Optical filter opacity control for reducing temporal aliasing in motion picture capture
The present invention comprises a system for and method of frequency prefiltering comprising a camera shutter capable of continuously variable illumination during a single exposure of the sensor. The shutter comprises a continuously variable exposure effector which in disposed in an image path, either in front of a lens or between a lens and a sensor. The system for frequency prefiltering further comprises a synchronization cable that synchronizes a drive system with a sensor or with film. The shutter further comprises a postfilter. The postfilter comprises a digital finite impulse response convolutional filter.
1. A system comprising:
an image sensor configured to generate a plurality of consecutive image frames;
an optical filter having an opacity that is adjustable, the optical filter being disposed in an optical path of the image sensor and configured to vary an amount of light incident on the image sensor responsive to a control signal; and
a controller configured to generate the control signal to vary the opacity during individual image frame periods of the plurality of consecutive image frames so that:
the opacity decreases during a first part of the individual image frame periods and increases during a second part of the individual image frame periods, and
object motion above a Nyquist frequency is filtered by the optical filter prior to detection by the image sensor.
2. The system of claim 1 , wherein the controller is configured to generate the control signal to vary the opacity during the individual image frame periods so that the decrease in the opacity during the first part of the individual image frame periods and the increase in the opacity during the second part of the individual image frame periods are substantially symmetric about a minimum opacity level of the opacity during the individual image frame periods.
3. The system of claim 1 , wherein the first part of the individual image frame periods and the second part of the individual image frame periods have the same duration.
4. The system of claim 1 , wherein the opacity is substantially the same across an entire filter surface of the optical filter at a first time during the individual image frame periods.
5. The system of claim 1 , wherein the controller is configured to generate the control signal to vary the opacity during the individual image frame periods so that a minimum opacity level of the opacity occurs mid-way during the individual image frame periods.
6. The system of claim 1 , wherein the controller is configured to generate the control signal to vary the opacity during the individual image frame periods so that the opacity transitions from a first opacity level to a 50% opacity level and from the 50% opacity level to a second opacity level, the first opacity level being below the 50% opacity level and the second opacity level being above the 50% opacity level.
7. The system of claim 1 , wherein the optical filter comprises a variable opacity liquid crystal.
8. The system of claim 1 , wherein an amplitude of the control signal varies over time, and a level of the opacity during the individual image frame periods corresponds to the amplitude during the individual image frame periods.
9. The system of claim 1 , wherein the control signal is a square wave.
10. The system of claim 1 , wherein the optical filter is disposed before the image sensor in the optical path and after a lens in the optical path so that the light passes through the lens and the optical filter before reaching the image sensor.
11. The system of claim 1 , further comprising a device housing configured to enclose the image sensor and the controller, the optical filter being disposed external to the device housing.
12. A method comprising:
generating, with a processor, a control signal to control an opacity of an optical filter disposed in an optical path of an image sensor;
varying the opacity during individual image frame periods of a plurality of consecutive image frames responsive to the control signal so that:
an amount of light incident on the image sensor varies during the individual image frame periods,
the opacity decreases during a first part of the individual image frame periods and increases during a second part of the individual image frame periods, and
the opacity varies during the individual image frame periods in a way that diminishes an impact of temporal aliasing that is capturable by the image sensor; and
generating, with the image sensor, the plurality of consecutive image frames responsive to the light incident on the image sensor.
13. The method of claim 12 , wherein said varying comprises varying the opacity during the individual image frame periods so that the decrease in the opacity during the first part of the individual image frame periods and the increase in the opacity during the second part of the individual image frame periods are substantially symmetric about a minimum opacity level of the opacity during the individual image frame periods.
14. The method of claim 12 , wherein the first part of the individual image frame periods and the second part of the individual image frame periods have the same duration.
15. The method of claim 12 , wherein said varying comprises varying the opacity during the individual image frame periods so that the opacity is substantially the same across an entire filter surface of the optical filter at a first time during the individual image frame periods.
16. The method of claim 12 , wherein said varying comprises varying the opacity during the individual image frame periods so that a minimum opacity level of the opacity occurs mid-way during the individual image frame periods.
17. The method of claim 12 , wherein said varying comprises varying the opacity during the individual image frame periods so that the opacity transitions from a first opacity level to a 50% opacity level and from the 50% opacity level to a second opacity level, the first opacity level being below the 50% opacity level and the second opacity level being above the 50% opacity level.
18. The method of claim 12 , wherein the optical filter comprises a variable opacity liquid crystal.
19. The method of claim 12 , wherein an amplitude of the control signal varies over time, and said varying comprises varying the opacity during the individual image frame periods so that a level of the opacity during the individual image frame periods corresponds to the amplitude during the individual image frame periods.
20. The method of claim 12 , wherein the control signal is a square wave.
21. The method of claim 12 , further comprising receiving the light through a lens and then the optical filter prior to the light reaching the image sensor.
22. The method of claim 12 , wherein the optical filter is disposed external to a device housing that encloses the image sensor and the controller.
23. A method comprising:
generating, with a processor, a control signal to control an opacity of an optical filter disposed in an optical path of an image sensor;
varying the opacity during individual image frame periods of a plurality of consecutive image frames responsive to the control signal so that:
an amount of light incident on the image sensor varies during the individual image frame periods,
the opacity decreases during a first part of the individual image frame periods and increases during a second part of the individual image frame periods, and
object motion above a Nyquist frequency is filtered by the optical filter prior to detection by the image sensor; and
generating, with the image sensor, the plurality of consecutive image frames responsive to the light incident on the image sensor.
24. A system comprising:
an image sensor configured to generate a plurality of consecutive image frames;
an optical filter having an opacity that is adjustable, the optical filter being disposed in an optical path of the image sensor and configured to vary an amount of light incident on the image sensor responsive to a control signal; and
a controller configured to generate the control signal to vary the opacity during individual image frame periods of the plurality of consecutive image frames so that:
the opacity decreases during a first part of the individual image frame periods and increases during a second part of the individual image frame periods, and
the opacity varies during the individual image frame periods in a way that diminishes an impact of temporal aliasing that is capturable by the image sensor.