Getting Contrast Value of Every Pixel in Image - c++

Question

I would like to get average contrast value of an image and I found working code in processing .I tried to apply this in qt-creator as in c++, however when I try to run my code I get this error:
cannot convert ‘cv::Mat’ to ‘uint8_t* {aka unsigned char*}’ in initialization
uint8_t* rowPtr = foo.row(i);<
In these parts:
uint8_t* rowPtr = foo.row(i);>
Here is the code I am trying to run;
bool normalizeRange = true; // normalize results to range of 0-1
float contrast = 0;
float brightness =0;
Mat foo = imread("1.png");
int cn = foo.channels();
Scalar_<uint8_t> bgrPixel;
for(int i = 0; i < foo.rows; i++)
{
uint8_t* rowPtr = foo.row(i);
for(int j = 0; j < foo.cols; j++)
{
bgrPixel.val[0] = rowPtr[j*cn + 0]; // B
bgrPixel.val[1] = rowPtr[j*cn + 1]; // G
bgrPixel.val[2] = rowPtr[j*cn + 2]; // R
// do something with BGR values...
}
}
brightness += (0.2126 * bgrPixel.val[2]) + (0.7152 * bgrPixel.val[1]) + (0.0722 * bgrPixel.val[0]); // scales RGB to perceived brightness
if (normalizeRange) {
brightness /= 255.0; // normalize to 0-1
}
brightness /= sizeof(bgrPixel); // average result
qDebug() << "Average brightness:" << brightness;
// find contrast by comparing average brightness with current value
for(int i = 0; i < foo.rows; i++)
{
uint8_t* rowPtr = foo.row(i);
for(int j = 0; j < foo.cols; j++)
{
bgrPixel.val[0] = rowPtr[j*cn + 0]; // B
bgrPixel.val[1] = rowPtr[j*cn + 1]; // G
bgrPixel.val[2] = rowPtr[j*cn + 2]; // R
// do something with BGR values...
}
}
float pxIntensity = (0.2126 * bgrPixel.val[2]) + (0.7152 * bgrPixel.val[1]) + (0.0722 * bgrPixel.val[2]);
if (normalizeRange) {
pxIntensity /= 255.0; // normalizes to range 0-1
}
contrast += pow((brightness - pxIntensity), 2);
contrast /= sizeof(bgrPixel);
qDebug() << "Average cotrast:" << contrast;
}
Update: I have added the java code which I would like to run in C++
boolean normalizeRange = true; // normalize results to range of 0-1
PImage img;
float brightness = 0;
float contrast = 0;
void setup() {
img = loadImage(filename);
size(img.width, img.height);
image(img, 0,0);
loadPixels(); // load pixels into array, iterate!
// find average brightness across image
for (color c : pixels) {
float r = c >> 16 & 0xFF; // extract RGB values quickly (better than red(), etc)
float g = c >> 8 & 0xFF;
float b = c & 0xFF;
brightness += (0.2126 * r) + (0.7152 * g) + (0.0722 * b); // scales RGB to perceived brightness
if (normalizeRange) {
brightness /= 255.0; // normalize to 0-1
} }
brightness /= pixels.length; // average result
println("Average brightness: " + brightness);
// find contrast by comparing average brightness with current value
for (color c : pixels) {
float r = c >> 16 & 0xFF;
float g = c >> 8 & 0xFF;
float b = c & 0xFF;
float pxIntensity = (0.2126 * r) + (0.7152 * g) + (0.0722 * b);
if (normalizeRange) {
pxIntensity /= 255.0; // normalizes to range 0-1 }
contrast += pow((brightness - pxIntensity), 2);}
contrast /= pixels.length;
println("Average contrast: " + contrast);
}

Answer 1

Use cv::Mat::ptr() to obtain a pointer to the specified matrix row buffer.
cv::Mat::row() constructs a new cv::Mat header for submatrix that consists of that single row instead.
Iterating raw cv::Mat buffer may be more efficient but less readable way to iterate elements. Read about different ways and their comparison here.