WO2014118032A1 - Procédé et dispositif permettant de modifier la plage dynamique d'une image - Google Patents

Procédé et dispositif permettant de modifier la plage dynamique d'une image Download PDF

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Publication number
WO2014118032A1
WO2014118032A1 PCT/EP2014/051116 EP2014051116W WO2014118032A1 WO 2014118032 A1 WO2014118032 A1 WO 2014118032A1 EP 2014051116 W EP2014051116 W EP 2014051116W WO 2014118032 A1 WO2014118032 A1 WO 2014118032A1
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WIPO (PCT)
Prior art keywords
image
image region
pixels
key
dynamic range
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PCT/EP2014/051116
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English (en)
Inventor
Dominique Thoreau
Kadi BOUATOUCH
Rémi COZOT
Ronan BOITARD
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Thomson Licensing
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Publication of WO2014118032A1 publication Critical patent/WO2014118032A1/fr

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    • G06T5/94—
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T5/00—Image enhancement or restoration
    • G06T5/40—Image enhancement or restoration by the use of histogram techniques
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T7/00—Image analysis
    • G06T7/10—Segmentation; Edge detection
    • G06T7/136—Segmentation; Edge detection involving thresholding
    • 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
    • G06T2207/20004—Adaptive image processing
    • G06T2207/20012—Locally adaptive
    • 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
    • G06T2207/20021—Dividing image into blocks, subimages or windows
    • 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

Definitions

  • the invention relates to the general field of modifying the dynamic range of an image.
  • the invention relates to a device and a method for modifying the dynamic range of an image.
  • This device and method can be used, for example, to reduce the dynamic range of an image, that is to say to modify the luminance values of the pixels of this image which belong to a given dynamic value range so as to obtain luminance values which belong to a lower dynamic value range that the initial image.
  • tone mapping operators TMAs
  • tone reproducers to modify the dynamic range of an image called the original image which can be, for example, acquired by a high dynamic range camera so as to obtain an image whose dynamic range is lower (a low dynamic range image) so as to adapt the dynamic range of the original image to that, for example, of a screen on which this image is displayed.
  • the luminance component of this adapted image is quantised and encoded so as to be compatible with a display standard (BT 709, etc.).
  • BT 709, etc. a display standard
  • luminance for its part, corresponds to a physical unit expressed in cd/m 2 .
  • the invention is equally applicable to a luminance component and a luma component.
  • TMOs One of these TMOs is that developed by Reinhard which is commonly called a PTR operator (Reinhard, E., Stark, M., Shirley, P., and Ferwerda, J., ⁇ Photographic tone reproduction for digital images, " ACM Transactions on Graphics 21 (July 2002)) .
  • L white is a luminance value used to ignore zones with high luminance values
  • L d is a matrix whose size is that of the image and which comprises the luminance values of the pixels of the image which are expressed in a lower dynamic value range than that of the original image
  • L s is a matrix whose size is that of the image and which comprises the luminance values obtained by equation (2):
  • N is the number of pixels in the image
  • 5 is a value which avoids any singularity
  • L w (i) is the luminance value of a pixel i of the luminance component L w of the image.
  • the values a and L white are two parameters of this TMO which are fixed, for example, at 18% for parameter a and at the maximum luminance value of the original image for parameter L white .
  • An extension of this operator makes it possible to avoid using a matching curve and instead to modify each luminance component Lw of an original image so as to obtain a modified luminance component L d of the image by using a weighted average over the spatial neighbourhood of each pixel.
  • a pyramid called a Gaussian pyramid
  • a Gaussian pyramid corresponds to such a weighted average. Its successive application corresponds to doubling the size of the spatial neighbourhood. It is defined by the equation:
  • s is the index of the current level of the pyramid and L is a matrix whose size is that of the image and which comprises the averaged luminance values.
  • G corresponds to the weighting applied to each pixel during the averaging.
  • a difference of Gaussian (DoG) is thus defined by:
  • H S L - L _, (5) where Hs is the difference of Gaussian and s is the index of the neighbourhood size.
  • the size of the neighbourhood used is determined when the weighted difference of Gaussian exceeds a certain threshold:
  • Vs is the weighted difference of Gaussian at level s
  • a is the exposure chosen in equation (2)
  • ⁇ is a parameter which controls the degree of conservation of the fronts.
  • TMOs of the prior art use a spatial neighbourhood to modify the dynamic range of a pixel (Kuang, J., Johnson, G. M., & Fairchild, M. D. (2007).
  • iCAM06 A refined image appearance model for HDR image rendering. Journal of Visual Communication and Image Representation, 18(5), 406-414, Li, Y., Sharan, L, & Adelson, E. H. (2005). Compressing and companding high dynamic range images with subband architectures.
  • the invention aims to overcome the disadvantages of the prior art.
  • it aims to define a method for modifying the dynamic image of an image which preserves the spatial coherence of the brightness of the image while preserving the brightness contrasts which exist in the image.
  • the purpose of the invention is to overcome at least one of the disadvantages of the prior art.
  • the invention relates to a method for modifying the dynamic range of an image (IM) by multiplying the luminance values of the pixels of the image by a modification coefficient determined from a value, called a key, which defines an indication of the brightness of the pixels.
  • the method is characterised in that it comprises the following steps:
  • FIG. 1 shows a diagram of the method for modifying the dynamic range of an image according to a preferred embodiment of the invention
  • FIG. 2 shows a diagram of an example of a method for segmenting an image into regions
  • FIG. 3 shows an example of a histogram of the luminance values of an image
  • - Figure 4 shows an example of the internal architecture of a device according to a preferred embodiment of the invention
  • - Figure 5 shows a block diagram of a device for converting an image which implements a method according to the invention.
  • the invention relates to a method for modifying the dynamic range of an image IM by multiplying the luminance values of the pixels of the image by a modification coefficient determined from a value, called a key, which defines an indication of the brightness of the pixels of the image.
  • the method comprises a step 10 of obtaining at least one image region
  • At least one image region Rj is obtained from a memory. This implies that at least one image region Rj has been previously determined and stored.
  • Each image region groups together pixels of image IM which are homogeneous in the sense of a criterion. More than one image region can thus be obtained for the image.
  • At least one image region Rj is determined by image segmentation.
  • the criterion for homogeneity of the pixels of a same image region is the luminance value of these pixels and the segmentation of the image is thus based on luminance values of the pixels of this image.
  • Segmenting the image based on the luminance values of the pixels rather than on spatial characteristics of the images such as salience points or other contours ensures that two image portions spatially separated from each other and which have the same intensity will undergo a same dynamic range correction. Moreover, segmentation based on the intensity of the pixels avoids certain imperfections of the spatial segmentation methods such as the occurrence of outlier values in segmented zones which would influence the dynamic range modification.
  • a histogram H(Lw) of the luminance values (luminance component Lw) of the image to be segmented is calculated (step 1 10) in the logarithmic domain.
  • Local maxima P1 to P4 are then determined (step 1 1 1 ).
  • a local maximum is determined when the number of occurrences exceeds a threshold T1 equal, for example, to 5% of the total number of pixels in the image. Those local maxima which are too close to other local maxima in the sense of a metric are removed.
  • a local maximum for example P2
  • another local maximum for example P3
  • T2 expressed in the logarithmic scale
  • this local maximum here P2
  • the local maxima P1 , P3 and P4 are conserved.
  • Local minima are then determined between each pair of local maxima (step 1 12).
  • two local minima are determined, one between the local maxima P1 and P3 and the other between P3 and P4.
  • These local minima define delimitations of image regions Ri.
  • three image regions R1 , R2 and R3 are determined. They are delimited by dashed vertical lines defined, for example, at the first local minimum found between the two local maxima.
  • the thresholds T1 and T2 are parameters of the segmentation and can be chosen by a user.
  • the segmentation of the image is not restricted to that above which uses a histogram H(Lw) of the image luminance values. Indeed, it can also extend to any other segmentation which does not use a histogram. As an example, it can also extend to segmentations which use other types of histograms but also to any method which defines region delimiters from local maxima, or which defines local maxima from which are defined the image region delimiters.
  • the method also comprises, as shown in Fig. 1 , a step 20 of associating a value, called a key kj, with each image region obtained.
  • a key kj associated with an image region defines an indication of the brightness of the pixels belonging to this image region Rj.
  • a key kj associated with an image region Rj is defined by:
  • Nj is the number of pixels in image region Rj and L J w (i) is the luminance value of a pixel of image region Rj originating from luminance component L w of image IM.
  • the method also comprises a step 30 of modifying the dynamic range of the pixels of at least one image region by multiplying the luminance values of these pixels by a modification coefficient CRj determined from key kj associated with this image region Rj:
  • Lj is the luminance component of image region Rj of image IM and Lj is the modified luminance component of image region Rj.
  • the dynamic range of one or all of the regions of an image can thus be modified.
  • the luminance component L is given by:
  • x is an offset in the modification coefficient fixed for example by a user.
  • This variant makes it possible to modify the gradient of the modification function when the modification coefficient is too small.
  • step 30 it is determined if a pixel belonging to an image region Rj is close, in the sense of a metric, to another image region Rk.
  • the dynamic range of the image region Rj is modified by equation (10) or (1 1 ). This is referred to as direct modification.
  • the modification coefficient CRj,k is then equal to the weighted sum of the modification coefficients relating to each of the image regions Rj and Rk. This is referred to as weighted modification.
  • CRj and CR k are the modification coefficients relating to image regions Rj and Rk and Wj and W k are weighting coefficients given by:
  • W x ⁇ (13) where x designates either index j or index k, N is a normalisation factor and B designates a parameter corresponding to the width of an intersection band situated on either side of a border delimiting image regions Rj and Rk as shown in Figure 3, and ⁇ is
  • This embodiment makes it possible to enhance the spatial contrasts as the brightest image region then has a modification coefficient equal to 1 .
  • the modification coefficient CRj is given by the equation:
  • kj is the key calculated from equation (9) and k v is a key which gives an indication of the brightness of image IM.
  • the brightness indication of image IM is given by a key kv which is calculated by:
  • N the total number of pixels in image IM.
  • the modification coefficient CR t is given by:
  • kf DR is the key which is associated with an image region Rj and calculated according to equation (9) where L J w k) is the luminance value of a pixel of image region Rj originating from luminance component L w of image I M > is tne maximum key from among all keys associated with image region Rj, each of said keys being calculated according to equation (9) for luminance values of the pixels of the image regions originating from luminance component L w of image IM, k DR is the key which is associated with an image region Rj and calculated according to equation (9) where L J w k) is the luminance value of a pixel of image region Rj originating from the luminance component modified according to equation (10) or (1 1 ), and kf ax is the maximum key from among all keys associated with the image regions, each of said keys being calculated according to equation (9) for luminance values of the pixels of the image regions originating from the luminance component modified according to equation (10) or (1 1 ).
  • This latter embodiment ensures the spatial coherence of the brightness of the original image, that is to say that the relative brightnesses of the different modified image regions follow the original relative brightnesses of these regions of image IM.
  • This latter embodiment is particularly advantageous in the case where image IM is the outcome of the application of a TMO to an original image as any type of TMO which applies to an image can then be used.
  • the invention relates to a device 400 for modifying the dynamic range of an image IM described with reference to Figure 4.
  • Device 400 comprises the following elements, interconnected by a digital address and data bus 81 :
  • a calculation unit 43 also called a central processing unit
  • a network interface 44 for interconnections between device 400 and other remote devices connected via a connection 41 ;
  • the calculation unit 43 can be implemented by a (possibly dedicated) microprocessor, a (possibly also dedicated) microcontroller, etc.
  • the memory 45 can be implemented in a volatile and/or non-volatile form such as a RAM (random access memory), a hard disc, an EPROM (erasable programmable ROM), etc.
  • Device 400 is configured to implement a method according to the invention described in relation to Figures 1 to 3.
  • means 43, 44 and possibly 45 cooperate with each other to obtain at least one image region, in order to associate a key with an image region, each key defining an indication of the brightness of the pixels belonging to this image region.
  • Means 43, 44 and possibly 45 also cooperate with each other to modify the dynamic range of the pixels belonging to an image region by multiplying the luminance values of these pixels belonging to this image region by a coefficient determined from the key associated with this image region.
  • the invention relates to a device CONV for converting an image whose luminance values belong to a given dynamic value range (HDR) to an image whose luminance values belong to a lower dynamic value range (LDR) than that of the original image.
  • HDR given dynamic value range
  • LDR lower dynamic value range
  • FIG. 5 shows a block diagram of such an image conversion device which implements an image modification method according to the invention.
  • Device CONV comprises a dynamic range conversion operator TMO which is applied to luminance component Lw of an original image SIO to obtain a modified image whose luminance values belong to a lower dynamic value range LDR than that of image SIO (HDR).
  • TMO dynamic range conversion operator
  • device CONV comprises means GLW for obtaining the luminance component Lw of this colour image SIO.
  • the image is transformed in order to be expressed in the ( ⁇ , ⁇ , ⁇ ) colour space so as to recover the Y channel of the ( ⁇ , ⁇ , ⁇ ) space which forms the luminance component Lw. It is widely known to use such colour space transformation means.
  • Other examples of means GLW can be used without leaving the scope of the invention.
  • Device CONV also comprises means DIV and MULT for the purpose of conserving a constant saturation and the hue of the colours.
  • These means DIV are configured to divide the R, G and B colour components corresponding to a colour image SIO by component Lw and means MULT are configured to multiply the R, G and B colour components thus modified by the modified luminance component.
  • the three components originating from this multiplication are then expressed in floating values.
  • these three components originating from this multiplication are submitted at the input of means Ftol of device CONV and undergo a conversion of their values to whole values which belong to a dynamic value range for the screen on which the modified colour image must be displayed.
  • the operator TMO can be any TMO of the prior art which applies to a still image.
  • the PTR operator described in the introductory section can for example be used.
  • the modified luminance component is obtained from equation (5) or (6).
  • the conversion device also comprises means for modifying the dynamic range represented on the diagram by module C and means IAN for obtaining characteristics of the luminance component Lw. These means are configured so that the dynamic range of the values of the modified luminance component is in turn modified by one of the methods described in relation to one of Figures 1 to 3.
  • the modules shown are functional units that may or may not correspond to physically distinguishable units.
  • these modules or some of them can be grouped together in a single component or circuit, or constitute functions of the same software.
  • some modules may be composed of separate physical entities.
  • the inter-image prediction devices compatible with the invention are implemented according to a purely hardware embodiment, for example in the form of a dedicated component (for example in an ASIC (application specific integrated circuit) or FPGA (field-programmable gate array) or VLSI (very large scale integration) or of several electronic components integrated into a device or even in the form of a mixture of hardware elements and software elements.
  • ASIC application specific integrated circuit
  • FPGA field-programmable gate array
  • VLSI very large scale integration

Abstract

L'invention concerne un procédé et un dispositif permettant de modifier la plage dynamique d'une image (IM) par la multiplication des valeurs de luminance des pixels de l'image grâce à un coefficient de modification déterminé à partir d'une valeur, appelée clé, qui définit une indication de la luminosité des pixels. Le procédé est caractérisé en ce qu'il comprend les étapes consistant à : - obtenir (10) au moins une région d'image (Rj), - associer (20) une clé (kj) à chaque région d'image, ladite clé définissant une indication de la luminosité des pixels appartenant à cette région d'image, et - modifier (30) la plage dynamique des pixels d'au moins une région d'image par la multiplication des valeurs de luminance de ces pixels par un coefficient déterminé à partir de la clé ainsi associée à cette région d'image.
PCT/EP2014/051116 2013-01-29 2014-01-21 Procédé et dispositif permettant de modifier la plage dynamique d'une image WO2014118032A1 (fr)

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EP1857975A2 (fr) * 2006-05-17 2007-11-21 Xerox Corporation Réglage de l'histogramme pour une cartographie d'image de plage hautement dynamique
EP2372638A1 (fr) * 2010-03-04 2011-10-05 Vestel Elektronik Sanayi ve Ticaret A.S. Procédé d'étirement en noir et blanc pour extension de plage dynamique
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EP1857975A2 (fr) * 2006-05-17 2007-11-21 Xerox Corporation Réglage de l'histogramme pour une cartographie d'image de plage hautement dynamique
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