Time-of-flight histogram super-resolution shape fitting
A method of ranging using a time-of-flight (ToF) ranging system includes: receiving, by a processor, a histogram generated by a ToF imager of the ToF ranging system, where the ToF imager is configured to transmit a light pulse for ranging purpose; finding a rising edge of a pulse region in the histogram, where the pulse region corresponds to a reflected light pulse from a target; fine-tuning a location of the rising edge by performing a fitting process between the rising edge and a pre-stored high-solution rising edge; and calculating an estimate of a distance of the target by adding a pre-determined offset to a distance of the rising edge after fine-tuning the location of the rising edge.
1 . A method of operating a time-of-flight (ToF) ranging system to obtain target locations with an improved timing resolution than a native timing resolution of a histogram generated by a ToF imager of the ToF ranging system, the method comprising:
receiving, by a processor, the histogram generated by the ToF imager of the ToF ranging system, wherein the ToF imager is configured to transmit a light pulse for ranging purposes and to generate the histogram having the native timing resolution;
finding a rising edge of a pulse region in the histogram, wherein the pulse region corresponds to a reflected light pulse from a target;
fine-tuning a location of the rising edge by performing a fitting process between the rising edge and a pre-stored high-resolution rising edge, wherein the fine-tuning adjusts the location of the rising edge by a fraction of a bin width of the histogram to achieve the improved timing resolution for the location of the rising edge, wherein the pre-stored high-resolution rising edge corresponds to a measured characteristic of a transmitted light pulse from the ToF imager, wherein the pre-stored high-resolution rising edge has a higher timing resolution than the native timing resolution of the histogram;
calculating an estimate of a distance of the target by adding a pre-determined offset to a distance of the rising edge after fine-tuning the location of the rising edge;
outputting the estimate of the distance of the target as a target location, wherein the target location has the improved timing resolution than the native timing resolution of the histogram.
2 . The method of claim 1 , wherein the pre-stored high-resolution rising edge corresponds to a rising edge of a high-resolution histogram of the transmitted light pulse from the ToF imager, wherein a timing resolution of the high-resolution histogram is N times that of the histogram.
3 . The method of claim 2 , wherein each histogram bin of the histogram corresponds to N adjacent histogram bins of the high-resolution histogram, the N adjacent histogram bins representing N subphases of each histogram bin, wherein performing the fitting process comprises:
down-sampling the pre-stored high-resolution rising edge by a factor of N at each of the N subphases to generate N coarse rising edges;
generating N error metrics by computing differences between the rising edge and the N coarse rising edges; and
finding a subphase of the N subphases that has a lowest error metric of the N error metrics.
4 . The method of claim 3 , wherein the location of the rising edge is indicated by an index of a histogram bin of the histogram, wherein fine-tuning the location of the rising edge comprises adjusting the index by a fractional number indicated by the subphase having the lowest error metric.
5 . The method of claim 1 , further comprising:
finding a falling edge in the histogram that corresponds to a falling edge of a reflected light pulse from a furthest target;
fine-tuning a location of the falling edge by performing another fitting process between the falling edge and a pre-stored high-resolution falling edge; and
calculating an estimate of a distance of the furthest target by subtracting another pre-determined offset from a distance of the falling edge after fine-tuning the location of the falling edge.
6 . The method of claim 1 , wherein finding the rising edge comprises:
generating a differential histogram by computing differences between adjacent histogram bins of the histogram and assigning the computed differences as values of respective histogram bins of the differential histogram; and
finding a histogram bin in the pulse region of the histogram, wherein a gradient of the histogram bin, indicated by a value of a corresponding histogram bin of the differential histogram, is larger than a pre-determined threshold, or is a maximum value of the gradients of the histogram bins of the histogram in the pulse region.
7 . The method of claim 6 , wherein the pre-determined threshold is determined by an ambient noise level in the histogram and a user-specified confidence level.
8 . The method of claim 6 , wherein the pre-determined offset is determined by a shape of the light pulse transmitted by the ToF imager, and corresponds to half of a width of the light pulse.
9 . The method of claim 1 , further comprising performing a calibration process for the ToF imager, comprising:
measuring a shape of the light pulse transmitted by the ToF imager at different temperatures;
computing a plurality of values for the pre-determined offset at the different temperatures; and
storing the plurality of values for the pre-determined offset at the different temperatures.
10 . The method of claim 9 , further comprising:
obtaining a measurement of a present temperature of the ToF imager; and
determining a present value for the pre-determined offset based on the measurement of the present temperature and the stored plurality of values for the pre-determined offset at the different temperatures.
11 . A method of operating a time-of-flight (ToF) ranging system to obtain a target location having a timing resolution higher than a native timing resolution of a histogram generated by the ToF ranging system, the method comprising:
transmitting, by an emitter of the ToF ranging system, a light signal toward one or more targets;
receiving, by a ToF sensor of the ToF ranging system, the light signal reflected by the one or more targets;
generating, by a histogram generation circuit of the ToF ranging system, a histogram based on the received light signal, wherein the histogram has a first timing resolution;
estimating gradients of histogram bins of the histogram by computing differences between adjacent histogram bins of the histogram;
identifying a location of a rising edge of the histogram by finding a histogram bin of the histogram, wherein the histogram bin of the histogram has a gradient larger than a pre-determined threshold or has a maximum gradient of a pulse region of the histogram;
fine-tuning the location of the rising edge by performing a fitting process between the rising edge and a high-resolution rising edge, wherein the fine-tuning adjusts the location of the rising edge by a fraction of a bin width of the histogram bin to achieve a second timing resolution for the location of the rising edge, wherein the second timing resolution is higher than the first timing resolution, wherein the high-resolution rising edge corresponds to a rising edge of a high-resolution histogram of a light pulse transmitted by the emitter, wherein a timing resolution of the high-resolution histogram is higher than that of the histogram; and
computing an estimate of a distance of a target by adding a pre-determined offset to a distance of the histogram bin.
12 . The method of claim 11 , wherein the histogram bin is a leftmost histogram bin of the histogram having the gradient larger than the pre-determined threshold or having the maximum gradient of the pulse region of the histogram.
13 . The method of claim 12 , wherein the timing resolution of the high-resolution histogram is N times that of the histogram such that each histogram bin of the histogram corresponds to N adjacent histogram bins of the high-resolution histogram, the N adjacent histogram bins representing N subphases of each histogram bin, wherein performing the fitting process comprises:
down-sampling the high-resolution rising edge by a factor of N at each of the N subphases to generate N coarse rising edges;
generating N error metrics by computing differences between the rising edge and the N coarse rising edges; and
finding a subphase of the N subphases that has a lowest error metric of the N error metrics.
14 . The method of claim 13 , wherein fine-tuning the location of the rising edge comprises adjusting an index of the histogram bin by an amount indicated by the subphase having the lowest error metric.
15 . The method of claim 11 , wherein the pre-determined offset is proportional to half of a width of the light pulse transmitted by the emitter.
16 . The method of claim 15 , wherein the width of the light pulse transmitted by the emitter varies with temperature, wherein the method further comprises adjusting the pre-determined offset in accordance with a present temperature of the emitter.
17 . A time-of-flight (ToF) ranging system with improved accuracy in target distance measurement, the ToF ranging system comprising:
a light source configured to transmit light pulses for illuminating one or more targets;
a ToF sensor configured to receive reflected light pulses from the one or more targets and to generate a histogram based on the reflected light pulses, wherein the histogram has a first timing resolution; and
a processor configured to:
find a location of a rising edge in the histogram that corresponds to a rising edge of a reflected light pulse reflected by a target;
fine-tune the location of the rising edge by performing a fitting process between the rising edge in the histogram and a pre-stored high-resolution rising edge, wherein the fine-tuning adjusts the location of the rising edge by a fraction of a bin width of the histogram to achieve a second timing resolution for the location of the rising edge, wherein the second timing resolution is higher than the first timing resolution, wherein the pre-stored high-resolution rising edge corresponds to a rising edge of a high-resolution histogram of a transmitted light pulse from the light source; and
calculate an estimate of a distance of the target by adding a pre-determined offset to a distance of the rising edge in the histogram after the fine-tuning.
18 . The ToF ranging system of claim 17 , wherein the processor is configured to find the location of the rising edge by:
computing differences between adjacent histogram bins of the histogram and assigning the computed differences as gradients of respective histogram bins of the histogram; and
finding a first histogram bin in a pulse region of the histogram, wherein the first histogram bin has a first value that is a maximum value of the computed differences in the pulse region, or is larger than a pre-determined threshold value.
19 . The ToF ranging system of claim 18 , wherein a timing resolution of the high-resolution histogram is N times that of the histogram such that each histogram bin of the histogram corresponds to N adjacent histogram bins of the high-resolution histogram representing N subphases of each histogram bin, wherein performing the fitting process comprises:
down-sampling the pre-stored high-resolution rising edge by a factor of N at each of the N subphases to generate N coarse rising edges;
generating N error metrics by computing differences between the rising edge in the histogram and the N coarse rising edges; and
finding a subphase of the N subphases that has a lowest error metric of the N error metrics.
20 . The ToF ranging system of claim 17 , wherein the pre-determined offset is proportional to half of a width of the transmitted light pulse.