Auto flicker detector
An auto flicker detector (AFD) includes a difference generator that generates a difference image representing difference between consecutive frames of at least one pair of captured images from an image sensor illuminated by a light source; and a correlator operatively performed on the difference image to generate correlations associated with a first frequency and a second frequency respectively. A frequency at which the light source flickers is determined according to the correlations.
1 . An auto flicker detector (AFD), comprising:
a difference generator that generates a difference image representing difference between consecutive frames of at least one pair of captured images from an image sensor illuminated by a light source; and
a correlator operatively performed on the difference image to generate correlations associated with a first frequency and a second frequency respectively;
wherein a frequency at which the light source flickers is determined by comparing a correlation associated with the first frequency and a correlation associated with the second frequency; and
wherein the determined frequency is used by an imaging system to adjust a rolling-shutter readout parameter to reduce flicker artifacts.
2 . The AFD of claim 1 , further comprising:
an extractor that extracts a luma image for each of the captured images before generating the difference image.
3 . The AFD of claim 2 , wherein the extractor converts the captured images from RGB to YUV color space, where Y component represents the luma image.
4 . The AFD of claim 1 , wherein the image sensor converts light emitted from the light source via a rolling shutter into electrical signals, thereby generating the captured images.
5 . The AFD of claim 1 , wherein the light source flickers at the first frequency if the correlation associated with the first frequency is statistically greater than the correlation associated with the second frequency.
6 . The AFD of claim 1 , wherein the correlator transforms the difference image from time domain to frequency domain by domain transformation, and transformed components of multiples of the first frequency and the second frequency are used as the correlations associated with the first frequency and the second frequency respectively.
7 . The AFD of claim 6 , wherein the domain transformation comprises discrete Fourier transform (DFT).
8 . The AFD of claim 1 , further comprising:
a comparator that compares the correlation associated with the first frequency and the correlation associated with the second frequency for consecutive frames of a pair.
9 . The AFD of claim 8 , wherein the difference generator receives and performs on plural pairs of consecutive frames.
10 . The AFD of claim 9 , wherein only a correlation with larger value is accumulatively counted by the comparator, thereby obtaining final counts of the correlations associated with the first frequency and the second frequency respectively.
11 . The AFD of claim 10 , wherein the light source is determined to be flickering at the first frequency when the final count of the correlations associated with the first frequency is greater than the final count of the correlations associated with the second frequency; and the light source is determined to be flickering at the second frequency when the final count of the correlations associated with the second frequency is greater than the final count of the correlations associated with the first frequency.
12 . The AFD of claim 10 , wherein the light source is determined to be flickering at the first frequency when the final count of the correlations associated with the first frequency is greater than the final count of the correlations associated with the second frequency and is greater than a predetermined threshold; and the light source is determined to be flickering at the second frequency when the final count of the correlations associated with the second frequency is greater than the final count of the correlations associated with the first frequency and is greater than the predetermined threshold.
13 . The AFD of claim 9 , wherein a flicker detection (FD) window is defined in a frame and is performed by the difference generator.
14 . The AFD of claim 13 , wherein the FD window is divided into plural sub-windows, and corresponding sub-windows are used to generate the difference image for each pair of consecutive frames.
15 . The AFD of claim 14 , wherein the sub-windows are moved through entire or part of the FD window for generating difference images of the plural pairs of consecutive frames.
16 . The AFD of claim 15 , wherein the sub-windows are alternatively moved from both ends toward center until entire frame is covered.
17 . The AFD of claim 15 , wherein one or more sub-windows are skipped each time when the sub-windows are moved.