ILLUMINANT ESTIMATION

• US 20100172574A1
• Filed: 11/08/2007
• Published: 07/08/2010
• Est. Priority Date: 11/08/2006
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Finlayson, Graham D., Fredembach, Clement
• Granted Patent: US 8,385,637 B2
• Time in Patent Office: Days
• US Class Current: 382/162
• CPC Class Codes: G06T 5/50 using two or more images, e.g. averaging or subtraction
• CPC Class Codes: G06T 7/90 Determination of colour characteristics
• CPC Class Codes: G06T 7/97 Determining parameters from multiple pictures depth or shape recovery from multiple images G06T7/55; stereo camera calibration G06T7/85

Abstract

In a method of chromagenic illuminant estimation pixels from mutually-corresponding images with different filtering (e.g. a filtered image and an unfiltered image) are compared, a fraction of the brightest pixels being selected for a subsequent chromagenic estimation. The pixels may be at corresponding locations or they may correspond in that their mean brightness is in the same rank order. In one method, in which, in a first preprocessing stage, for a database of m lights E_i (λ) and n surfaces S_j (λ) there is calculated T_i˜Q^F Q⁺ where Q₁^F and Q_F represent the matrices of unfiltered and filtered sensor responses to the n surfaces under the i th light and + denotes an inverse, and in a second operation stage, given P surfaces in an image and 3×P matrices Q and Q^F, from these matrices there are chosen the r% brightest pixels giving the matrices Q′ and Q′^F, and the scene illuminant P_est is estimated where formula (I) and (II).

Claims

1. A method of chromagenic illuminant estimation in which, from mutually corresponding images with different sets of spectral components, a fraction of the brightest pixels are selected for subsequent chromagenic estimation.

2. A method according to claim 1 wherein the images have a different filtering.

3. A method according to claim 2, wherein the images comprise a filtered image and an unfiltered image.

4. A method according to claim 1, wherein there are compared pixels with their mean brightness in the same rank order.

5. A method according to claim 1, wherein there are compared pixels in the images which are in the same pixel location.

6. A method according to claim 1, wherein 0.5 to 20% of the brightest pixels are selected.

7. A method according to claim 6, wherein 1 to 3% of the brightest pixels are selected.

8. A method according to claim 1 employing a chromagenic algorithm which works by comparing m responses in one image to a corresponding n responses in another image.

9. A method according to claim 1 wherein:

a. in a first preprocessing stage, for a database of m lights E_i (λ

) and n surfaces S_j (λ

) there is calculated T_i≈

Q_i^FQ_i⁺ where Q_i and Q_i^F represent the matrices of unfiltered and filtered sensor responses to the n surfaces under the i th light and + denotes an inverse, and

b. in a second operation stage, given P surfaces in an image and 3×

P matrices Q and Q^F, from these matrices there are chosen the r% brightest pixels giving the matrices Q′ and

Q′

^F and the scene illuminant ρ

_est is estimated where

$\mathrm{est}=\underset{i}{\min }<br><br>\left({\mathrm{err}}_i\right)<br><br>\left(i=1,2,\mathrm{\dots <br><br>}<br><br>,m\right)$ $\mathrm{and}$ ${\mathrm{err}}_i=<br><br>T_i<br><br>Q^{\mathrm{\prime <br><br>}}-Q^{\mathrm{\prime <br><br>}<br><br><br><br>F}<br><br>$

13. An image treatment system comprising means for estimating an illuminant in accordance with claim 1, and means for using the estimate obtained to remove from the image being treated the colour bias due to the illuminant.

14. A method of chromagenic illuminant estimation according to claim 1, further including the steps of:

a. removing the colour bias due to illumination from one of the images, and

b. rendering the image.

15. A method of chromagenic illuminant estimation according to claim 1 combined with a gamut mapping process.

10. A method of chromagenic illuminant estimation wherein:

a. in a first preprocessing stage, for a database of m lights E_i (λ

) and n surfaces S_j (λ

) there is calculated T_i≈

Q_i^FQ_i⁺ where Q_i and Q_i^F represent the matrices of unfiltered and filtered sensor responses to the n surfaces under the i th light and + denotes an inverse, and

b. in a second operation stage, given P surfaces in an image and 3×

P matrices Q and Q^F, from these matrices there are chosen the r% brightest pixels giving the matrices Q′ and

Q′

^F, and the scene illuminant ρ

_est is estimated where

$\mathrm{est}=\underset{i}{\min }<br><br>\left({\mathrm{err}}_i\right)<br><br>\left(i=1,2,\mathrm{\dots <br><br>}<br><br>,m\right)$ $\mathrm{and}$ ${\mathrm{err}}_i=<br><br>T_i<br><br>Q^{\mathrm{\prime <br><br>}}-Q^{\mathrm{\prime <br><br>}<br><br><br><br>F}<br><br>$

11. A method of chromagenic illuminant estimation according to claim 10 combined with a gamut mapping process.

12. A method of chromagenic illuminant estimation according to claim 10, further including the step of removing from the images the colour bias due to illumination.