CN103973941B - Method and system for adjusting dynamic contrast of digital image or video - Google Patents
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Abstract
The invention discloses a method for adjusting the dynamic contrast of a digital image or video. The method comprises the following steps that a frame of image in the digital image or video is read; a pixel brightness value probability cumulative distribution function cdf() of the image is calculated; when the image is a gray level image, the contrast of the current gray level image is adjusted with a relational expression (please see the specific formula in the specification); when the image is a color image, the contrast of the current color image is adjusted with a relational expression (please see the specific formula in the specification), wherein zi=f(Ei), zi is the output pixel value of a pixel point at exposure brightness Ei, f() is a mapping function, Yi is a brightness value, and Ci is one channel in RGB three color channels. The invention further discloses a system for adjusting the dynamic contrast of the digital image or video.
Description
Technical field
The present invention relates to digital image field, the dynamic contrast method of adjustment of more particularly to a kind of digital image and it is
System.
Background technology
Contrast refers to the measurement of the white different brightness levels between most dark black most bright in image, difference
The bigger contrast of scope is bigger.Because the dynamic range of actual scene is usually in more than 100dB, and traditional digital cameras are adopted
Sensor element (CCD or CMOS) can only typically have the dynamic range of about 60dB, this allows for the photometric system of camera
Affected by highlighted or low clear zone, compressed the quantization ash exponent number in other regions, cause overall contrast to decline, details is lost
Lose.
The enhancing of existing picture contrast can be adjusted using Gamma (gamma) parameters of display, by changing
Become grey scale signal with the relation of brightness to adjust picture contrast.But single contrast settings simultaneously cannot be applied to all images.
For example, the image higher for an overall brightness, user may select the big Gamma parameters of brightness or change right
Picture superposition is made than the setting of degree, but it is such when setting for the relatively low image of brightness, image low-light level can be made
Part it is too dark, and losing details makes image quality be deteriorated.
The content of the invention
Present invention solves the technical problem that a kind of dynamic contrast method of adjustment and system of digital image is to provide, can
Effectively according to the histogram distribution of current scene, adaptively control, enhancing contrast ratio.
To solve above-mentioned technical problem, the invention provides a kind of dynamic contrast method of adjustment of digital image, its bag
Include following steps：
Read piece image or the two field picture in video；
Calculate probability cumulative distribution function cdf () of the pixel brightness value of described image；
Relational expression is adopted when image is gray scale image：The contrast of current gray image is entered
Row adjustment；When image is coloured image, using relational expression：The contrast of current color image is entered
Row adjustment, wherein, zi=f (Ei), ziIt is that pixel is exposing brightness EiOutput pixel value, f () is mapping function, YiIt is bright
Angle value, CiIt is a passage in tri- color channels of RGB.
Wherein, pixel brightness value probability cumulative distribution function cdf () for calculating described image includes：
The brightness histogram of described image is calculated, probability density function pdf () is obtained, calculates right using following relational expression
Probability density function pdf () makees index normalization：
A is exponential factor；
Probability density function pdf () normalization is distributed using following relational expression：
Probability cumulative distribution function is calculated using following relational expression：
Wherein, step is further included：
Adjust exponential factor a to control the intensity of contrast lifting.
Wherein, step is further included：Image is carried out to keep away sudden strain of a muscle process.
Wherein, it is described that image is carried out keeping away sudden strain of a muscle and processing to include：
Using to cdf () amplitude limit, if zi-cdf(Zi) ＞ T (zi), then zi-cdf(Zi)=T (Zi)；If zi-cdf
(Zi) ＜-T (zi), then zi-cdf(Zi)=- T (Zi), wherein T (Zi) be and ziRelevant threshold function table.
Wherein, it is described that image is carried out keeping away sudden strain of a muscle and processing to include：
By the way of interframe is smooth, using following relational expression：Image is carried out to keep away at sudden strain of a muscle
Reason, wherein α be interframe smoothing factor, f (Zi) it is GTG mapping function.
To solve above-mentioned technical problem, the present invention provides a kind of dynamic contrast adjustment system of digital image, the system
System includes：
One reading unit, for the two field picture in reading piece image or video；
One probability cumulative distribution computing unit, for calculating the pixel brightness value probability cumulative distribution function of described image
cdf(·)；
One contrast adjustment unit, when image is gray scale image, the setting contrast unit adopts relational expression：The contrast of current gray image is adjusted；When image is coloured image, the contrast
Adjustment unit adopts relational expression：The contrast of current color image is adjusted, wherein, zi=f
(Ei), ziIt is that pixel is exposing brightness EiOutput pixel value, f () is linear function, YiIt is brightness value, CiIt is RGB tri-
A passage in color channel.
Wherein, the system further includes probability density computing unit and a normalization unit, the probability density
Computing unit is used to calculate the brightness histogram of described image, obtains probability density function pdf (), and the normalization unit is adopted
Calculated with following relational expression and index normalization is made to probability density function pdf ()：
A is exponential factor；
The normalization unit is distributed probability density function pdf () normalization using following relational expression：
The probability cumulative distribution computing unit calculates probability cumulative distribution function using following relational expression：
Wherein, the system further includes a parameter adjustment unit, controls contrast for adjusting exponential factor a and carries
The intensity for rising.
Wherein, the system further includes that one keeps away sudden strain of a muscle processing unit, and the sudden strain of a muscle processing unit of keeping away is used to avoid video from broadcasting
Put, the problem of the film flicker that real-time adjustment is brought.
Wherein, the sudden strain of a muscle processing unit of keeping away carries out keeping away sudden strain of a muscle process to cdf () amplitude limit to image, works as zi-cdf(Zi) ＞ T
(zi) when, then zi-cdf(Zi)=T (Zi)；Work as zi-cdf(Zi) ＜-T (zi) when, then zi-cdf(Zi)=- T (Zi), wherein T (Zi)
It is and ziRelevant threshold function table.
Wherein, described keeping away dodges processing unit by the way of interframe is smooth, using following relational expression：
Carry out keeping away sudden strain of a muscle to image to process, wherein α is interframe smoothing factor, f (Zi) it is GTG mapping function.
Relative to prior art, by the dynamic contrast method of adjustment and system of above-mentioned digital image, can be effectively
Strengthen the dynamic contrast of digital image.
Description of the drawings
Fig. 1 is the dynamic contrast method of adjustment schematic flow sheet of the digital image of the embodiment of the present invention；
Fig. 2 is that the effect before and after the dynamic contrast method of adjustment of the digital image of the embodiment of the present invention compares signal
Figure；
Fig. 3 is the module diagram that the dynamic contrast of the digital image of the embodiment of the present invention adjusts system.
Specific embodiment
Refer to Fig. 1, the schematic flow sheet of the dynamic contrast method of adjustment of digital image provided in an embodiment of the present invention.
As shown in figure 1, the dynamic contrast method of adjustment of the digital image comprises the steps：
S101：Piece image or a certain frame in video are read, I is designated as.
S102：The brightness histogram of I is calculated, probability density function pdf () is obtained.
Described calculating brightness histogram, namely probability density function pdf () refer to calculate I in corresponding grey scale level picture
The number of times that element occurs, forms distribution of the pixel value in whole luminance dynamic range in I.Wherein rectangular histogram can be designated as one to
Amount H=[p1, p2..., pn].The vector meets：
pi=# (x, y) | I (x, y)=xi}；
Wherein xiRepresent any gray level.# (x, y) | I(x, y)=xiRepresent that pixel of the brightness value equal to xi is constituted in I
Set in element number.I(x, y)Represent the brightness value positioned at coordinate (x, y) place pixel.This set is designated as into Xi={ (x, y)
|I(x, y)=xi}.The luminance dynamic range refers to that all possible brightness value determined by the number of bits of pixel value in I goes out
Existing scope.If pixel value is a k bit number, dynamic range is 0 to 2k-1.In the present embodiment, it is considered to 8 bit images,
Then dynamic range 0 to 255.
For gray-scale maps, need to only rectangular histogram H be calculated to the luminance channel of I.For cromogram, need respectively to RGB tri-
Path computation goes out three rectangular histograms Hc, c=R, G, B.
S103：The index normalization of probability density function pdf () is calculated, and by probability density function pdf () normalizing
Change distribution.
In the present embodiment, the index normalization of probability density function pdf () is calculated using following relational expression：
A is exponential factor, and it can be arranged according to demand and arbitrarily.
In the present embodiment, probability density function pdf () normalization is distributed using following relational expression：
S104：Calculate probability cumulative distribution function cdf ().
In the present embodiment, probability cumulative distribution function is calculated using following relational expression：
Cdf () is probability cumulative distribution function, i.e. Probability (Z ＜ zi)；zi=f (Ei), ziExist for pixel
Exposure brightness EiOutput pixel value.
It is understood that also can step S103 omit, and directly calculate probability cumulative distribution function cdf (), not
It is limited to the present embodiment.
S105：Following GTG mapping function is adopted when image is gray scale imageTo current gray
The contrast of image is adjusted；When image is coloured image, adopt with minor functionTo current color
The contrast of color image is adjusted.
Wherein, f () is mapping function, and Yi is brightness value, CiIt is a passage in tri- color channels of RGB.Due to
Sample space is image itself, GTG mapping functionItself it is adaptive.Can see, work as zi=
When 1,And work as ziWhen=0,So the GTG mapping functionItself be bounded and
It is normalized.In the present embodiment, ziNormalize to 0～1, it is considered to 8 bit images, then 0≤i≤255, with one 255 take advantage of because
Son quantifies to 8bit.
In S105, whole digital image, i.e. each pixel of image I are traveled through.When image is RGB color image, then
Brightness value Y is asked for, then using relational expressionCalculate new color pixel values.When image is gray-scale figure,
Using relational expressionGTG mapping value is calculated as new pixel value.
In the present embodiment, following two step is further included：
S106：Regulation parameter, controls the intensity that contrast is lifted.
Without loss of generality, in the present embodiment, by making index normalization to probability density function pdf (), so as to letter
Number cdf () is corrected, and accomplishes more preferable parameter regulation.
By relational expression：Relational expression：And relational expression：Can see, a is more big, cdf more tends to linear, therefore exponential factor a can be adjusted
To control the intensity of contrast lifting.
S107：Image is carried out to keep away sudden strain of a muscle process.
Consider video playback, the film flicker that real-time adjustment should be avoided to bring.The present embodiment is adopted to cdf () amplitude limit
The problem of film flicker of the method to avoid real-time adjustment from bringing.If i.e. zi-cdf(Zi) ＞ T (zi), then zi-cdf(Zi)=T
(Zi)；If zi-cdf(Zi) ＜-T (zi), then zi-cdf(Zi)=- T (Zi), wherein T (Zi) be and ziRelevant threshold function table,
A kind of special case is T (Zi)=T0。
It is understood that the film flicker for avoiding real-time adjustment from bringing can also be reached by the way of interframe is smooth
Problem.Such as adopt following relational expression：Image is carried out to keep away sudden strain of a muscle process, wherein α is smooth for interframe
The factor, f (Zi) it is GTG mapping function.
In other embodiments, if computing resource is less, step S103 can be omitted；If computing resource is enough,
The present embodiment controls the intensity of contrast lifting using step S103.If only carrying out contrast lifting to single frames picture,
Step S106 is not required, if carrying out contrast lifting to video, step S107 can effectively avoid film flicker.And
It is not limited to the present embodiment.
By Fig. 2, effect contrast figure before and after dynamic contrast regulation, it can be seen that by the dynamic right of above-mentioned digital image
Than degree method of adjustment, can effectively strengthen the dynamic contrast of digital image.
Fig. 3 is referred to, Fig. 3 is the structural representation of the adjustment system of the dynamic contrast of the digital image of the embodiment of the present invention
Figure.As shown in figure 3, the adjustment system 20 includes：One reading unit 21, probability density computing unit 22, normalization unit 23,
The contrast adjustment unit 25 of probability cumulative distribution computing unit 24 and.
In embodiments of the present invention, the reading unit 21 is used to read piece image or a certain frame in video, is designated as
I.The probability density computing unit 22 is connected with the reading unit 21, and the probability density computing unit 22 is used to calculate I
Brightness histogram, obtain probability density function pdf ().The normalization unit 23 and the probability density computing unit 22
Connection, the normalization unit 23 is used to calculating the index normalization of probability density function pdf (), and by probability density function
Pdf () normalization is distributed.The probability cumulative distribution computing unit 24 is connected with the normalization unit 23, and the probability tires out
Integration cloth computing unit 24 is used to calculate probability cumulative distribution function cdf ().The setting contrast unit 25 is general with described
Rate cumulative distribution function computing unit 24 connects, and the setting contrast unit 25 is used to be adjusted the contrast of image,
That is, when image is gray scale image, the setting contrast unit 25 adopts following GTG mapping functionThe contrast of current gray image is adjusted；When image is coloured image, the contrast
Adjustment unit 25 adopts relationship belowThe contrast of current color image is adjusted.
In embodiments of the present invention, described calculating brightness histogram, namely probability density function pdf () refers to calculating
The number of times that the pixel of corresponding grey scale level occurs in I, forms distribution of the pixel value in whole luminance dynamic range in I.It is wherein straight
Square figure can be designated as a vector H=[p1, p2..., pn].The vector meets：
pi=# (x, y) | I(x, y)=xi}；
Wherein xiRepresent any gray level.# (x, y) | I(x, y)=xiRepresent that pixel of the brightness value equal to xi is constituted in I
Set in element number.I(x, y)Represent the brightness value positioned at coordinate (x, y) place pixel.This set is designated as into Xi={ (x, y)
|I(x, y)=xi}.The luminance dynamic range refers to that all possible brightness value determined by the number of bits of pixel value in I goes out
Existing scope.If pixel value is a k bit number, dynamic range is 0 to 2k-1.In the present embodiment, it is considered to 8 bit images,
Then dynamic range 0 to 255.
For gray-scale maps, need to only rectangular histogram H be calculated to the luminance channel of I.For cromogram, need respectively to RGB tri-
Path computation goes out three rectangular histograms Hc, c=R, G, B.
In the present embodiment, the normalization unit 23 is adopted to calculating probability density function pdf using relationship below
The index normalization of ()：
A is exponential factor, and it can be arranged according to demand and arbitrarily.
In the present embodiment, the normalization unit 23 adopts following relational expression by probability density function pdf () normalization
Distribution：
In the present embodiment, the probability cumulative distribution computing unit 24 calculates probability cumulative distribution letter using following relational expression
Number：
Cdf () is probability function, i.e. Probability (Z ＜ zi)；zi=f (Ei), ziIt is that pixel is exposing brightness Ei
Output pixel value.
In the present embodiment, when image is gray scale image, the setting contrast unit 25 maps letter using following GTG
NumberThe contrast of current gray image is adjusted；When image is coloured image, the contrast
Degree adjustment unit 25 is adopted with minor function f (Ci)=The contrast of current color image is adjusted.
Wherein, f () is mapping function, YiIt is brightness value, CiIt is a passage in tri- color channels of RGB.Due to sample
This space is image itself, GTG mapping functionItself it is adaptive.Can see, work as zi=1
When,And work as ziWhen=0,So the GTG mapping functionItself it is bounded and returns
One change.
In the present embodiment, ziNormalize to 0～1, it is considered to 8 bit images, then 0≤i≤255, with one 255 the factor is taken advantage of
Quantify to 8bit.
In the present embodiment, in order to control the intensity of contrast lifting, the adjustment system 20 of the dynamic contrast is further
Including a parameter adjustment unit 26.
The parameter adjustment unit 26 and the setting contrast unit 25.The adjustment system 20 of the dynamic contrast is led to
Cross and index normalization is made to probability density function pdf (), so as to correct to function cdf (), accomplish that more preferable parameter is adjusted
Section.
By relational expression：Relational expression：And relational expression：Can see, a is more big, cdf more tends to linear, therefore the parameter adjustment unit
26 control the intensity of contrast lifting by adjusting exponential factor a.
In the present embodiment, in order to avoid video playback, the problem of the film flicker that real-time adjustment is brought, the dynamic contrast
The adjustment system 20 of degree further includes that one keeps away sudden strain of a muscle processing unit 27.It is described keep away sudden strain of a muscle processing unit 27 also with the setting contrast
Unit 25.Described keeping away dodges the film flicker that processing unit 27 avoids real-time adjustment from bringing using the method to cdf () amplitude limit
Problem.If i.e. zi-cdf(zi) ＞ T (zi), then zi-cdf(zi)=T (zi)；If zi-cdf(zi) ＜-T (zi), then zi-
cdf(zi)=- T (zi), wherein T (zi) be and ziA kind of relevant threshold function table, special case is T (zi)=T0。
It is understood that it is described keep away sudden strain of a muscle processing unit 27 can also using interframe it is smooth by the way of come reach avoid it is real-time
The problem of the film flicker that adjustment brings.Such as adopt following relational expression：αzi+(1-α)f(Zi), image is carried out to keep away at sudden strain of a muscle
Reason, wherein α be interframe smoothing factor, f (Zi) it is GTG mapping function.
In other embodiments, the rate density computing unit 22 and the normalization unit 23 can be omitted, and by institute
State reading unit 21 to be directly connected with the probability cumulative distribution computing unit 24, the probability cumulative distribution computing unit 24 is straight
Connect and obtain probability cumulative distribution function, however it is not limited to the present embodiment.
In several embodiments provided by the present invention, it should be understood that disclosed system, apparatus and method can be with
Realize by another way.For example, device embodiment described above be only it is schematic, for example, the module or
The division of unit, only a kind of division of logic function can have other dividing mode, such as multiple units when actually realizing
Or component can with reference to or be desirably integrated into another system, or some features can be ignored, or not perform.It is another, institute
The coupling each other for showing or discussing or direct-coupling or communication connection can be by some interfaces, device or unit
INDIRECT COUPLING or communication connection, can be electrical, mechanical or other forms.
The unit as separating component explanation can be or may not be it is physically separate, it is aobvious as unit
The part for showing can be or may not be physical location, you can with positioned at a place, or can also be distributed to multiple
On NE.Some or all of unit therein can according to the actual needs be selected to realize the mesh of this embodiment scheme
's.
In addition, each functional unit in each embodiment of the invention can be integrated in a processing unit, it is also possible to
It is that unit is individually physically present, it is also possible to which two or more units are integrated in a unit.Above-mentioned integrated list
Unit both can be realized in the form of hardware, it would however also be possible to employ the form of SFU software functional unit is realized.
If the integrated unit is realized using in the form of SFU software functional unit and as independent production marketing or used
When, during a computer read/write memory medium can be stored in.Based on such understanding, technical scheme is substantially
The part for contributing to prior art in other words or all or part of the technical scheme can be in the form of software products
Embody, the computer software product is stored in a storage medium, including some instructions are used so that a computer
Equipment (can be personal computer, server, or network equipment etc.) or processor (processor) perform the present invention each
The all or part of step of embodiment methods described.And aforesaid storage medium includes：USB flash disk, portable hard drive, read only memory
(ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), magnetic disc or CD
Etc. it is various can be with the medium of store program codes.
Embodiments of the invention are the foregoing is only, the scope of the claims of the present invention is not thereby limited, it is every using this
Equivalent structure or equivalent flow conversion that bright description and accompanying drawing content are made, or directly or indirectly it is used in other related skills
Art field, is included within the scope of the present invention.
Claims (12)
1. the dynamic contrast method of adjustment of a kind of digital image, it comprises the steps：
Read piece image or the two field picture in video；
Calculate pixel brightness value probability cumulative distribution function cdf () of described image；
Relational expression is adopted when image is gray scale image：The contrast of current gray image is adjusted
It is whole；When image is coloured image, using relational expression：The contrast of current color image is adjusted
It is whole, wherein, Zi=f (Ei), ZiIt is that pixel is exposing brightness EiOutput pixel value, f () is mapping function, YiIt is brightness
Value, CiIt is a passage in tri- color channels of RGB.
2. method according to claim 1, it is characterised in that the pixel brightness value probability accumulation of the calculating described image
Distribution function cdf () includes：
The brightness histogram of described image is calculated, probability density function pdf () is obtained, is calculated to probability using following relational expression
Density function pdf () makees index normalization：
A is exponential factor；
Probability density function pdf () normalization is distributed using following relational expression：
Probability cumulative distribution function is calculated using following relational expression：
3. method according to claim 2, it is characterised in that further include step：
Adjust exponential factor a to control the intensity of contrast lifting.
4. method according to claim 2, it is characterised in that further include step：Image is carried out to keep away sudden strain of a muscle process.
5. method according to claim 4, it is characterised in that described to carry out keeping away sudden strain of a muscle and processing to image including：
Using to cdf () amplitude limit, if Zi-cdf(Zi) ＞ T (Zi), then Zi-cdf(Zi)=T (Zi)；If Zi-cdf(Zi)
＜-T (Zi), then Zi-cdf(Zi)=- T (Zi), wherein T (Zi) be and ZiRelevant threshold function table.
6. method according to claim 4, it is characterised in that described to carry out keeping away sudden strain of a muscle and processing to image including：
By the way of interframe is smooth, using following relational expression：Z′i=α Zi+(1-α)f(Zi), image is carried out to keep away sudden strain of a muscle process,
Wherein α be interframe smoothing factor, f (Zi) it is GTG mapping function.
7. a kind of dynamic contrast of digital image adjusts system, and the system includes：
One reading unit, for the two field picture in reading piece image or video；
One probability cumulative distribution computing unit, for calculating the pixel brightness value probability cumulative distribution function cdf of described image
(·)；
One contrast adjustment unit, when image is gray scale image, the setting contrast unit adopts relational expression：The contrast of current gray image is adjusted；When image is coloured image, the contrast
Adjustment unit adopts relational expression：The contrast of current color image is adjusted, wherein, Zi=f
(Ei), ZiIt is that pixel is exposing brightness EiOutput pixel value, f () is linear function, YiIt is brightness value, CiIt is RGB tri-
A passage in color channel.
8. system according to claim 7, it is characterised in that the system further includes probability density computing unit
With a normalization unit, the probability density computing unit is used to calculate the brightness histogram of described image, obtains probability density
Function pdf (), the normalization unit is calculated using following relational expression makees index normalizing to probability density function pdf ()
Change：
A is exponential factor；
The normalization unit is distributed probability density function pdf () normalization using following relational expression：
The probability cumulative distribution computing unit calculates probability cumulative distribution function using following relational expression：
9. system according to claim 8, it is characterised in that the system further includes a parameter adjustment unit, uses
The intensity of contrast lifting is controlled in exponential factor a is adjusted.
10. system according to claim 8, it is characterised in that the system further includes that keeps away sudden strain of a muscle processing unit, institute
State and keep away sudden strain of a muscle processing unit for avoiding video playback, the problem of the film flicker that real-time adjustment is brought.
11. systems according to claim 10, it is characterised in that described to keep away sudden strain of a muscle processing unit to cdf () amplitude limit to figure
As carrying out keeping away sudden strain of a muscle process, work as Zi-cdf(Zi) ＞ T (Zi) when, then Zi-cdf(Zi)=T (Zi)；Work as Zi-cdf(Zi) ＜-T (Zi) when,
Then Zi-cdf(Zi)=- T (Zi), wherein T (Zi) be and ZiRelevant threshold function table.
12. systems according to claim 10, it is characterised in that described keeping away dodges the side that processing unit is smoothed using interframe
Formula, using following relational expression：Z′i=α Zi+(1-α)f(Zi), carrying out keeping away sudden strain of a muscle to image and process, wherein α is interframe smoothing factor, f
(Zi) it is GTG mapping function.
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| CN105791790B (en) * | 2014-12-25 | 2018-09-04 | 小米科技有限责任公司 | Image processing method and device |
| CN106557729B (en) * | 2015-09-30 | 2021-12-21 | 日本电气株式会社 | Apparatus and method for processing face image |
| CN109493291A (en) * | 2018-10-29 | 2019-03-19 | 南京邮电大学 | A kind of method for enhancing color image contrast ratio of adaptive gamma correction |
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