Background light subtraction for infra-red images
A method of performing background light subtraction in an infra-red (IR) illuminated image depicting a scene, comprises: providing a rolling shutter image sensor; providing an IR light source configured to be turned on and off; changing an on-off status of the IR light source a plurality of times while capturing an image; capturing two or more image frames each image frame comprising: a first set of lines of pixels comprising image data captured with the IR light turned on; a second set of lines of pixels comprising image data captured with the IR light turned off; creating an IR-illuminated image; creating a non-IR-illuminated image; subtracting background light from the IR-illuminated image using pixel values in the non-IR-illuminated image.
1. A method of performing background light subtraction in an infra-red (IR) illuminated image depicting a scene, the method comprising:
providing a rolling shutter image sensor with a plurality lines of pixels, wherein the rolling shutter image sensor during capture of an image reads out pixel data from one line of pixels at a time;
providing an IR light source configured to be turned on and off, wherein when being turned on, the IR light is configured to illuminate the scene;
changing an on-off status of the IR light source a plurality of times while capturing an image with the rolling shutter image sensor;
capturing three image frames using the rolling shutter image sensor, the three image frames being captured in a chronological order and comprising a first image frame in chronological order and a last image frame in a chronological order, the rolling shutter image sensor being configured to read out image data to the three image frames interleaved in time, such that read-out of a first line of pixels for the last image frame is performed prior to read-out of a last line of pixels to the first image frame, each image frame comprising:
a first set of lines of pixels exposed only with the IR light turned on;
a second set of lines of pixels exposed only with the IR light turned off;
a third set of lines of pixels exposed with the IR light turned both on and off;
creating an IR-illuminated image using only first set of lines of the three frames;
creating a non-IR-illuminated image using only the second set of lines of the three image frames;
subtracting background light from the IR-illuminated image using pixel values in the non-IR-illuminated image, thereby creating a modified IR-illuminated image,
wherein the step of changing an on-off status of the IR light comprises completing an on-off cycle of the IR light during a second time span, tspan 2 ,
wherein the IR light is turned on during a first half of the second time span and turned off during a second half of the second time span and wherein
t
span
2
=>
3
times the exposure time of any line of pixels in the three image frames;
wherein the step of capturing three image frames comprises:
capturing a first image frame, a second image frame and a third image frame;
reading out, by the rolling shutter image sensor, a first line of pixels for the first image frame at a first point in time, t;
reading out, by the rolling shutter image sensor, the first line of pixels for the second image frame at
t
+
x
*
(
t
span
_
2
3
)
;
and
reading out, by the rolling shutter image sensor, the first line of pixels for the third image frame at
t
+
y
*
(
2
×
t
span
_
2
3
)
;
wherein x and y are positive natural numbers.
2. The method of claim 1 , wherein the step of subtracting comprises, for each pixel coordinate of at least a sub-set of pixel coordinates in the non-IR-illuminated image, subtracting the pixel value at the pixel coordinate in the non-IR-illuminated image from the pixel value at a corresponding pixel coordinate in of the IR-illuminated image.
3. The method of claim 2 , further comprising the step of selecting the sub-set of pixel coordinates based on luminosity of the pixel values in the non-IR-illuminated image.
4. The method of claim 1 , wherein the step of subtracting comprises, for each pixel coordinate of at least a sub-set of pixel coordinates in the non-IR-illuminated image, multiplying the pixel value at the pixel coordinate in the non-IR-illuminated image with a weight value, and subtracting the weighted pixel value at the pixel coordinate in the non-IR-illuminated image from the pixel value at a corresponding pixel coordinate in the IR-illuminated image.
5. The method of claim 1 , comprising an initial step of determining a luminosity value of natural light in the scene, wherein, upon determining that the luminosity value exceeds a threshold luminosity, the method is aborted, and upon determining that the luminosity value not exceeding a threshold luminosity, the method is continued.
6. The method of claim 1 , wherein the rolling shutter image sensor is configured to have a same exposure time for all lines of pixels.
7. The method of any one of claim 1 , further comprising the step of:
detecting a license plate in the modified IR illuminated image.
8. The method of claim 1 , wherein the rolling shutter image sensor is provided in a camera capturing a scene comprising a road.
9. A system comprising:
a rolling shutter image sensor with a plurality lines of pixels, wherein the rolling shutter image sensor during capture of an image reads out pixel data from one line of pixels at a time;
an IR light configured to be turned on and off, wherein when being turned on, the IR light is configured to illuminate the scene;
an IR light controlling unit configured to changing an on-off status of the IR light a plurality of times while capturing an image with the rolling shutter image sensor;
wherein the rolling shutter image sensor being configured for capturing three image frames, the three image frames being captured in a chronological order and comprising a first image frame in chronological order and a last image frame in a chronological order, wherein the rolling shutter image sensor being configured to read out image data to the three image frames interleaved in time, such that read-out of a first line of pixels for the last image frame is performed prior to read-out of a last line of pixels to the first image frame, each image frame comprising:
a first set of lines of pixels exposed only with the IR light turned on;
a second set of lines of pixels exposed only with the IR light turned off;
a third set of lines of pixels exposed with the IR light turned both on and off;
wherein the device further comprising an image stitching unit configured to:
create an IR-illuminated image using only the first set of lines of the three image frames;
create a non-IR-illuminated image using only the second set of lines of the three frames; and
subtract background light from the IR-illuminated image using pixel values in the non-IR-illuminated image, thereby creating a modified IR-illuminated image
wherein the IR light controlling unit is configured to complete an on-off cycle of the IR light during a second time span, t span 2 , wherein
t
span
2
=>
3
times the exposure time of any line of pixels in the three image frames, wherein the IR light is turned on during a first half of the second time span and turned off during a second half of the second time span;
wherein the rolling shutter image sensor being configured for:
capturing a first image frame, a second image frame and a third image frame;
reading out a first line of pixels for the first image frame at a first point in time, t;
reading out the first line of pixels for the second image frame at
t
+
x
*
(
t
span
_
2
3
)
;
and
reading out the first line of pixels for the third image frame at
t
+
y
*
(
t
span
_
2
3
)
;
wherein x and y are positive natural numbers.
10. The system of claim 9 , further comprising:
an image analysis unit configured to detect a license plate in the modified IR illuminated image.
11. A non-transitory computer-readable storage medium having stored thereon instructions for implementing a method of performing background light subtraction in an infra-red (IR) illuminated image depicting a scene, when executed on a device having processing capabilities, the method comprising:
providing a rolling shutter image sensor with a plurality lines of pixels, wherein the rolling shutter image sensor during capture of an image reads out pixel data from one line of pixels at a time;
providing an IR light source configured to be turned on and off, wherein when being turned on, the IR light is configured to illuminate the scene;
changing an on-off status of the IR light source a plurality of times while capturing an image with the rolling shutter image sensor;
capturing three image frames using the rolling shutter image sensor, the three image frames being captured in a chronological order and comprising a first image frame in chronological order and a last image frame in a chronological order, the rolling shutter image sensor being configured to read out image data to the three image frames interleaved in time, such that read-out of a first line of pixels for the last image frame is performed prior to read-out of a last line of pixels to the first image frame, each image frame comprising:
a first set of lines of pixels exposed only with the IR light turned on;
a second set of lines of pixels exposed only with the IR light turned off;
a third set of lines of pixels exposed with the IR light turned both on and off;
creating an IR-illuminated image using only first set of lines of the three frames;
creating a non-IR-illuminated image using only the second set of lines of the three image frames;
subtracting background light from the IR-illuminated image using pixel values in the non-IR-illuminated image, thereby creating a modified IR-illuminated image,
wherein the step of changing an on-off status of the IR light comprises completing an on-off cycle of the IR light during a second time span, t span 2 ,
wherein the IR light is turned on during a first half of the second time span and turned off during a second half of the second time span and wherein
t
span
2
=>
3
times the exposure time of any line of pixels in the three image frames;
wherein the step of capturing three image frames comprises:
capturing a first image frame, a second image frame and a third image frame;
reading out, by the rolling shutter image sensor, a first line of pixels for the first image frame at a first point in time, t;
reading out, by the rolling shutter image sensor, the first line of pixels for the second image frame at
t
+
x
*
(
t
span
_
2
3
)
;
reading out, by the rolling shutter image sensor, the first line of pixels for the third image frame at
t
+
y
*
(
2
×
t
span
_
2
3
)
;
wherein x and y are positive natural numbers.
12. The method of claim 1 , wherein x=1 and y=1.