Method for automatic exposure control within a video capture device
View Patent ↗On embodiment of the present invention sets forth a technique for automatically computing exposure parameters for a digital video camera (DVC) system. The exposure parameters include exposure gain, analog gain, and digital gain. Each frame is analyzed with respect to an image luma value. Exposure gain and analog gain are adjusted to reduce differences between a target image luma value and measured image luma values over sequential frames. Digital gain is adjusted with respect to each frame to reduce visually abrupt changes in sequentially captured video frames.
1. A method for computing image statistics associated with a current video frame for controlling changes in recorded image intensity, the method comprising:
computing an average luma value for the current video frame;
computing a luma histogram based on luma data associated with the current video frame;
determining a number of overexposed pixels associated with the current video frame based on the luma histogram;
computing an offset value based on the number of overexposed pixels; and
computing a final luma value for the current video frame based on the average luma value for the current video frame and the offset value, wherein the final luma value is used in determining a system gain in a digital video recorder to control the changes in the recorded image intensity.
2. The method of claim 1 , wherein the offset value is linearly related to the number of overexposed pixels.
3. The method of claim 1 , wherein the step of determining the number of overexposed pixels comprises performing an occurrence count of overexposed pixels in the luma histogram.
4. The method of claim 1 , wherein the step of computing the final luma value comprises determining that the offset value is greater than a dark limit, and adding the dark limit to the average luma value for the current video frame to produce the final luma value.
5. The method of claim 1 , wherein the step of computing the final luma value comprises determining that the offset value is less than a bright limit, and adding the bright limit to the average luma value for the current video frame to produce the final luma value.
6. The method of claim 1 , wherein the step of computing the final luma value comprises determining that the offset value is less than or equal to a dark limit, determining that the offset value is greater than or equal to a bright limit, and adding the offset value to the average luma value for the current video frame to produce the final luma value.
7. A computer readable medium including instructions that, when executed by a processing unit, cause the processing unit to compute image statistics associated with a current video frame for controlling changes in recorded image intensity for the current video frame, by performing the steps of:
computing an average luma value for the current video frame;
computing a luma histogram based on luma data associated with the current video frame;
determining a number of overexposed pixels associated with the current video frame based on the luma histogram;
computing an offset value based on the number of overexposed pixels; and
computing a final luma value for the current video frame based on the average luma value for the current video frame and the offset value, wherein the final luma value is used in determining a system gain in a digital video recorder to control the changes in recorded image intensity.
8. The computer readable medium of claim 7 , wherein the offset value is linearly related to the number of overexposed pixels.
9. The computer readable medium of claim 7 , wherein the step of determining the number of overexposed pixels comprises performing an occurrence count of overexposed pixels in the luma histogram.
10. The computer readable medium of claim 7 , wherein the step of computing the final luma value comprises determining that the offset value is greater than a dark limit, and adding the dark limit to the average luma value for the current video frame to produce the final luma value.
11. The computer readable medium of claim 7 , wherein the step of computing the final luma value comprises determining that the offset value is less than a bright limit, and adding the bright limit to the average luma value for the current video frame to produce the final luma value.
12. The computer readable medium of claim 7 , wherein the step of computing the final luma value comprises determining that the offset value is less than or equal to a dark limit, determining that the offset value is greater than or equal to a bright limit, and adding the offset value to the average luma value for the current video frame to produce the final luma value.