajectory being defined such that:

[0044] - the parameter trajectories projected into the hue and chroma planes follow one or two ellipses; and

[0045] -The dynamic range of perceived brightness for grayscale values ​​follows one or two straight lines.

[0046] Preferably, the parameter trajectory follows the following relationship:

[0047]

[0048] The first semi-axis (a ell ) corresponds to the peak chromaticity (c c * ) is divided by 2, and the second semi-axis (b ell ) is defined as the first semi-axis (a ell ) function:

[0049] If the eccentricity (e) of the ellipse is positive; and

[0050] If the eccentricity (e) of the ellipse is negative.

[0051] Using these precise construction parameters, the present invention enables to obtain a color palette designed linearly in the CIE CAM02-UCS space to ensure good linearity of brightness perception.

[0052] The color palette thus defined can then be modified by the user according to his / her own expectations. To this end, the method may also include the following steps:

[0053] - displaying the rendered image on a screen visible to a user; and

[0054] -Restore user-intended shading modifications from the human-machine interface.

[0055] Preferably, the human-machine interface enables modification of the peak hue, peak chroma, dynamic range of polar angle vectors, eccentricity of the at least one ellipse projected into the hue and chroma planes, and / or dynamic range of perceived brightness. These parameters enable modification of the color palette with concepts that are understandable to those skilled in the art who desire to improve the image color spectrum.

[0056] In the sense of the present invention, a hue or chromaticity peak is a point whose polar angle corresponds to π. In the case of positive ellipticity, this peak corresponds to a chromaticity maximum.

[0057] Preferably, the dynamic range of the polar angle vector is limited to between 0 and 2π or between 2π and 0.

[0058] Furthermore, in order to simplify the selection of a color palette for a specific application, the human-machine interface may also enable the selection of a predefined color palette.

[0059] For example, to evoke the idea of ​​thermal imaging while avoiding unsightly purple tones and overly aggressive white and yellow tones, the first predefined color palette preferably has a peak hue ranging from 20° to 30°, a peak chromaticity ranging from 30 to 40, a dynamic range of polar angle vectors ranging from 0 to 1.8π, an eccentricity of the ellipse projected into the hue and chromaticity plane ranging from -0.35 to -0.25, and a dynamic range of perceived brightness ranging from 5 to 100. The first predefined color palette avoids pure white and too bright yellow for representing warm colors to limit aggressiveness and visual fatigue effects.

[0060] For night vision applications, green monochromatic representations of night images are referenced to phosphor scintillators used in light enhancement tubes, which have a spectral reemission length centered around green. The use of green has become iconic and evocative of public resonance for nighttime imaging because it is conveyed, in particular, through military propaganda, via movies and video games. The use of green also has advantages in terms of sensitivity to light, since the spectral sensitivity curve is a maximum at about 555 nanometers, at least under good photometric conditions.

[0061] In night vision applications of this type, the display is usually set to have a very low luminosity. This allows, on the one hand, to reduce the adaptation time of the user's vision when he stops looking at the screen showing the colored infrared image and looks directly at his environment, and on the other hand, to reduce the light reflected on the face for stealth reasons.

[0062] Now, in scotopic vision, the spectral sensitivity curve is adapted to darkness by involving rods rather than cones, then it is scotopic vision with the sensitivity curve centered around 507 nanometers.

[0063] The second predefined color palette is designed to facilitate viewing of infrared images in dark conditions while limiting visual fatigue in the case of long-term viewing of video streams. To this end, the second predefined color palette has a peak hue ranging from -170° to -160°, a peak chromaticity ranging from 10 to 20, a dynamic range of polar angle vectors ranging from 0 to 2π, an eccentricity of the ellipse projected into the hue and chromaticity plane ranging from 0.2 to 0.4, and a dynamic range of perceived brightness ranging from 100 to 20.

[0064] With this second predefined color palette, the image appears smooth and rounded while remaining sharp and prominent for the objects of interest. The background is displayed in a light color (especially for the sky), which reduces the aperture of the pupil and the effort to interpret both (especially through the familiarity of daytime scenes with clear skies). The aesthetic appearance of the colored image provides a smooth paper print appearance, which is considered quite pleasing.

[0065] The third predefined palette is formed by a threshold palette, i.e. a palette constructed by a cascade of two palettes, with a grayscale threshold according to which the transition between the two palettes is performed. Preferably, color continuity is ensured at the level of the threshold. The two palettes enable the separation of pixels of the two families / classes by coloring them differently and by maximizing the dynamic range of perceived brightness for each class. The pixels of the two families / classes can be distinguished by different hue or chromaticity variations.

[0066] In order to extract the hot areas from the ambient temperature of the scene, by maximizing the details about the two areas, the third predefined color palette preferably includes: a first part having a peak hue in the range from -110° to -100°, a peak chromaticity in the range from 15 to 20, a dynamic range of polar angle vectors in the range from 2π to 0, an eccentricity of the first ellipse projected into the hue and chromaticity planes in the range from 0.5 to 0.7, and a dynamic range of perceived brightness in the range from 100 to 5; and then a second part having a peak hue in the range from 25° to 40°, a peak chromaticity in the range from 30 to 40, a dynamic range of polar angle vectors in the range from 0 to 1.8π, an eccentricity of the second ellipse projected into the hue and chromaticity planes in the range from 0.3 to 0.4, and a dynamic range of perceived brightness in the range from 5 to 99, and the threshold between the two parts is set between 45% and 55% grayscale values.

[0067] In this third predefined palette, the representation of the object of interest is achieved by a palette close to the first predefined palette, except that the dynamic range of tones is slightly reduced to avoid image overload.

[0068] Thus, objects of interest are generally colored, which aids in the interpretation of the nature of the image. The perceived brightness has a full increased dynamic range to ensure linearity of color changes to the user.

[0069] The representation of the background is also done via an elliptical color palette with a peak of slate blue and a perceived brightness with a full reduced dynamic range. The peak color is chosen to have a low chroma because it is desired to draw less attention to the background and more attention to the object of interest. Similarly, with the third predefined color palette, it is desired to limit hue variations via highly elliptical trajectories. These choices also result in reduced sensitivity to small intensity variations, partially limiting the perception of noise, where the signal-to-noise ratio for the ambient background is typically lower than that for the thermal object of interest.

[0070] The reduced perceived brightness enables the background to be brightened to improve the saliency for the object of interest. The saliency is emphasized by the black outline at the threshold and by the chromaticity contrast between the pixels of the two classes. For this purpose, the pixels around the threshold are colored as a dark color with low chromaticity, usually a color close to black. These pixels are usually located at the boundary between the background and the object of interest.

[0071] Finally, the use of blue to represent the background is justified, on the one hand, because blue is a hue that evokes cold temperatures, and on the other hand, because blue is the color that evokes the sky, which is a reassuring representation. By the same token, the use of blue creates the illusion of depth by reminding of the horizon, which was at least a technique used by the Impressionists to create this effect.

[0072] In a fourth predefined palette, it is desirable to define a palette that includes a grayscale portion with a full dynamic range of perceived brightness and a brightly colored portion with a red hue starting from a thermal threshold.

[0073] For hunting applications, the hotspot is usually an animal, and it is desirable to highlight the animal in the image. In such applications, the environment is also very important. The environment is usually formed by complex elements such as vegetation, so it is important to represent the environment in the full grayscale dynamic range. Other applications (such as for ADAS, or "Advanced Driver Assistance System" applications) may require user alerts, including alerting the driver to the presence of pedestrians for braking assistance.

[0074] To this purpose, the fourth predefined color palette preferably has: a first part having zero chromaticity and a dynamic range of perceived brightness in the range from 0 to 100; and then a second part having a peak hue in the range from 30° to 35°, a peak chromaticity in the range from 30 to 40, a dynamic range of polar angle vectors in the range from 2π to 0, an eccentricity of the ellipse projected into the hue and chromaticity plane in the range from 0.8 to 0.9, and a dynamic range of perceived brightness in the range from 100 to 30, the threshold between the two parts being set between 75% and 85% of the grayscale value. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The manner of implementing the invention and the advantages obtained will be apparent from the references given as non-limiting indications. Figures 1 to 7 This becomes apparent in the following examples, where:

[0076] Figure 1 is a simplified representation of a method for colorizing an infrared image according to an embodiment of the present invention;

[0077] Figure 2 It is based on the ellipse projected into the hue and chromaticity plane. Figure 1 A simplified representation of the method's palette parameters generated;

[0078] Figure 3 shows the ellipses projected into the hue and chromaticity planes according to Figure 1 The method generates three variations of the palette parameters according to the variation of the eccentricity of the ellipse;

[0079] Figure 4 Shown for Figure 1 The method first predefines the dynamic range of the perceived brightness of the color palette and the dynamic range of the ellipse projected into the hue and chroma planes;

[0080] Figure 5 Shown for Figure 1 a dynamic range of perceived brightness of a second predefined color palette of the method and a dynamic range of the ellipse projected into the hue and chroma planes;

[0081] Figure 6 Shown for Figure 1 The dynamic range of perceived brightness of a third predefined color palette of the method and the dynamic range of the two ellipses projected into the hue and chroma planes; and

[0082] Figure 7 Shown for Figure 1 A fourth method of predefining a dynamic range of perceived brightness of a color palette and a dynamic range of an ellipse projected into the hue and chroma planes. DETAILED DESCRIPTION

[0083] Figure 1 A method of colorizing an infrared image 11 obtained from an infrared sensor 10 comprising a set of elementary detectors is shown. Typically, the elementary detectors are formed by microbolometers arranged in an array.

[0084] For each row or column of microbolometers, a readout circuit makes it possible to measure the amount of infrared radiation captured by each elementary detector and thus forming a pixel of the infrared image.

[0085] The values ​​of the individual pixels originating from the infrared image can be coded in grayscale NG on 8, 11 or 14 bits without modifying the invention. Furthermore, a pre-processing can be performed on the infrared image 11 before implementing the colorization method of the invention.

[0086] More specifically, the invention provides for the conversion of an infrared image 11 coded in grayscale NG into a rendered image 13 using a palette 12, the individual pixels of which are coded by a color system according to the sRGB standard, for example.

[0087] According to the present invention, a color palette 12 is defined in the CIE CAM02-UCS space by defining each color by the following six characteristics: luminosity, absolute brightness, coloration, chromaticity c*, saturation, and hue h*. These six characteristics are interdependent and can be encoded by a triple of three values: j*, a*, and b*, where the parameter j* corresponds to the perceived brightness, a* corresponds to the blue to yellow color axis, and b* corresponds to the green to red color axis.

[0088] In this CIE CAM02-UCS space, chromaticity c* is defined by:

[0089]

[0090] Hue h* is defined by:

[0091]

[0092] More specifically, the present invention provides for generating a color palette 12 in CIE CAM02-UCS space using a parameter trajectory by a color palette generation module 20, in which the parameter is a polar angle vector Φ. The parameter definition is constrained so that the projection onto the hue and chromaticity plane (O, a c *,b c *) follows one or two ellipses; and the dynamic range of perceived brightness j* for grayscale value NG follows one or two straight lines.

[0093] Figure 2 A representation of an ellipse in the hue and chroma plane (O, ac*, bc*) generated by the parameters of the palette 12 is shown. In this example, the ellipse is formulated according to a parameter trajectory whose parameters are a length of 2 n The polar angle vector φ = [0, .., φmax], where n corresponds to the depth of the digitized pixel value. Therefore, the various gray values ​​NG range from 0 to 2 n-1 Preferably, the depth φmax=1.8. Preferably, the dynamic range of the polar angle vector Φ is limited to between 0 and 2π or between 2π and 0.

[0094] exist Figure 2In the example, the ellipse does not complete a full rotation, that is, it is not closed, to avoid the color plate ending on pure white (which is considered aggressive).

[0095] An ellipse is defined as follows:

[0096]

[0097] where, in a reference frame (O, a′, b′) aligned with the axes of the ellipse, the ellipse has a peak chromaticity c c * Divide by 2 corresponding to the first semi-axis a ell , and the second semi-axis b ell Depends on the first semi-axis a ell and the eccentricity e of the ellipse.

[0098] More specifically, if the eccentricity is positive, the second semi-axis b can be derived from the desired eccentricity e using the following formula: ell :

[0099]

[0100] When the eccentricity is negative, the second semi-axis follows the following formula:

[0101]

[0102] When the eccentricity is positive, the first semi-axis a ell Greater than the second semi-axis b ell , and when the eccentricity is negative, the first semi-axis a ell Smaller than the second semi-axis b ell .

[0103] exist Figure 3 The effect of a variation in the eccentricity e is shown in , where the parameter trajectory with a small eccentricity enables exploitation of purple and orange tones, while the parameter trajectory with a high eccentricity avoids such tones at the risk of giving the image a monochrome appearance.

[0104] For aesthetic reasons and to facilitate perception of detail via a good dynamic range of purple / orange hues, it is preferred to choose a low eccentricity, typically an eccentricity in the range from -0.2 to -0.4, for example e = -0.3.

[0105] Regardless of the chosen eccentricity value, a change of reference frame can be performed to change the angle A simple azimuthal rotation of , i.e., by the hue of the desired peak, writes the ellipse as Figure 2 The reference frame (O,a * ,b * )middle:

[0106]

[0107] The peak hue angle can be written as And the perceived brightness curve j*(φ) is just a straight line, the limit of its dynamic range can be set. The peak hue h can be selected according to the typology of the desired palette c * Value, peak chromaticity c c *, the value of the dynamic range of the polar angle vector Φ, the value of the eccentricity e and / or the value of the dynamic range of the perceived brightness j*. For example, the tonal dynamic range can be increased by reducing the eccentricity e of the ellipse.

[0108] With these precise parameters of the definition of the palette 12, a table associating the various gray values ​​NG with triples in the sRGB space can be obtained. For example, this conversion between the CIE CAM02-UCS space and the sRGB space can be performed by a "colorspacious" module coded in the Python computer language. This table can be called a LUT, or "Look Up Table".

[0109] In this palette 12, each gray value NG is thus associated with a color triplet in the sRGB space, enabling the infrared image 11 to be transformed into a rendered image 13. The rendered image 13 can then be displayed on a screen 14, allowing the user to efficiently detect salient elements of the scene captured by the infrared sensor 10.

[0110] To detect specific elements in various applications, a user may use the human-machine interface 15 to select a predefined color palette.

[0111] Furthermore, the human-machine interface 15 may also enable the user to adjust the configuration values ​​of the color palette 12 .

[0112] In fact, even if the palette 12 is defined with a peak hue h c * Value, peak chromaticity c c *, the dynamic range of the polar angle vector Φ, the eccentricity e and / or the dynamic range of the perceived brightness j*, the user may also change these values ​​via the human-machine interface 15. For this purpose, the user can request an increment or decrement of the respective values ​​in order to observe the evolution of these changes directly on the screen 14 used to view the rendered image 13.

[0113] Regarding the predefined color palettes, four color palettes may be provided to the user via the human-machine interface 15: Lifeinred TM Color, Life in red TM Serenity, Life in red TM Contrast, and Life inredTM Tracker.

[0114] For example, the predefined color palette Lifeinred TM Color has a peak hue h in the range from 20° to 30° c *, Peak chromaticity c in the range from 30 to 40 c *, the dynamic range of the polar angle vector Φ in the range from 0 to 1.8π, the eccentricity e of the ellipse projected into the hue and chromaticity planes (O, a*, b*) in the range from -0.35 to -0.25, and the dynamic range of the perceived brightness j* in the range from 5 to 100.

[0115] like Figure 4 As shown, the predefined color palette Lifeinred TM Color includes a straight line for perceived brightness j* and increasing dynamic range and ellipses in the hue and chromaticity planes (O, a*, b*). Point P1 corresponds to coral red, point P2 corresponds to vanilla or peach, and point P3 corresponds to wine red.

[0116] Predefined color palette Lifeinred TM Serenity has a peak hue in the range from -170° to -160° c *, Peak chromaticity c in the range from 10 to 20 c *, the dynamic range of the polar angle vector Φ in the range from 0 to 2π, the eccentricity e of the ellipse projected into the hue and chromaticity planes (O, a*, b*) in the range from 0.2 to 0.4, and the dynamic range of the perceived brightness j* in the range from 100 to 20.

[0117] like Figure 5 As shown, the predefined color palette Lifeinred TM Serenity includes a straight line and reduced dynamic range of perceived brightness j*, with a reduced dynamic range Dr; and ellipses in the hue and chroma planes (O, a*, b*). Point P6 corresponds to aqua, while point P5 corresponds to slate gray, and point P7 corresponds to cyan.

[0118] Predefined color palette Lifeinred TM Contrast has a first portion having a peak hue h in the range from -110° to -100° c *, Peak chromaticity c in the range from 15 to 20 c*, the dynamic range of the polar angle vector Φ in the range from 2π to 0, the eccentricity e of the first ellipse projected into the plane (O, a*, b*) in the range from 0.5 to 0.7, and the dynamic range of the perceived brightness j* in the range from 100 to 5.

[0119] Predefined color palette Lifeinred TM Contrast also has a second portion having a peak hue h in the range from 25° to 40°. c *, Peak chromaticity c in the range from 30 to 40 c *, a dynamic range of the polar angle vector Φ in the range from 0 to 1.8π, an eccentricity e of the second ellipse projected into the plane (O, a*, b*) in the range from 0.3 to 0.4, and a dynamic range of the perceived brightness j* in the range from 5 to 100. The threshold S1 between the two parts is set between the grayscale values ​​NG of 45% and 55%, for example 50%.

[0120] like Figure 6 As shown, the predefined color palette Lifeinred TM Contrast includes: a linear dynamic range of perceived brightness j* decreasing down to threshold S1, and then a linear and increasing dynamic range of perceived brightness j*. Two ellipses are formed in the plane (O, a*, b*). Point P1 corresponds to coral red, which is also used in the predefined color palette Lifeinred TM Color, and point P5 corresponds to slate blue.

[0121] Predefined color palette Lifeinred TM Tracker has a first portion with zero chromaticity and a dynamic range of perceived brightness j* ranging from 0 to 100, and then a second portion with a peak hue h ranging from 30° to 35°. c *, Peak chromaticity c in the range from 30 to 40 c *, a dynamic range of the polar angle vector Φ in the range from 2π to 0, an eccentricity e of the ellipse projected into the plane (O, a*, b*) in the range from 0.8 to 0.9, and a dynamic range of the perceived brightness j* in the range from 100 to 30. The threshold S2 between the two parts is set between the grayscale values ​​NG of 75% and 85%, for example 80%.

[0122] like Figure 7 As shown, the predefined color palette Lifeinred TMTracker comprises a linear dynamic range of perceived brightness j* increasing up to threshold S2, and then a linear and decreasing dynamic range of perceived brightness j*. An ellipse is formed in plane ab, where point P8 corresponds to the red brick color.

[0123] Using these various predefined palettes and a human-machine interface 15 enabling the user to modify the parameters generating the palette 12, the present invention enables the user to obtain a linear aspect according to his / her personal perception. Thus, the present invention enables a clearer observation of an object of interest captured by an infrared detector.

Claims

1. A method for coloring an infrared image (11), the method comprising the following steps: Acquiring an infrared image (11) obtained from an infrared sensor (10) comprising a set of elementary detectors, the infrared image (11) comprising, for each elementary detector, a pixel whose value is coded in grayscale NG corresponding to a value representing infrared radiation received by the elementary detector; as well as Generating a rendered image (13) by associating each pixel value encoded in grayscale NG with a pixel value encoded in color space by means of a color palette (12); The method is characterized in that the color palette (12) is defined in the CIECAM02-UCS space by a module (20) for generating a color palette using a parameter trajectory, in which the parameter is the polar angle vector Φ of the parameter trajectory, and the parameter trajectory is defined such that: The parameter trajectories projected into the hue and chrominance planes (O, a*, b*) follow one or two ellipses; and The dynamic range of perceived brightness j* for gray value NG follows one or two straight lines.

2. The method for coloring an infrared image according to claim 1, wherein: The parameter trajectory follows the following relationship: Among them, the first semi-axis a ell Corresponding to the peak chromaticity c c * divided by 2, and the second semi-axis b is converted into ell Defined as the first semi-axis a ell Function: If the eccentricity e of the ellipse is positive; and If the eccentricity e of the ellipse is negative.

3. The method for coloring an infrared image according to claim 1 or 2, wherein: The method further comprises the following steps: Displaying the rendered image (13) on a screen (14) visible to a user; and The coloring change expected by the user is restored from the human-machine interface (15).

4. The method for coloring an infrared image according to claim 3, wherein: The human-machine interface (15) enables modification of the peak hue h c *, Peak chromaticity c c *, the dynamic range of the polar angle vector Φ, the eccentricity e of at least one ellipse projected into the hue and chrominance plane (O, a*, b*) and / or the dynamic range of the perceived brightness j*.

5. The method for coloring an infrared image according to any one of claims 1 to 4, wherein: The dynamic range of the polar angle vector Φ is limited to between 0 and 2π or between 2π and 0.

6. The method for coloring an infrared image according to any one of claims 3 to 5, wherein: The human-machine interface (15) enables the selection of a predefined color palette.

7. The method for coloring an infrared image according to claim 6, wherein: The first predefined palette (Lifeinred TM Color) has a peak hue h in the range from 20° to 30° c *, Peak chromaticity c in the range from 30 to 40 c *, the dynamic range of the polar angle vector Φ in the range from 0 to 1.8π, the eccentricity e of the ellipse projected into the hue and chromaticity plane (O, a*, b*) in the range from -0.35 to -0.25, and the dynamic range of the perceived brightness j* in the range from 5 to 100.

8. The method for coloring an infrared image according to claim 6 or 7, wherein: The second predefined palette (Lifeinred TM Serenity) has a peak hue in the range from -170° to -160° c *, Peak chromaticity c in the range from 10 to 20 c *, the dynamic range of the polar angle vector Φ in the range from 0 to 2π, the eccentricity e of the ellipse projected into the hue and chromaticity plane (O, a*, b*) in the range from 0.2 to 0.4, and the dynamic range of the perceived brightness j* in the range from 100 to 20.

9. The method for coloring an infrared image according to any one of claims 6 to 8, wherein: The third predefined palette (Lifeinred TM Contrast) has a first portion having a peak hue h in the range from -110° to -100° c *, Peak chromaticity c in the range from 15 to 20 c *, a dynamic range of the polar angle vector Φ in the range of 2π to 0, an eccentricity e of the first ellipse projected into the hue and chromaticity plane (O, a*, b*) in the range of from 0.5 to 0.7, and a dynamic range of the perceived brightness j* in the range of from 100 to 5; and then a second portion having a peak hue h in the range from 25° to 40° c *, Peak chromaticity c in the range from 30 to 40 c *, the dynamic range of the polar angle vector Φ in the range from 0 to 1.8π, the eccentricity e of the second ellipse projected into the hue and chromaticity plane (O, a*, b*) in the range from 0.3 to 0.4, and the dynamic range of the perceived brightness j* in the range from 5 to 100, and the threshold S1 between the two parts is set between the grayscale value NG of 45% and 55%.

10. The method for coloring an infrared image according to any one of claims 6 to 9, wherein: The fourth predefined palette (Lifeinred TM Tracker) having: a first portion having zero chrominance and a dynamic range of perceived brightness j* ranging from 0 to 100; and then a second portion having a peak hue h in the range from 30° to 35° c *, Peak chromaticity c in the range from 30 to 40 c *, the dynamic range of the polar angle vector Φ in the range from 2π to 0, the eccentricity e of the ellipse projected into the hue and chromaticity plane (O, a*, b*) in the range from 0.8 to 0.9, and the dynamic range of the perceived brightness j* in the range from 100 to 30, and the threshold S2 between the two parts is set between 75% and 85% of the grayscale value NG.

Images