Electronically controlled graduated density filters in stacked image sensors

In a digital camera having an imaging array formed in a semiconductor body and including a plurality of pixels arranged in rows and columns, each pixel including first, second, and third color sensors aligned with each other at different depths in the semiconductor body, the digital camera further having a mechanical shutter having an open position allowing light to illuminate the imaging array and a closed position blocking light from illuminating the imaging array, a method for performing color density filtering of images captured by the imaging array, the method comprising:

• opening the mechanical shutter;
• operating each row in the array by:
• resetting all of the color sensors in each pixel in the row by asserting a row reset signal common to all color sensors in all pixels in the row and then, for the first, second, and third color sensors in each pixel in the row separately asserting first, second, and third color-select signals for the first, second, and third color sensors, respectively, of all of the pixel sensors in the row at times after the mechanical shutter is opened;
• starting exposure for the first color sensor in each pixel in the row at a first color sensor exposure start time by then de-asserting the first-color select signal for all of the first color sensors for all pixels in the row, the exposure start time for the first color sensor in each row in the array being a function of a density filter function for the first color applied to an image to be captured by the array;
• starting exposure for the second color sensor in each pixel in the row at a second color sensor exposure start time by then de-asserting the second-color select signal for all of the second color sensors for all pixels in the row, the exposure start time for the second color sensor in each row in the array being a function of a density filter function for the second color applied to an image to be captured by the array;
• starting exposure for the third color sensor in each pixel in the row at a third color sensor exposure start time by then de-asserting the third-color select signal for all of the third color sensors for all pixels in the row, the exposure start time for the third color sensor in each row in the array being a function of a density filter function for the third color applied to an image to be captured by the array;
• ending the exposure for the first, second, and third color sensors for all of the pixel sensors in the row by closing the mechanical shutter;
• and
• reading color exposure values from the first, second, and third color sensors in each row of the pixel array by separately asserting the first, second, and third color-select signals for all of the pixel sensors in the row at first, second, and third color read times after the mechanical shutter has closed, the first, second, and third color read times being independent of one another and being unrelated to the start times for first, second, and third color sensors;
• wherein the first, second, and third color exposure start times are independent of one another.