Systems, methods, and devices for flash exposure control using preflash statistics

• US 8,730,384 B2
• Filed: 06/03/2010
• Issued: 05/20/2014
• Est. Priority Date: 06/03/2010
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Guo, Haitao, Kuo, David Daming
• Primary Examiner(s): Daniels, Anthony J
• Application Number:
  US12/792,916
• Publication Number:
  US 20110298947A1
• Time in Patent Office: 1,447 Days
• Field of Search: 348/221.1, 348/234, 348/362, 348/372
• US Class Current: 348/372
• CPC Class Codes: H04N 23/74 by influencing the scene brightness using illuminating means

Abstract

Systems, methods, and devices for obtaining a properly exposed strobe-illuminated image are provided. One method for doing so may include, for example, gathering image capture statistics during a first period when a strobe is not emitting light and during a second period when the strobe emits a preflash. These image capture statistics may include distinct image capture control statistics and luma values associated with the periods. Final image capture control statistics then may be determined based at least in part on the first luma value normalized to the first image capture control statistics and the second luma value normalized to the second image capture control statistics. Thereafter, the final image capture control statistics may be used to capture a properly exposed strobe-illuminated image when the strobe emits a main flash.

Claims

1. A method comprising:

gathering, using data processing circuitry, first image capture statistics based at least in part on first image data captured by image capture circuitry while a strobe is not emitting light, wherein the first image capture statistics include a first luma value and first autoexposure control statistics;

emitting a preflash using the strobe;

gathering, using the data processing circuitry, second image capture statistics based at least in part on second image data captured by the image capture circuitry while the preflash is being emitted, wherein the second image capture statistics include a second luma value and second autoexposure control statistics;

determining, using the data processing circuitry, third autoexposure control statistics based at least in part on the first luma value normalized to the first autoexposure control statistics and the second luma value normalized to the second autoexposure control statistics;

emitting a main flash using the strobe; and

capturing, using the image capture circuitry, an image based at least in part on the third autoexposure control statistics while the main flash is being emitted.

2. The method of claim 1, wherein at least one of the first autoexposure control statistics differs from a corresponding one of the second autoexposure control statistics.

3. The method of claim 1, wherein the first luma value and the second luma value are average luma values respectively associated with the first image data and the second image data.

4. The method of claim 1, wherein the first image capture statistics include an image focus position and wherein a power of the preflash depends at least in part on the image focus position.

5. The method of claim 1, wherein an intensity of the main flash depends at least in part on an intensity of the preflash.

6. The method of claim 1, comprising determining a power of the main flash based at least in part on the first luma value normalized to the first autoexposure control statistics and the second luma value normalized to the second autoexposure control statistics.

7. An electronic device comprising:

a strobe configured not to emit light during an off period, to emit a first amount of light during a preflash period, and to emit a second amount of light during a main flash period, wherein the first amount of light is less than the second amount of light;

image capture circuitry configured to capture image data during the off period, the preflash period, and the main flash period;

a statistics engine configured to detect first image capture statistics associated with the image data captured during the off period and second image capture statistics associated with the image data captured during the preflash period, wherein the first image capture statistics include first autoexposure control statistics and the second image capture statistics include second autoexposure control statistics, wherein the first autoexposure control statistics differ from the second autoexposure control statistics; and

an image capture controller configured to cause the strobe to emit the first amount of light during the preflash period and the second amount of light during the main flash period, and to control the image capture circuitry based at least in part on the first autoexposure control statistics during the off period, the second autoexposure control statistics during the preflash period, and third autoexposure control statistics during the main flash period, wherein the image capture controller is configured to determine the third autoexposure control statistics based at least in part on the first image capture statistics and the second image capture statistics.

8. The electronic device of claim 7, wherein the image capture controller is configured to determine the third autoexposure control statistics by setting initial values of the third autoexposure control statistics equal to the first autoexposure control statistics and changing the third autoexposure control statistics in a certain order until the third autoexposure control statistics are expected to produce a suitably exposed image during the main flash period.

9. The electronic device of claim 8, wherein, when the initial values of the third autoexposure control statistics are expected to produce an overexposed image during the main flash period, a digital gain control statistic of the third autoexposure control statistics is changed before an analog gain control statistic of the third autoexposure control statistics.

10. The electronic device of claim 8, wherein, when the initial values of the third autoexposure control statistics are expected to produce an underexposed image during the main flash period, an analog gain control statistic of the third autoexposure control statistics is changed before a digital gain control statistic of the third autoexposure control statistics.

11. The electronic device of claim 7, wherein the image capture controller is configured to determine the third autoexposure control statistics by comparing an expected main flash image data brightness to a main flash autoexposure target brightness, wherein the expected main flash image data brightness is determined based at least in part on the first image capture statistics and the second image capture statistics.

12. An article of manufacture comprising:

one or more tangible, machine-readable storage media having instructions encoded thereon for execution by a processor, the instructions comprising;

instructions to receive a first luma value and first autoexposure control statistics associated with image data captured while a strobe is not emitting light;

instructions to receive a second luma value and second autoexposure control statistics associated with image data captured while the strobe is emitting a preflash;

instructions to normalize the first luma value to the first autoexposure control statistics to obtain a first normalized luma value;

instructions to normalize the second luma value to the second autoexposure control statistics to obtain a second normalized luma value;

instructions to determine a third luma value that is expected to be associated with image data captured when the strobe emits a main flash, based at least in part on the first normalized luma value and the second normalized luma value; and

instructions to determine third autoexposure control statistics configured to control image capture when the strobe emits the main flash, based at least in part on the third luma value.

13. The article of manufacture of claim 12, wherein the instructions to determine the third luma value comprise instructions to determine a third normalized luma value based at least in part on the first normalized luma value and the second normalized luma value and instructions to denormalize the third normalized luma value based at least in part on the first autoexposure control statistics to obtain the third luma value.

14. The article of manufacture of claim 13, wherein the first autoexposure control statistics include exposure time, analog gain, sensor digital gain, or image signal processor digital gain, or a combination thereof.

15. The article of manufacture of claim 13, wherein the instructions to determine the third normalized luma value comprise instructions to determine a strobe-only component of the third normalized luma value attributable to light emitted by the strobe when the strobe emits the main flash and instructions to add the strobe-only component of the third normalized luma value to the first normalized luma value to obtain the third normalized luma value.

16. The article of manufacture of claim 15, wherein the instructions to determine the strobe-only component comprise instructions to determine a difference between the second normalized luma and the first not normalized luma that represents a strobe-only luma value attributable to light emitted by the strobe when the strobe is emitting the preflash and instructions to multiply the difference by an expected proportional increase in power between the preflash and the main flash.

17. The article of manufacture of claim 15, comprising instructions to determine a white balance associated with the image capture when the strobe emits the main flash, based at least in part on a ratio of the strobe-only component of the third normalized luma value to the third normalized luma value.

18. The article of manufacture of claim 12, wherein the instructions to determine the third autoexposure control statistics comprise instructions to compare the third luma value to an autoexposure target luma and instructions to vary one or more of the first autoexposure control statistics to obtain the third autoexposure control statistics.

19. A method comprising:

determining, using a processor, an expected brightness of a main-flash-illuminated scene based at least in part on first image capture statistics associated with a non-flash-illuminated scene and second image capture statistics associated with a preflash-illuminated scene;

determining, using the processor, a degree to which an image of the main-flash-illuminated scene is expected to be overexposed or underexposed when the image is captured according to main flash image capture control parameters set to initial values and a main flash strobe intensity set to an initial value, based at least in part on a comparison between the expected brightness of the main-flash-illuminated scene and a main flash autoexposure target brightness;

when the image is expected to be overexposed, reducing at least one of the main flash image capture control parameters down to a reduced value that is expected to cause the image to be properly exposed or to be less overexposed, using the processor; and

when the image is expected to be overexposed despite the reduction of the at least one of the main flash image capture control parameters to the reduced value, reducing the main flash strobe intensity down to a reduced intensity that is expected to cause the image to be properly exposed or to be less overexposed, using the processor.

20. The method of claim 19, wherein the initial values of the main flash image capture control parameters are equal to non-flash image capture control parameters associated with the non-flash-illuminated scene.