Method and system in digital imaging for adjusting exposure and a corresponding device

1. A method for adjusting exposure in digital imaging, the method comprising:

capturing a raw image frame corresponding to given exposure parameters by a camera sensor;

forming a histogram from the raw image frame;

specifying a plurality of characteristic values from the histogram, including at least a first characteristic value being at least partly based on a mean value of the histogram and a second characteristic value indicating substantially the bright end of the histogram;

setting at least a minimum target level for the first characteristic value and a dynamic target interval for the second characteristic value;

comparing the first characteristic value to its minimum target level and the second characteristic value to its dynamic target interval;

in response to either the first characteristic value not exceeding its minimum target level or the second characteristic value not locating within its dynamic target interval, calculating a total offset required to guide the first characteristic value to exceed its minimum target level and the second characteristic value to locate within its dynamic target interval; and

adjusting the exposure parameters on the basis of the first and second characteristic value biased by the total offset.

2. A method according to claim 1 , further comprising

specifying a third characteristic value from the histogram, indicating the point of the histogram wherein substantially 5-35%, preferably 20% of the pixels in the histogram are brighter than said point; and

setting a dynamic target interval for the third characteristic value.

3. A method according to claim 2 further comprising

setting upper and lower limits specifying the dynamic target intervals.

4. A method according to claim 1 , wherein the dynamic target intervals are specified according to the dynamic characteristics of the camera sensor.

5. A method according to claim 1 , wherein said characteristic values of the histogram are specified according to its pixel portions/distribution.

6. A method according to claim 1 further comprising

specifying a first offset for the second characteristic value, by means of which it is guided to the target interval set for it,

estimating a mean value resulting from the said guiding, and comparing it with a target interval set for it,

if the estimated mean value is not guided to the target interval, specifying a second offset for the second characteristic value, such that the estimated mean value is guided to the target interval set for it;

wherein the sum of the first offset and the second offset represents the total offset used in adjusting the exposure parameters.

7. A method according to claim 1 , wherein prior to the adjustment of the exposure parameters, the method further comprises

making an examination as to whether a saturation condition set as a result of the guiding made for the second characteristic value is met;

if it is met, then the total offset is specified for the third characteristic value, by means of which

the third characteristic value is guided to the target interval set for it and which

total offset meets saturation criteria set for the target interval, and after which

the said exposure parameters are specified on the basis of the third characteristic value and of the offset specified for it.

8. A method according to claim 7 , wherein

if the saturation condition is not met, then the total offset is specified for the second characteristic value, by means of which it is guided to upper limit of the target interval set for it, and after which

the said exposure parameters are specified on the basis of the second characteristic value and of the offset specified for it.

9. A method according to claim 1 , wherein the histogram is formed from the raw image frame, only 1/10- 9/10 part of the data is recorded, preferably ⅛ of the data.

10. A method according to claim 9 , wherein the brightness value of the brightest pixel from a predefined pixel neighbourhood is recorded from the raw image frame into the histogram.

11. A method according to claim 1 , wherein the second characteristic value is specified in such a way that 95-99.9% of the pixels of the histogram have a brightness value that is preferably darker than the second characteristic value.

12. A method according to claim 1 , wherein the dynamic target intervals and their upper and lower limits target are modified according to the ambient light level.

13. A method according to claim 12 , wherein the ambient light level is estimated on the basis of the previous exposure parameters.

14. A system in a digital imaging chain, the system including:

a camera sensor for capturing a raw image frame corresponding to given exposure parameters;

a plurality of modular components for processing the image data, including an exposure-control module for adjusting the exposure parameters; the system being arranged to

form a histogram from the raw image frame;

specify a plurality of characteristic values from the histogram, including at least a first characteristic value being at least partly based on a mean value of the histogram and a second characteristic value indicating substantially the bright end of the histogram;

set at least a minimum target level for the first characteristic value and a dynamic target interval for the second characteristic value;

compare the first characteristic value to its minimum target level and the second characteristic value to its dynamic target interval;

in response to either the first characteristic value not exceeding its minimum target level or the second characteristic value not locating within its dynamic target interval, calculate a total offset required to guide the first characteristic value to exceed its minimum target level and the second characteristic value to locate within its dynamic target interval; and

adjust the exposure parameters on the basis of the first and second characteristic value biased by the total offset.

15. A digital imaging device comprising the system according to claim 14 .