Is there a way to detect if an image is blurry?

Question

I was wondering if there is a way to determine if an image is blurry or not by analyzing the image data.

Answer 1

Building off of Nike's answer. Its straightforward to implement the laplacian based method with opencv:

short GetSharpness(char* data, unsigned int width, unsigned int height)
{
    // assumes that your image is already in planner yuv or 8 bit greyscale
    IplImage* in = cvCreateImage(cvSize(width,height),IPL_DEPTH_8U,1);
    IplImage* out = cvCreateImage(cvSize(width,height),IPL_DEPTH_16S,1);
    memcpy(in->imageData,data,width*height);

    // aperture size of 1 corresponds to the correct matrix
    cvLaplace(in, out, 1);

    short maxLap = -32767;
    short* imgData = (short*)out->imageData;
    for(int i =0;i<(out->imageSize/2);i++)
    {
        if(imgData[i] > maxLap) maxLap = imgData[i];
    }

    cvReleaseImage(&in);
    cvReleaseImage(&out);
    return maxLap;
}

Will return a short indicating the maximum sharpness detected, which based on my tests on real world samples, is a pretty good indicator of if a camera is in focus or not. Not surprisingly, normal values are scene dependent but much less so than the FFT method which has to high of a false positive rate to be useful in my application.