Object detection and tracking with reduced error due to background illumination
View Patent ↗An image sensor frame rate can be increased by “interlaced” mode operation whereby only half the number of lines of an image is transported to the readout circuitry. This halves the integration time but also halves the resolution of the sensor. Accordingly, in one embodiment, an image sensor operated in an interlaced fashion is first exposed to a scene under a first form of illumination (e.g., narrowband illumination), and a first set of alternating (horizontal or vertical) lines constituting half of the pixels is read out of the array; the sensor is then exposed to the same scene under a second form of illumination (e.g., existing ambient illumination with the illumination source turned off), and a second set of alternating lines, representing the other half of the pixel array, is read out. The two images are compared and noise removed from the image obtained under narrowband illumination.
1. An image capture and analysis system including:
a camera including an image sensor and a field of view, wherein the image sensor operates in an interlaced mode that alternately reads out sets of odd lines and even lines of pixels arranged in adjoining rows or columns;
one or more light sources directed toward the field of view with existing ambient illumination; and
at least one processor that generates control signals to coordinate activated and deactivated intervals of the light sources with the interlaced mode read out of the image sensor;
wherein the control signals initiate the interlaced mode readout so that a successive pair of reads from the sets of the odd and even lines capture a non-illuminated image from a deactivated interval and an illuminated image from an activated interval;
wherein the non-illuminated image from the deactivated interval is illuminated by the existing ambient illumination;
wherein the processor further compares adjoining lines from the non-illuminated image with lines from one or more illuminated images and uses the comparison to modify the illuminated image to remove noise based on ratios of pixel values between the pixel values in illuminated and non-illuminated images, whereby signal-to-noise ratio between foreground object illumination and background object illumination or background lighting improves.
2. The system of claim 1 , wherein the processor further detects motion of a foreground object from successive images after noise removal.
3. The system of claim 1 , wherein the noise is removed by subtraction.
4. The system of claim 1 , wherein the noise is removed by thresholding.
5. The system of claim 1 , wherein all pixels of the image sensor respond to light during activation and deactivation time intervals.
6. The system of claim 1 , wherein only pixels in the set of odd lines respond to light during activation time intervals and only pixels in the set of even lines respond to light during deactivation time intervals.
7. The system of claim 1 , wherein the image sensor is a charge-coupled device (CCD) sensor.
8. A method of image capture and analysis using an image sensor in a camera having a field of view with existing ambient illumination, the image sensor operating in an interlaced mode that alternately reads out sets of odd lines and even lines of pixels arranged in adjoining rows or columns, the method including:
at least one processor generating control signals to coordinate activated and deactivated intervals of one or more light sources directed to the field of view with the interlaced mode read out of the image sensor;
the control signals initiating an interlaced mode readout from the image sensor so that a successive pair of reads from the sets of the odd and even lines capture a non-illuminated image from a deactivated interval and an illuminated image from an activated interval;
wherein the non-illuminated image from the deactivated interval is illuminated by the existing ambient illumination; and
comparing lines from the non-illuminated image with lines from one or more illuminated images and using comparison to modify the illuminated image to remove noise based on ratios of pixel values between the pixel values in illuminated and non-illuminated images.
9. The method claim 8 , further including detecting motion of a foreground object from successive images after noise removal.
10. The method of claim 8 , further including removing the noise by subtraction.
11. The method of claim 8 , further including removing the noise by thresholding.
12. The method of claim 8 , wherein all pixels of the image sensor respond to light during activation and deactivation time intervals.
13. The method of claim 8 , wherein only pixels in the set odd of lines respond to light during activation time intervals and only pixels in the set of even lines respond to light during deactivation time intervals.
14. The method of claim 8 , wherein the image sensor is a charge-coupled device (CCD) sensor.