Coded light
A coded light signal is embedded into visible light emitted from the light source, to be received by a rolling-shutter camera which captures frames ( 16 ) by exposing a plurality of lines ( 18, 24 ) of each frame in sequence, the camera having an exposure time with each line being exposed for the exposure time. The coded light signal is formatted according to a format whereby the coded light signal comprises at least one message and the message is repeated multiple times with a timing such that, when samples of the coded light signal are obtained from a substantially smaller number of lines ( 24 ) than exposed by the camera in each frame and the message is longer than this number of lines, a different part of the message is seen by the camera in each of a plurality of different ones said frames.
1. A device adapted to provide a coded light signal to a rolling-shutter camera, comprising:
a driver for controlling a light source based on a controller output to embed the coded light signal into visible light emitted from the light source for reception by the camera, the camera being configured to capture frames by exposing a plurality of lines of each frame in sequence, the camera having an exposure time being the time for which each line is exposed; and
a controller configured to generate the controller output to generate the coded light signal according to a format whereby the coded light signal comprises at least one message and the message is repeated multiple times with a timing that is configured relative to the exposure time of the camera; wherein:
the message is of a duration longer than an amount of time for capturing one frame, wherein a different part of the message is emitted during different captured frames; and
the message comprises one or more packets comprising different data content, wherein each of the packets of the message is followed by an inter-packet idle period, and wherein the repetitions of the message are separated by an inter-message idle period different than the inter-packet idle period.
2. The device of claim 1 , wherein the message is repeated such that the whole message will be emitted over a plurality of frames.
3. The device of claim 1 , wherein said number of lines is less than or equal to 14% of the lines of each frame.
4. A system comprising the device of claim 1 , the light source, and the camera.
5. The device of claim 1 , wherein the inter-packet idle period is greater than or equal to the exposure time or a maximum anticipated value of the exposure time.
6. The device of claim 1 , wherein the inter-message idle period is selected to obtain said timing whereby a different part of the message is emitted in each of a plurality of different ones of said frames.
7. The device of claim 1 , wherein the exposure time is less than or equal to ( 1/30)s, less than or equal to ( 1/60)s, or less than or equal to ( 1/120)s.
8. The device of claim 1 , wherein the at least one message is formed of at least three packets per message.
9. The device of claim 1 , wherein each of the packets is of a length less than or equal to 17 bits long, less than or equal to 12 bits long, or less than or equal to 9 bits long.
10. The device of claim 9 , wherein the packet length is 9 bits, consisting of a byte of content and a synchronization bit.
11. The device of claim 10 , wherein the controller is configured to encode the coded light signal with a symbol rate of said symbols being 1 kHz, 2 kHz or 4 kHz.
12. The device of claim 1 , wherein the controller is configured to encode the coded light signal according to a ternary Manchester modulation coding scheme whereby data bits of the signal are represented by being mapped to ternary Manchester symbols.
13. The device of claim 12 , wherein the inter-message idle period has a duration of at least 4 of said symbols.
14. The device of claim 13 , wherein each of the packets is 19 of said symbols long, the inter-packet idle period has a duration of 33 of said symbols, and the inter-message idle period has a duration of 5 of said symbols.
15. The device of claim 1 , wherein the controller is configured to receive an indication of the exposure time from the camera, and to adapt the format of the message based on the exposure time.
16. The device of claim 15 , wherein the controller is configured to perform said adaptation by selecting one of more parameters such that a different part of the message is emitted in each of a plurality of different ones said frames, the one or more parameters comprising: the inter-packet idle period, inter-message idle period, number of packets per message, and/or symbol rate.
17. The device of claim 16 , wherein the controller is configured to adapt the format by selecting between a plurality of different predetermined combinations of said parameters.
18. A method for providing a coded light signal to a rolling-shutter camera, comprising:
controlling a light source to embed the coded light signal into visible light emitted from the light source for reception by the camera, the camera being configured to capture frames by exposing a plurality of lines of each frame in sequence, the camera having an exposure time being the time for which each line is exposed; and
generating the coded light signal according to a format whereby the coded light signal comprises at least one message and the message is repeated multiple times with a timing that is configured relative to the exposure time of the camera; wherein:
the message is of a duration longer than an amount of time for capturing one frame, wherein a different part of the message is emitted during different captured frames; and
the message comprises one or more packets comprising different data content, wherein each of the packets of the message is followed by an inter-packet idle period, and wherein the repetitions of the message are separated by an inter-message idle period different than the inter-packet idle period.
19. A computer program product comprising code embodied on a computer-readable storage medium and configured so as, when executed on a device comprising a driver for controlling a light source based on a controller output to provide a coded light signal to a rolling-shutter camera, to perform operations of:
controlling the light source based on the controller output to the driver to embed the coded light signal into visible light emitted from the light source, to be received by the camera, the camera being configured to capture frames by exposing a plurality of lines of each frame in sequence, the camera having an exposure time being the time for which each line is exposed; and
generating the controller output for output to the driver to generate the coded light signal according to a format whereby the coded light signal comprises at least one message and the message is repeated multiple times with a timing that is configured relative to the exposure time of the camera; wherein:
the message is of a duration longer than an amount of time for capturing one frame, wherein a different part of the message is emitted during different captured frames; and
the message comprises one or more packets comprising different data content, wherein each of the packets of the message is followed by an inter-packet idle period, and wherein the repetitions of the message are separated by an inter-message idle period different than the inter-packet idle period.