WO2014184244A1

WO2014184244A1 - Method for transfering the chromaticity of an example-image to the chromaticity of an image

Info

Publication number: WO2014184244A1
Application number: PCT/EP2014/059844
Authority: WO
Inventors: Oriel FRIGO; Vincent Demoulin; Neus SABATER; Pierre Hellier
Original assignee: Thomson Licensing
Priority date: 2013-05-16
Filing date: 2014-05-14
Publication date: 2014-11-20

Abstract

The invention relates to a method and apparatus for transfering the chromaticity of an example-image to the chromaticity of an image. The method is characterized in that it comprises the following steps: a) obtaining the white point of the example image, b) obtaining the white point of the image, and c) computing a chromatic adaptation transform in order to adapt the white point of the image to the white point of the example image.

Description

Method for transfering the chromaticity of an example-image to the chromaticity of an image 1. Field of invention.

The present invention generally relates to the transfer of colors from one image to another one. 2. Technical background.

This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

Traditionnally, image and video editing is performed using predefined filters that perform "known" operations on histograms or image pixels. The editing can be either automatic (image improvement, histogram equalization, noise reduction, etc.), or driven by the user (manual tuning of a gamma correction, contrast adjustment, etc.).

Color balancing is another editing technique which is an essential part of almost any image processing pipeline. People have the ability of perceptual color constancy. This means that the color of objects will appear the same to someone under a variety of different lighting conditions. For example, a piece of blank sheet of paper will appear white whether it is under the sun at noon or at sunset. The actual light coming from the sheet of paper is quite different in these two situations but to the observer the color of a sheet of paper remains the same. Color balancing can be thought of as a way to bring this ability to computers, i.e. to make an object in an image have the same color even under different illuminants. Color balancing comprises first an illuminant estimation of the image and then a chromatic adaptation of the image which changes the apparent illumination of the image in order to excite the same cone responses in the eye as with a targeted illuminant.

Typically, a chromatic adaptation method is implemented as a linear transformation of a source color represented in the XYZ space by the coordinates (X_S, Y_S, Z_S) in a targeted color X_T, Y_T, Z_T) by a linear transformation M which is dependent of both the source white point (.Xws, Yws, Z_Ws) and a predefined targeted white point (X_WT, Y_WT, Z_WT)

(1 )

where the matrix M is usually given by:

with

The matrix M_A transforms the XYZ color vectors of the image into a cone response domain ρ, γ, β) .

The colors vectors in the cone response domain are thus scaled by factors dependent upon both the source and the targeted whites point and an inverse transform is used to back to the initial XYZ color space.

Note that because the LMS space is a space that resembles the cone responses in human retina, a matrix M_A may also transform the XYZ color vectors into such a LMS space.

Such a chromatic adaptation is well-suited to adapt the colors of an image to a specific lighting source with spectral characteristics (the wavelength distribution of a sun light differs from a LED light for instance) that affect the look of the scene and causing the ambient light to be for instance either cold or warm.

However, color balancing is not suited to homogenize, with an example-image, a set of several images of a same scene which have been captured under different lighting conditions and/or using different capture parameters and/or different capture devices.

One of the problem solved by the invention is overcome this drawback.

3. Summary of the invention.

Generally speaking, the invention adapts the chromaticity of an image in order to adapt the white point of the image to the white point directly obtained from an example-image.

This is different of the usual color balancing which adapts the colors of an image by adapting the white point of the image to a white point of a so called canonical illuminant (D65, A, etc.), i.e. a white point defined according to the wavelength distribution of a specific light source such the sun light or a LED light.

The invention guarantees a smooth example based chromatic adaptation transform and leads to an homogenization of the colors of an image with the colors of an example-image independently of the devices or parameters used to capture such images.

According to one of its aspects, the invention relates to a method for transfering the chromaticity of an example-image to the chromaticity of an image. The method is characterized in that it comprises the following steps: a) obtaining the white point of the example-image,

b) obtaining the white point of the image, and

c) computing a chromatic adaptation transform in order to adapt the white point of the image to the white point of the example-image.

According to an embodiment of the method, the steps b) and c) are repeated until a determined criterium is reached.

Using iterations enables to adjust more precisely the degree of adaptation of the illuminant of the image to the one of the example image. According to another aspects, the invention relates to an apparatus which comprises means configured to implement the method.

The specific nature of the invention as well as other objects, advantages, features and uses of the invention will become evident from the following description of a preferred embodiment taken in conjunction with the accompanying drawings.

4. List of figures. The embodiments will be described with reference to the following figures:

Fig. 1 shows a diagram of an embodiment of the steps of the method. Fig. 2 shows a diagram of an internal structure of an apparatus configured to implement the method.

5. Detailed description of a preferred embodiment of the invention.

An image comprises pixels or image points with each of which is associated at least one item of image data. An item of image data is for example an item of luminance data or an item of chrominance data.

Let I be an input image and E be an example image. The method to transfer the chromaticity (also called dominant colors) of the example image

E to the input image I comprises a step 1 1 a in Fig. 1 in the course of which the white point of the input image I is obtained and a step step 1 1 b in Fig. 1 in the course of which the white point of the example-image E is obtained.

Then, a chromatic adaptation transform is computed (step 12 in Fig. 1 ) in order to adapt the white point of the input image I to the white point of the example-image E.

According to an embodiment, the step 1 1 a and 12 are repeated until a criterium is reached.

According to an embodiment, the criterium is reached when a maximum number of iterations is reached or when no significant changes of the white point of the modified input image since a previous iteration occur. More precisely, the white point of an example image E is obtained. Next, at each iteration, the white point of the input image I is estimated (step 1 1 a), a chromatic adaptation transform (matrix M) is performed, the colors of the input image I are then adapted according to such a transform and the criterium is evaluated.

If the criterium indicates that a new iteration is required, a new white point of the input image I is estimated (step 1 1 a), a new chromatic adaptation transform (step 12) is performed, the colors of the input image I (modified at a previous step) are then adapted according to such the new transform and the criterium is evaluated once again.

When the criterium is reached, the resulting image is an illuminant adjusted image which has the same geometry of the input image I, but with colors adapted to the white point of the example image E.

According to an embodiment, the white point of an image (E or I) is obtained from a memory or from a remote device via a communication network.

According to an embodiment, the white point of an image (E or I) is estimated following a method defined by J. Huo, Y. Chang, J. Wang, and X. Wei, "Robust Automatic White Balance Algorithm using Gray Color Points in Images," 2006, pp. 541 -546. More precisely, the white point of an image is estimated as being the mean of selected gray points of this image. A gray point of the image having the coordinates (Y,U,V) in the YUV color space is selected when the following condition is verified:

\u\+W\ _{< T}

Y

where T is a threshold between 0 and 1 . In the same way, a gray point of the image having the coordinates (L,a,b) in the Lab color space is selected when the following condition is verified:

\a\ + \b\

——— < T

L

and a gray point of the image having the coordinates (R,G,B) in the RGB color space is selected when the following condition is verified: 2(R² + G² + B² - RG - RB - GB)

< T

The invention is not limited to any specific estimation of a white point of an image and may extend to any other white point estimation approach defined in any space.

According to an embodiment, the chromatic adaptation method is a linear transformation given by equation (1 ) with any matrix M_A such as a well- known CAT matrix or any other one to transforms the XYZ color vectors of the image into any space such as LMS or the cone response domain (ρ,γ, β).

According to an embodiment of the step 12, the chromatic adaptation transform is a linear transform defined by equation (1 ).

The matrix M_A may be, for example, a CAT matrix such as CAT02 (Moroney, N.; Fairchild, M. ; Hunt, R. ; Li, C; Luo, R; Newman, T. (November 12 2002) The CIECAM02 Color Appearance Model. IS&T/SID Tenth Color Imaging Conference. Scottsdale, Arizona: The Society for Imaging Science and Technology. ISBN 0-89208-241-0), a CMCCAT2000 matrix or the CAT matrices defined by Bradford or Sharp, etc.

Thus, a simple rescaling of color values of the image based on the ratio of input and example white points adapt the colors of the input image I to appear to have the same illuminant of the example-image E.

The invention is not limited to any kind of linear transform (matrix) to transform the color vectors of the input image (and example image) into another specific space for defining the color adaptation transform.

On Fig. 1 , the modules are functional units, which may or not be in relation with distinguishable physical units. For example, these modules or some of them may be brought together in a unique component or circuit, or contribute to functionalities of a software. A contrario, some modules may potentially be composed of separate physical entities. The apparatus which are compatible with the invention are implemented using either pure hardware, for example using dedicated hardware such ASIC or FPGA or VLSI, respectively « Application Specific Integrated Circuit », « Field- Programmable Gate Array », « Very Large Scale Integration », or from several integrated electronic components embedded in a device or from a brend of hardware and software components.

Figure 2 shows an apparatus 200 that can be used in a system that implements the method of the invention. The apparatus comprises the following components, interconnected by a digital data- and address bus 20:

- a processing unit 23 (or CPU for Central Processing Unit); - a memory 25 ;

- a network interface 24, for interconnection of device 200 to other devices connected in a network via connection 21 .

Processing unit 23 can be implemented as a microprocessor, a custom chip, a dedicated (micro-) controller, and so on. Memory 25 can be implemented in any form of volatile and/or non-volatile memory, such as a RAM (Random Access Memory), hard disk drive, non-volatile random-access memory, EPROM (Erasable Programmable ROM), and so on. Apparatus 200 is suited for implementing a data processing apparatus according to the method of the invention described in relation with the Fig. 1.

Reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one implementation of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments.

Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

While not explicitly described, the present embodiments and variants may be employed in any combination or sub-combination.

Claims

1 . Method for transfering the chromaticity of an example image to the chromaticity of an image, characterized in that it comprises the following steps:

a) obtaining the white point of the example image

b) obtaining the white point of the image, and

c) computing a chromatic adaptation transform in order to adapt the white point of the image to the white point of the example image.

2. Method according to the claim 1 , wherein the steps b) and c) are repeated until a determined criterium is reached.

3. Method according to the claim 2, wherein the criterium is reached when a maximum number of iterations is reached or when no significant changes of the white point of the modified input image since a previous iteration occur.

4. Apparatus for transfering the chromaticity of an example-image to the chromaticity of an image, characterized in that it comprises the following means for:

- obtaining the white point of the image and the white point of the example-image,

- computing a chromatic adaptation transform in order to adapt the white point of the image to the white point of the example-image.

5. Apparatus according to the claim 4, where the means are configured to implement a method according to one of the claims 1 to 3.