How to detect dark photos in Android [closed]

Question

I have an Android app where user takes a photo of himself with the front camera and then the photo is being uploaded to my server. I notice that many photos comes to my server too dark (sometimes almost impossible to cleary see the user face).

I would like to filter out such photos and show notification (eg. "Photo is too dark. Take one more picture") to the user in the app side. How I could accomplish such task in Android?

EDIT:

I have found out how to calculate brightness for one single pixel (thank's to this answer: https://stackoverflow.com/a/16313099/2999943):

private boolean isPixelColorBright(int color) {
    if (android.R.color.transparent == color)
        return true;

    boolean rtnValue = false;

    int[] rgb = {Color.red(color), Color.green(color), Color.blue(color)};

    int brightness = (int) Math.sqrt(rgb[0] * rgb[0] * .299 + rgb[1]
            * rgb[1] * .587 + rgb[2] * rgb[2] * .114);

    if (brightness >= 200) {    // light color
        rtnValue = true;
    }

    return rtnValue;
}

But still I don't have clear idea how to determine whole image brightness "status". Any suggestions?

Answer 1

As a variant you can build a brightness histogram of your photo. Calculate brightness as described here Formula to determine brightness of RGB color. Then initialize array of size 256 and increment by one an array element which index is a brightness of every pixel.

Then look if too much values are on the left side or right one, it means that your picture is too light/dark. For example you can look at 10 right and left values.

Code example:

int histogram[256];
for (int i=0;i<256;i++) {
     histogram[i] = 0;
}

for (int x = 0; x < a.getWidth(); x++) {
    for(int y = 0; y < a.getHeight(); y++) {
        int color = a.getRGB(x, y);

        int r = Color.red(pixel);
        int g = Color.green(pixel);
        int b = Color.blue(pixel);

        int brightness = (int) (0.2126*r + 0.7152*g + 0.0722*b);
        histogram[brightness]++;
    }
}

int allPixelsCount = a.getWidth() * a.getHeight();

// Count pixels with brightness less then 10
int darkPixelCount = 0;
for (int i=0;i<10;i++) {
    darkPixelCount += histogram[i];
}

if (darkPixelCount > allPixelCount * 0.25) // Dark picture. Play with a percentage
else // Light picture.