HISTOGRAM AND CHROMINANCE PROCESSING

• US 20090244386A1
• Filed: 03/27/2008
• Published: 10/01/2009
• Est. Priority Date: 03/27/2008
• Status: Active Grant

• Current Assignee: Bang & Olufsen A/S
• Original Assignee: Bang & Olufsen A/S
• Inventors: Norgaard, Lars
• Granted Patent: US 8,159,616 B2
• Time in Patent Office: Days
• US Class Current: 348/672
• CPC Class Codes: G06T 5/40 using histogram techniques
• CPC Class Codes: G06T 5/92 based on global image properties
• CPC Class Codes: H04N 1/6027 Correction or control of colour gradation or colour contrast H04N1/6058 takes precedence
• CPC Class Codes: H04N 1/6088 by viewing conditions, i.e. conditions at picture output
• CPC Class Codes: H04N 21/44008 involving operations for analysing video streams, e.g. detecting features or characteristics in the video stream arrangements characterised by components specially adapted for monitoring, identification or recognition of video in broadcast systems H04H60/59
• CPC Class Codes: H04N 21/4402 involving reformatting operations of video signals for household redistribution, storage or real-time display details of conversion of video standards at pixel level H04N7/01; video transcoding H04N19/40; adapting incoming signals to the display format of the display terminal G09G5/005; media handling at the source in data packet switching networks H04L65/764
• CPC Class Codes: H04N 5/58 in dependence upon ambient light
• CPC Class Codes: H04N 9/68 for controlling the amplitude of colour signals, e.g. automatic chroma control circuits H04N9/71, H04N9/73 take precedence; camera processing pipelines for controlling the colour saturation of colour signals H04N23/86

Abstract

The present invention relates to a method and corresponding system for improving the perceived visual quality of images for instance displayed on a colour television screen. According to the invention, there is provided a means for carrying out histogram processing (13) for scaling an input video signal (6) such that the maximum dynamic range is utilised and where the quality of the perceived image is also optimised. According to the invention, the histogram processing (13) provides a transfer function for the image pixels to obtain enhanced image dynamics and also to provide a chrominance gain (g_c) for scaling the chrominance components, when the luminance has been changed by the histogram processing. According to the invention, the histogram processing can also take variations of the ambient light into account (5). The method and system according to the invention can be implemented by a histogram processing block (13) providing a luminance transfer function followed by a chrominance processing block (14), where the latter represents a selective chroma adjustment considering each individual pixel of an image.

Claims

1. A method for improving the perceived visual quality of images for instance displayed on a colour television screen, the method comprising:

provision of an input video signal (Y′

, U, V) (6);

carrying out a histogram processing (13) on said incoming video signal (6), whereby a luminance transfer function T(k) is obtained and based on said luminance transfer function T(k) calculating a global chrominance gain g_c (16);

for individual pixels of the image carrying out a chrominance processing (14) that associates a local gain g(U_p,V_p) for each individual pixel having the colour components (U_p, V_p);

provision of an output video signal (Y′

, U, V)_OUT2 (18) for individual pixels wherein;

the luminance component Y′

_OUT2 is determined by said luminance transfer function T(k);

the first chrominance component U_OUT2 is given by the expression;

U_OUT2=g_cg(U_p,V_p)U_p; and

the second chrominance component V_OUT2 is given by the expression;

V_OUT2=g_cg(U_p,V_p)V_p;

whereby the chrominance components of the individual pixels of the image are modified by a global gain factor g_c relating to the characteristics of the entire image and a local gain factor g relating to the individual pixels.

2. A method according to claim 1, wherein said luminance transfer function T(k) is a function of the ambient light intensity or corresponding physical quantity τ

(5).

3. A method according to claim 1, wherein said luminance transfer function T(k) is given by the expression:

T(k)=s(p₃(k)+x₂{circumflex over (T)}(k))

wherein s and c₂ are constants;

wherein p₃(k) is a cumulative luminance histogram that is derived from an original, unprocessed luminance histogram p(k) by multiplication hereof with a weight function w(k) that is a function of the bin number k and/or by clipping c(k); and

wherein {circumflex over (T)}(k) is a function of the ambient light intensity or other related physical quantity τ

.

4. A method according to claim 3, wherein said weight function w(k) is the power function given by:

w(k)=k^α

5. A method according to claim 1, wherein an average luminance level Y_AVG over all histogram bins k is determined (21) and wherein said chrominance gain g, is determined as:

g_c= T(Y_AVG)/T(Y_AVG)

where T(Y_AVG) is an identity mapping minus an offset bl;

i.e. T(Y_AVG) is given by the expression;

T(Y_AVG)=Y_AVG−

bl

6. A method according to claim 1, wherein said offset bl is calculated as the sum of two offsets bl₁ and bl₂, where bl₁ is determined from a second cumulative histogram p_c(k) determined by the expression:

$p_C<br><br>\left(k\right)=\underset{i=o}{\overset{k}{\sum <br><br>}}<br><br>p<br><br>\left(i\right),k\in <br><br>\left\{0,\mathrm{\dots <br><br>}<br><br>,N-1\right\}$

where p(i) is the unprocessed histogram and where said bl₁ is then calculated as the luminance level, for which the cumulative proportion exceeds a predetermined, preferably small and fixed, percentage of the sample counts and

where bl₂ is calculated according to the expression;

bl₂=c₁Y_AVG^β

where c₁ and β

are constants and where said total offset bl is then calculated according to the expression;

bl=min[bl₁+bl₂,b_max]

where b_max is a predetermined threshold value.

7. A method according to claim 1, wherein said chrominance processing for individual pixels with chroma components (U_p, V_p) of an image comprises associating a local gain g(U_p, V_p), such that the individual chroma components U_p and V_p for each given pixel is multiplied by said local gain such that processed output chroma components (UV)_OUT1 are determined by the expression:

U_OUT1=g(U_p,V_p)U_p and V_OUT1=g(U_p,V_p)V_p

where said local gain values g are provided by a look-up table (34) using the chroma coordinates (Up, Vp) as index variable in the table.

8. A method according to claim 1, wherein said chrominance processing for individual pixels with chroma components (U_p, V_p) of an image comprises associating a local gain g(U_p, V_p), such that the individual chroma components U_p and V_p for each given pixel is multiplied by said local gain such that processed output chroma components (UV)_OUT1 are determined by the expression:

U_OUT1=g(U_p,V_p)U_p and V_OUT1=g(U_p,V_p)V_p

where said local gain values g are provided by a mathematical function (35) with the chroma coordinates (Up, Vp) as independent variables.

9. A histogram processing method yielding a luminance transfer function T(k) comprising:

the provision of an original, un-processed luminance histogram H(k)=n_k, where k is the bin number;

based on said original, un-processed luminance histogram calculating a normalised histogram p(k)=H(k)/n=n_k/n;

weighting said normalised histogram by a suitable weight function w(k) to obtain a weighted histogram p₁(k)=p(k)w(k);

normalising said weighted histogram, such that;

$\underset{k=0}{\overset{N-1}{\sum <br><br>}}<br><br>p_1<br><br>\left(k\right)=1$

determining a clipped histogram p₂(k)=min[p1(k), c(k)], where c(k) is a function setting the clipping level for bin k;

calculating a first cumulative histogram p₃(k) by the expression;

$p_3<br><br>\left(k\right)=\underset{i=o}{\overset{k}{\sum <br><br>}}<br><br>p_2<br><br>\left(i\right),k\in <br><br>\left\{0,\mathrm{\dots <br><br>}<br><br>,N-1\right\}$

based on the normalised histogram p(k) determining a second cumulative histogram p_c(k) by the expression;

$p_C<br><br>\left(k\right)=\underset{i=o}{\overset{k}{\sum <br><br>}}<br><br>p<br><br>\left(i\right),k\in <br><br>\left\{0,\mathrm{\dots <br><br>}<br><br>,N-1\right\}$

from the normalised histogram determining an average luminance level according to the expression;

$Y_{\mathrm{AVG}}=\underset{k=0}{\overset{N-1}{\sum <br><br>}}<br><br>a<br><br>\left(k\right)<br><br>p<br><br>\left(k\right)$

where a(k) is the average luminance level at bin k;

using said second cumulative histogram p_C(k) and said average luminance level Y_AVG to determine a total off-set bl according to the procedure;

calculating a first off-set bl₁, which the luminance level, for which the cumulative proportion of p_C(k) exceeds a predetermined, preferably small and fixed, percentage of the sample counts;

based on said average luminance level Y_AVG determining a second off-set bl₂ as

bl₂=c₁Y_AVG^β

 

where c₁ and β

are constants;

calculating said total off-set bl as;

bl=min[bl₁+bl₂, b_max], where bl_max is a threshold value;

based on a signal τ

from an ambient light sensor and from said total off-set bl deriving a transfer function {circumflex over (T)}(k) according to the following procedure;

T₁(k)=(k−

bl)^β

and T₂(k)=(k−

bl)^1/β

and

{circumflex over (T)}(k)=τ

T₁(k)+(1−

τ

)T₂(k), τ

ε

[0,1], kε

{0, . . . , N−

1}

based on said first cumulative histogram p₃(k) and said transfer function {circumflex over (T)}(k) determining the final luminance transfer function;

T(k)=s(p₃(k)+c₂{circumflex over (T)}(k))

where s and c₂ are constants.

10. A histogram processing method according to claim 9, where said weight function w(k) is a power function:

w(k)=k^α

.

11. A histogram processing method according to claim 9, wherein a global chrominance gain factor g_c is determined by the expression:

g_c= T(Y_AVG)/T(Y_avg)

where Y_AVG is calculated by the expression;

$Y_{\mathrm{AVG}}=\underset{k=0}{\overset{N-1}{\sum <br><br>}}<br><br>a<br><br>\left(k\right)<br><br>p<br><br>\left(k\right)$

where a(k) is the average level represented by bin k and

where T(Y_AVG) is an identity mapping minus an offset bl;

i.e. T(Y_AVG) is given by the expression;

T(Y_AVG)=Y_AVG−

bl

12. A chrominance processing method comprising

receiving a global chrominance gain g_c;

based on the colour difference components (U, V)_p of individual pixels of an image determining a local chrominance gain g(U_p, V_p), where p denotes the individual pixels of the image, where said local chrominance gain g is either determined with reference to a look-up table comprising gain values designated by the chrominance components (Up, Vp) for the individual pixels or where said chrominance gain g is based on a mathematical function with the chrominance coordinates (Up, Vp) as independent variables;

based on said global gain g_c and said local gain g determining chrominance scaled pixels ps given by;

p_s=(Y′

_pg_cg(U_p,V_p)U_p,g_cg(U_p,V_p)V_p)

13. A system for improving the perceived visual quality of images for instance displayed on a colour television screen, the system comprising:

histogram processing means for providing a luminance transfer function T(k) and a global chrominance gain g_c for controlling the chroma components of individual pixels;

chrominance processing means for providing chroma processing of individual pixels of an image based on individual (local) chrominance gain values for the individual pixels and on said global chrominance gain g_c such that to each chroma component (U, V)p there is associated a total gain value that is the product of said local chrominance gain and said global chrominance gain.

14. A system according to claim 13 comprising means for storing local chrominance gain values for instance in a look-up table.

15. A system according to claim 13 comprising means for determining local chrominance gain g based on a mathematical expression using the chroma components of individual pixels as independent variables.

16. A system according to claim 13 furthermore comprising means for sensing the intensity—

or other related quantity—

of the ambient light and for using this intensity—

or other related quantity—

to modify said luminance transfer function T(k).