pass the pointer of image buffer to OpenCV and change the saturation? - c++

Question

I want to pass the pointer of my image buffer, change the saturation and see the result immediately. But the change is not applying in my buffer and it is not changing.
void changeSaturation(void* buffer,int width, int height)
{
Mat matObject(width, height, CV_8UC4, buffer);
m_matSource = matObject;
Mat newMat = m_matSource.clone();
// BGR to HSV
cvtColor(matSource, matSource, CV_BGR2HSV);
for(int i = 0; i < newMat.rows; ++i)
{
for(int j = 0; j < newMat.cols; ++j)
{
newMat.at<cv::Vec3b>(i, j)[1] = 255; //saturationValue;
}
}
// HSV back to BGR
cvtColor(newMat, m_matSource, CV_HSV2BGR); // here m_matSource->data change
}
How can I apply the change on my buffer?

Answer 1

I refactored your code when trying to reproduce your problem and in the process I fixed it. You cloned your source into newMat then changed the color space of your original image and then proceed to completely ignore your new modified image. Try this out:
void changeSaturation(Mat& image)
{
Mat result(image.rows, image.cols, image.type());
// BGR to HSV
cvtColor(image, result, CV_BGR2HSV);
for(int i = 0; i < result.rows; ++i)
{
for(int j = 0; j < result.cols; ++j)
result.at<cv::Vec3b>(i, j)[1] = 255; //saturationValue;
}
// HSV back to BGR
cvtColor(result, result, CV_HSV2BGR); // here m_matSource->data change
namedWindow("Original");
imshow("Original",image);
namedWindow("Duplicate");
imshow("Duplicate",result);
}
int main()
{
Mat image;
image = imread("C:/Users/Public/Pictures/Sample Pictures/Desert.jpg");
changeSaturation(image);
waitKey(0);
}
Edit
To modify the input image:
void changeSaturation(Mat& image)
{
// BGR to HSV
cvtColor(image, image, CV_BGR2HSV);
for(int i = 0; i < image.rows; ++i)
{
for(int j = 0; j < image.cols; ++j)
image.at<cv::Vec3b>(i, j)[1] = 255; //saturationValue;
}
// HSV back to BGR
cvtColor(image, image, CV_HSV2BGR); // here m_matSource->data change
}
Next Edit
This now has (almost) the original function signature:
void changeSaturation(uchar* buffer, int rows, int cols, int type)
{
Mat image(rows, cols, type, buffer);
Mat result;
// BGR to HSV
cvtColor(image, result, CV_BGR2HSV);
for(int i = 0; i < result.rows; ++i)
{
for(int j = 0; j < result.cols; ++j)
result.at<cv::Vec3b>(i, j)[1] = 255;
}
// HSV back to BGR
cvtColor(result, image, CV_HSV2BGR);
}
int main()
{
Mat image;
image = imread("C:/Users/Public/Pictures/Sample Pictures/Desert.jpg");
changeSaturation(image.data, image.rows, image.cols, image.type());
imshow("Original",image);
waitKey(0);
}

Answer 2

Your constructor Mat matObject(width, height, CV_8UC4, buffer); allocates matObject of size width and height at the location pointed by buffer.
In your function you are making changes to newMat, which is cloned from matObject. However, buffer doesn't point to newMat, it points to matObject and matObject is not changed by your function.