I was about to ask the same, but then found out, that similar question gave no satisfactory answers. All answers I've found on SO deal with human observation of a single pixel RGB vs HSV.
From my observations, the subjective brightness of an image also depends strongly on the pattern. A star in a dark sky may look more bright than a cloudy sky by day, while the average pixel value of the first image will be much smaller.
The images I use are grey-scale cell-images produced by a microscope. The forms vary considerably. Sometimes they are small bright dots on very black background, sometimes less bright bigger areas on not so dark background.
My approach is:
- Find histogram maximum (HMax) using threshold for removing hot pixels.
- Calculate mean values of all pixel between HMax * 2/3 and HMax
The ratio 2/3 could be also increased to 3/4 (which reduces the range of pixels considered as bright).
The approach works quite well, as different cell-patterns with same titration produce similar brightness.
P.S.: What I actually wanted to ask is, whether there is a similar function for such a calculation in OpenCV or SimpleCV. Many thanks for any comments!