How to convert image storage order from channel-height-width to height-width-channel? - c++

Question

I would like to know how to convert an image stored as a 1D std::vector<float> from CHW format (Channel, Height, Width) to HWC format (Height, Width, Channel) in C++. The format change is needed due to requirements of a neural network.
I used OpenCV to read and show the image as below:
cv::namedWindow("Screenshot", cv::WINDOW_AUTOSIZE );
cv::imshow("Screenshot", rgbImage);
Then I converted the cv::Mat rgbImage to a 1D std::vector<float> in format CHW:
size_t channels = 3;
std::vector<float> data(channels*ROS_IMAGE_HEIGHT*ROS_IMAGE_WIDTH);
for(size_t j=0; j<ROS_IMAGE_HEIGHT; j++){
for(size_t k=0; k<ROS_IMAGE_WIDTH; k++){
cv::Vec3b intensity = rgbImage.at<cv::Vec3b>(j, k);
for(size_t i=0; i<channels; i++){
data[i*ROS_IMAGE_HEIGHT*ROS_IMAGE_WIDTH + j*ROS_IMAGE_HEIGHT + k] = (float) intensity[i];
}
}
}
Now I want to convert the format of std::vector<float> data to HWC. How can I do this?

Answer 1

I found some description of the "CHW" and "HWC" formats here.
If the storage order is HWC, it means that
Each sample is stored as a column-major matrix (height, width) of float[numChannels] (r00, g00, b00, r10, g10, b10, r01, g01, b01, r11, g11, b11).
Thus a pixel (x, y, c) is found using
xStride = channels;
yStride = channels * width;
cStride = 1;
data[x*xStride + y*yStride + c*cStride]
If the storage order is CHW, it means that each channel is a different plane. A pixel (x, y, c) is found using
xStride = 1;
yStride = width;
cStride = width * height;
data[x*xStride + y*yStride + c*cStride]
Note that in the code in the question, data[i*ROS_IMAGE_HEIGHT*ROS_IMAGE_WIDTH + j*ROS_IMAGE_HEIGHT + k] is incorrect, j is the y-coordinate and should be multiplied by ROS_IMAGE_WIDTH.
The code in the question can be modified to yield a std::vector in the HWC format by replacing the line in the innermost loop by:
data[i + j*ROS_IMAGE_WIDTH*channels + k*channels] = (float) intensity[i];