Luminance changing image processing with color constraints

Patent metadata

Field	Value
Publication number	US-9230509-B2
Application number	US-201314402135-A
Country	US
Kind code	B2
Filing date	Sep 17, 2013
Priority date	Oct 8, 2012
Publication date	Jan 5, 2016
Grant date	Jan 5, 2016

Abstract

To be able to do good color mapping between color encodings for gamuts with considerably different luminance dynamic range while not introducing significant color errors, we describe an image color processing apparatus ( 201 ) arranged to transform an input color (L,x,y) of a pixel specified in a color representation corresponding to a first luminance dynamic range into an output color (L*,x,y) of a pixel specified in a color representation corresponding to a second luminance dynamic range, which first and second dynamic ranges differ in extent by at least a multiplicative factor 1.5, comprising a tone mapping deformation unit ( 203 ) arranged to determine on the basis of an input tone mapping ( 301 ) and a quantity linearly related to the luminance (L) of the input color, an adjusted output luminance (L*, 309 ), wherein the determining is arranged so that the adjusted output luminance (L*, 309 ) obtained by applying the input tone mapping to the input luminance (L) of all possible input luminances in its extent of valid values [0,1] giving the highest output (L*,L_HDR), is not higher than a maximum luminance Lmax(x,y) which for the chromatic coordinates (x,y) of the input color is maximally achievable in the gamut corresponding to the second luminance dynamic range for those chromatic coordinates (x,y).

First claim

The invention claimed is:

1. An image color processing apparatus comprising:
a tone mapping deformation unit arranged to determine an adjusted output luminance for the output color from an input luminance of the input color and on the basis of an input tone mapping defining output luminances as a function of input luminances, and a quantity linearly related to the input luminance, the quantity being indicative of a maximum luminance which for the chromatic coordinates of the input color is maximally achievable in at least one of a gamut corresponding to the first luminance dynamic range and a gamut corresponding to the second luminance dynamic range for those chromatic coordinates;
wherein the image color processing apparatus is arranged to transform an input color of a pixel specified in a color representation corresponding to a first luminance dynamic range into an output color of a pixel specified in a color representation corresponding to a second luminance dynamic range, which first and second dynamic ranges differ in extent by at least a multiplicative factor of 1.5,
wherein the tone mapping deformation unit is arranged to generate the adjusted output luminance by applying an adjusted tone mapping to the input luminance; and
the tone mapping deformation unit is arranged to determine the adjusted tone mapping by adapting the input tone mapping dependent on the quantity so that any adjusted output luminance obtained by applying the adjusted tone mapping to any input luminance of all possible input luminances in its range of valid values 0 to 1 is not higher than a maximum luminance which for the chromatic coordinates of the input color is maximally achievable in the gamut corresponding to the second luminance dynamic range for those chromatic coordinates.

2. An image color processing apparatus as claimed in claim 1 , in which the tone mapping deformation unit is further arranged to apply a monotonic transformation to the functional shape of the input tone mapping as compared to its axis, which is defined so that if the output luminances, obtained by applying the input tone mapping to two different input luminances, have a non-zero difference with a sign, then the adjusted output luminances for those input luminances also have a non-zero difference with the same sign.

3. An image color processing apparatus as claimed in claim 2 , in which the tone mapping deformation unit is further arranged to apply a smooth transformation to the functional shape of the input tone mapping as compared to its axis, determined to have successive luminance differences for neighboring chromatic coordinates between the adjusted output luminance and the output luminance obtained by applying the adjusted tone mapping to the input color luminance varying smoothly.

4. An image color processing apparatus as claimed in claim 1 , in which the tone mapping deformation unit is further arranged to perform the determining of the adjusted output luminance by applying an adjusted luminance as input to the input tone mapping, which adjusted luminance is derived by applying a function to the input color luminance, which function is defined based on the maximum luminance, so that an input luminance equal to this maximum luminance is mapped to the maximum possible value of the input luminance for the input tone mapping.

5. An image color processing apparatus as claimed in claim 4 , wherein the function applicable to the input color luminance is a linear scaling.

6. An image color processing apparatus as claimed in claim 1 wherein the tone mapping deformation unit is arranged to generate a scaled luminance by scaling the input luminance by a first scale factor determined from the quantity; determine a scaled adjusted luminance by applying the input tone mapping to the scaled luminance; and generating the adjusted output luminance by scaling the scaled adjusted luminance by a second scale factor corresponding to the reciprocal of the first scale factor.

7. An image color processing apparatus as claimed in claim 1 , wherein the tone mapping deformation unit is further arranged to use as the quantity linearly related to the luminance a maximum of a color channel component defining the input color.

8. An image color processing apparatus as claimed in claim 7 wherein the tone mapping deformation unit is arranged to generate an adjusted luminance by applying the input tone mapping to the maximum of the color channel components; and to generate the adjusted output luminance by scaling the adjusted luminance by a scale factor dependent on the input luminance and the maximum of the color channel components.

9. An image color processing apparatus as claimed in claim 7 wherein the input color is provided in a color channel representation and the apparatus further comprises a color representation converter for generating the input luminance as a luminance of a chromaticity and luminance representation of the input color by converting the color channel representation of the input color.

10. An image color processing apparatus as claimed in claim 7 further comprising an outputter arranged to generate a color channel representation output color by scaling the color channel components of the input color by a scale factor dependent on the adjusted output luminance and the input luminance.

11. An image color processing apparatus as claimed in claim 1 , further comprising a tone mapping determining unit arranged to determine the input tone mapping on the basis of at least one predetermined tone mapping.

12. An image encoder comprising an image color processing apparatus as claimed in claim 1 and a data formatter for outputting at least an output image comprising pixel output colors of chromatic coordinates and adjusted output luminances.

13. An image decoder comprising an image color processing apparatus as claimed in claim 1 , and comprising a data extractor arranged to obtain color data of pixels in an image.

14. An image decoder as claimed in claim 13 , in which the data extractor is further arranged to obtain a tone mapping.

15. A method of image color processing comprising:
transforming an input color of a pixel specified in a color representation corresponding to a first luminance dynamic range into an output color of a pixel specified in a color representation corresponding to a second luminance dynamic range, which first and second dynamic ranges differ in extent by at least a multiplicative factor 1.5, the transforming comprising;
determining an adjusted output luminance for the output color from an input luminance of the input color and on the basis of an input tone mapping defining output luminances as a function of input luminances, and a quantity linearly related to the input luminance, the quantity being indicative of a maximum luminance which for the chromatic coordinates of the input color is maximally achievable in at least one of a gamut corresponding to the first luminance dynamic range and a gamut corresponding to the second luminance dynamic range for those chromatic coordinates;
wherein the adjusted output luminance is determined by applying an adjusted tone mapping to the input luminance; and determining the adjusted tone mapping by adapting the input tone mapping dependent on the quantity so that any adjusted output luminance obtained by applying the adjusted tone mapping to any input luminance of all possible input luminances in its range of valid values 0 to 1 is not higher than a maximum luminance which for the chromatic coordinates of the input color is maximally achievable in the gamut corresponding to the second luminance dynamic range for those chromati

Assignees

• Koninkl Philips Nv

Inventors

• Van Der Vleuten Renatus Josephus

Classifications

• G09G5/02 Primary

  characterised by the way in which colour is displayed {(details of colour display specific for CRTs G09G1/28; specific for flat matrix panels other than liquid crystal displays G09G3/2003; specific for liquid crystal displays G09G3/3607)} · CPC title

• H04N1/60

  Colour correction or control {(H04N1/54 takes precedence)} · CPC title

• H04N1/6027

  Correction or control of colour gradation or colour contrast (H04N1/6058 takes precedence) · CPC title

• H04N5/57

  Control of contrast or brightness · CPC title

• G09G2320/0271

  Adjustment of the gradation levels within the range of the gradation scale, e.g. by redistribution or clipping · CPC title