pass matlab image to open3d three::Image in a mex script - matlab

Question

I am trying to load an image in a mex script and cast it to the corresponding format that the Open3D library uses, i.e. three::Image. I am using the following code:
uint8_t* rgb_image = (uint8_t*) mxGetPr(prhs[3]);
int* dims = (int*) mxGetDimensions(prhs[3]);
int height = dims[0];
int width = dims[2];
int channels = dims[4];
int imsize = height * width;
Image image;
image.PrepareImage(height, width, 3, sizeof(uint8_t)); // parameters: height, width, num_of_channels, bytes_per_channel
memcpy(image.data_.data(), rgb_image, image.data_.size());
The above works well when I give a grayscale image and specify num_of_channels to 1 but not for 3 channel images as you can notice below:
Then I tried to create a function where I am manually looping through the raw data and assigning them to the output image
auto image_ptr = std::make_shared<Image>();
image_ptr->PrepareImage(height, width, channels, sizeof(uint8_t));
for (int i = 0; i < height * width; i++) {
uint8_t *p = (uint8_t *)(image_ptr->data_.data() + i * channels * sizeof(uint8_t));
*p++ = *rgb_image++;
}
But now it seems that the color channels are wrongly assigned:
Any idea how to address this issue. The point is that it seems to be something easy but since my knowledge with C++ and pointers is quite limited I cannot figure it out straight forward.
I found this solution here (Reading image in matlab in a format acceptable to mex) as well but I am not sure how exactly I can use it. To be honest I am quite of confused.

Answer 1

ok the solution was quite straight forward as I was though in first place. It was just playing correctly with the pointers:
std::shared_ptr<Image> CreateRGBImageFromMat(uint8_t *mat_image, int width, int height, int channels)
{
auto open3d_image = std::make_shared<Image>();
open3d_image->PrepareImage(height, width, channels, sizeof(uint8_t));
for (int i = 0; i < height * width; i++) {
uint8_t *p = (uint8_t *)(open3d_image->data_.data() + i * channels * sizeof(uint8_t));
*p++ = *(mat_image + i);
*p++ = *(mat_image + i + height*width);
*p++ = *(mat_image + i + height*width*2);
}
return open3d_image;
}
since the three::Image expects the data in contiguous order row x col x channel while from matlab the image comes in blocks rows x cols x channel_1 (after you transpose the image since matlab is column major). My question though now is whether I can do the same with memcpy() or std::copy() where I can copy the bloc data to contiguous form so that I bypass the for loop.