standard deviation of a UIImage/CGImage - ios

Question

I need to calculate the standard deviation on an image I have inside a UIImage object.
I know already how to access all pixels of an image, one at a time, so somehow I can do it.
I'm wondering if there is somewhere in the framework a function to perform this in a better and more efficient way... I can't find it so maybe it doensn't exist.
Do anyone know how to do this?
bye

Answer 1

To further expand on my comment above. I would definitely look into using the Accelerate framework, especially depending on the size of your image. If you image is a few hundred pixels by a few hundred. You will have a ton of data to process and Accelerate along with vDSP will make all of that math a lot faster since it processes everything on the GPU. I will look into this a little more, and possibly put some code in a few minutes.
UPDATE
I will post some code to do standard deviation in a single dimension using vDSP, but this could definitely be extended to 2-D
float *imageR = [0.1,0.2,0.3,0.4,...]; // vector of values
int numValues = 100; // number of values in imageR
float mean = 0; // place holder for mean
vDSP_meanv(imageR,1,&mean,numValues); // find the mean of the vector
mean = -1*mean // Invert mean so when we add it is actually subtraction
float *subMeanVec = (float*)calloc(numValues,sizeof(float)); // placeholder vector
vDSP_vsadd(imageR,1,&mean,subMeanVec,1,numValues) // subtract mean from vector
free(imageR); // free memory
float *squared = (float*)calloc(numValues,sizeof(float)); // placeholder for squared vector
vDSP_vsq(subMeanVec,1,squared,1,numValues); // Square vector element by element
free(subMeanVec); // free some memory
float sum = 0; // place holder for sum
vDSP_sve(squared,1,&sum,numValues); sum entire vector
free(squared); // free squared vector
float stdDev = sqrt(sum/numValues); // calculated std deviation

Answer 2

Please explain your query so that can come up with specific reply.
If I am getting you right then you want to calculate standard deviation of RGB of pixel or HSV of color, you can frame your own method of standard deviation for circular quantities in case of HSV and RGB.
We can do this by wrapping the values.
For example: Average of [358, 2] degrees is (358+2)/2=180 degrees.
But this is not correct because its average or mean should be 0 degrees.
So we wrap 358 into -2.
Now the answer is 0.
So you have to apply wrapping and then you can calculate standard deviation from above link.
UPDATE:
Convert RGB to HSV
// r,g,b values are from 0 to 1 // h = [0,360], s = [0,1], v = [0,1]
// if s == 0, then h = -1 (undefined)
void RGBtoHSV( float r, float g, float b, float *h, float *s, float *v )
{
float min, max, delta;
min = MIN( r, MIN(g, b ));
max = MAX( r, MAX(g, b ));
*v = max;
delta = max - min;
if( max != 0 )
*s = delta / max;
else {
// r = g = b = 0
*s = 0;
*h = -1;
return;
}
if( r == max )
*h = ( g - b ) / delta;
else if( g == max )
*h=2+(b-r)/delta;
else
*h=4+(r-g)/delta;
*h *= 60;
if( *h < 0 )
*h += 360;
}
and then calculate standard deviation for hue value by this:
double calcStddev(ArrayList<Double> angles){
double sin = 0;
double cos = 0;
for(int i = 0; i < angles.size(); i++){
sin += Math.sin(angles.get(i) * (Math.PI/180.0));
cos += Math.cos(angles.get(i) * (Math.PI/180.0));
}
sin /= angles.size();
cos /= angles.size();
double stddev = Math.sqrt(-Math.log(sin*sin+cos*cos));
return stddev;
}