How to compare two RGB images in Qt? - c++

Question

I need to compare two color RBG images and get a result images of difference pixel by pixel. Is there any idea how can I do that in qt ?
I would appreciate for any help or advice.

Answer 1

Here is an alternative based on this QtForum question:
void substract(const QImage &left, const QImage &rigth, QImage &result)
{
int w=min(left.width(), rigth.width());
int h=min(left.height(),rigth.height();
w=min(w, result.width());
h=min(h, result.height();
//<-This ensures that you work only at the intersection of images areas
for(int i=0;i<h;i++){
QRgb *rgbLeft=(QRgb*)left.constScanLine(i);
QRgb *rgbRigth=(QRgb*)rigth.constScanLine(i);
QRgb *rgbResult=(QRgb*)result.constScanLine(i);
for(int j=0;j<w;j++){
rgbResult[j] = rgbLeft[j]-rgbRigth[j];
}
}
}

Answer 2

First, an RGB image is a 3-dimensional matrix containing the 3 channels (R,G,B)
To achieve the difference you can simply subtract the matrixes.
If you're using OpenCv consider the code below, otherwise you can traverse the matrixes and subtract each position separately.
#include <cv.h>
#include <highgui.h>
using namespace cv;
Mat img = imread("...");
Mat img2 = imread("...");
Mat diff_img = img - img2;

Answer 3

Using QImage you can iterate on pixel level, and simply output the RBG differences into a third image.
QRgb QImage::pixel(int x, int y) const
void QImage::setPixelColor(int x, int y, const QColor &color)
Keep in mind to iterate row sequentially for optimal performance. Meaning that the row should be your inner loop. A lot of the time people instinctively do the opposite, probably because most people prioritize width over height, therefore putting the row as the outer loop.

Answer 4

bool ImagesAreSimilar(QImage *img1,QImage *img2)
{
if (img1->isNull()||img2->isNull())
{
return false ;
}
if (img1->height()!=img2->height()) {
return false ;
}
if (img1->width()!=img2->width()) {
return false ;
}
auto pixel1 = img1->bits();
auto pixel2 = img2->bits();
bool similar=true;
for (int y = 0; y < img1->height(); y++)
{
for (int x = 0; x < img1->width(); x++)
{
if ( (pixel1[0]!=pixel2[0])||
(pixel1[1]!=pixel2[1])||
(pixel1[2]!=pixel2[2])||
(pixel1[3]!=pixel2[3])) {
return false ;
}
pixel1 += 4;
pixel2 += 4;
}
}
return similar;
}