Coding of concentric information

• US 7,031,539 B2
• Filed: 10/18/2002
• Issued: 04/18/2006
• Est. Priority Date: 10/19/2001
• Status: Active Grant

• Current Assignee: Stmicroelectronics France (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SA (STMicroelectronics NV)
• Inventors: Tisse, Christel-Loic, Barbeyrac, Damien
• Primary Examiner(s): Do, Anh Hong
• Application Number:
  US10/273,792
• Publication Number:
  US 20030076984A1
• Time in Patent Office: 1,278 Days
• Field of Search: 382/232, 382/248, 382/243, 359/599, 359/9, 359/25, 359/15, 355/67, 322/39, 327/3, 327/261, 377/43
• US Class Current: 382/243
• CPC Class Codes: G06V 10/431 Frequency domain transformation; Autocorrelation
• CPC Class Codes: G06V 10/478 Contour-based spectral representations or scale-space representations, e.g. by Fourier analysis, wavelet analysis or curvature scale-space [CSS]
• CPC Class Codes: G06V 40/193 Preprocessing; Feature extraction

Abstract

A method for coding in frequency, module and phase a digital representation, in the space field, of a ring-shaped element, including the steps of: applying to any point of the element a polar conversion at constant angle, whereby the element is unfolded in rectangular form; transferring, to the frequency field, any point of the converted rectangular shape by means of a Fourier transform; filtering the discrete data resulting from the transfer by means of at least one real, bidimensional, band-pass filter, oriented along the phase axis; applying a Hilbert transform to the filtering results; applying an inverse Fourier transform to the results of the Hilbert transform; and extracting phase and module information in the space field.

Claims

1. A method for coding in frequency, module and phase a digital representation, in the space field, of a ring-shaped element, including the steps of:

applying to any point of the element a polar conversion at constant angle, whereby the element is unfolded in rectangular form;

transferring, to the frequency field, any point of the converted rectangular shape by means of a Fourier transform;

filtering the discrete data resulting from the transfer by means of at least one real, bidimensional, band-pass filter, oriented along the phase axis;

applying a Hilbert transform to the filtering results;

applying an inverse Fourier transform to the results of the Hilbert transform; and

extracting phase and module information in the space field.

2. The method of claim 1, wherein the real, bidimensional, band-pass filter oriented along the phase axis is the product of two one-dimensional Hamming windows, the transfer function of each of these windows being zero everywhere, except in a restricted frequency field around a central frequency in which its analytic expression is the following:

$Y_{i,k}<br><br>\left(f_k\right)={\mathrm{\alpha <br><br>}}_{i,k}+\left(1-{\mathrm{\alpha <br><br>}}_{i,k}\right)<br><br>\cos <br><br>\frac{\mathrm{\pi <br><br>}<br><br>\left(f_k-f<br><br><br><br>q_{i,k}\right)}{{\mathrm{f0}}_{i,k}},$

where

i represents the index of the filter in a series of used filters;

k designates the direction of the considered Hamming window;

f_k is the current frequency;

α

_1,k is a non-zero real positive number strictly smaller than 1;

fq_i,k is the central frequency of the one-dimensional band-pass filter in direction k and of index i in the filter series; and

f0_1,k determines the spectral spreading centered on frequency fq_i,k.

3. The method of claim 2, wherein the Hamming windows have identical coefficients α

_1,k.

4. The method of claim 3, wherein the Hamming windows are Hanning windows for which the identical coefficients α

_1,k are 0.54.

5. The method of claim 2, wherein the Hamming windows have different coefficients α

_1,k.

6. The method of claim 1, wherein the real bidimensional band-pass filter oriented along the phase axis has a pass-band at −

3 dB with a width of one octave.

7. The method of claim 4, wherein the pass-band of each of the Hamming windows is defined by:

${\mathrm{f0}}_{i,k}=\frac{\mathrm{\pi <br><br>}}{3<br><br>\mathrm{arcos}\left(\frac{\frac{1}{2}-{\mathrm{\alpha <br><br>}}_{i,k}}{1-{\mathrm{\alpha <br><br>}}_{i,k}}\right)}<br><br>{\mathrm{fq}}_{i,k},$

where arcos designates the inverse cosine function.

8. The method of claim 1, comprising applying three real bidimensional band-pass filters oriented along the axis of the characteristic information of the rectangular image, of different central frequencies.

9. A method for coding, in a digital image, a texture of a ring-shaped element defined by including a first circle of relatively small radius in a second circle of relatively large radius, including implementing the method of claim 1.

10. The method of claim 9, wherein the ring is the iris of an eye, the first circle being the pupil of said eye and the second circle being the limit between the iris and the eye cornea.