Flatten array of structs efficiently - c++

Question

I'm looking for the most efficient way to flatten an array of structs in C++ for passing the flattend 1D array data as input to a cv::Mat. The struct looks as follows:
struct Color3
{
uint8_t red, green, blue;
}
My code then looks like this:
// Update color frame
cv::Mat colorMat = cv::Mat::zeros(cv::Size(1920, 1080), CV_8UC3)
const Color3* colorPtr = colorFrame->getData(); // Get Frame from Library
std::vector<uchar> vecColorData;
data.reserve(1920 * 1080 * 3);
for (int i = 0; i < 1920 * 1080; ++i)
{
auto color = *colorPtr;
vecColorData.push_back(color.red);
vecColorData.push_back(color.green);
vecColorData.push_back(color.blue);
vecColorData++;
}
colorMat.data = vecColorData.data();
Is there a more efficient way than creating an intermediate std::vector and looping over the entire array? I guess I'm looking for something like:
colorMat.data = colorFrame->getData()
However, I'm getting the following error: a value of type Color3* cannot be assigned to an entity of type uchar*.

Answer 1

you don't need an intermediate vector.
If I understood, you want to assign the same RGB triple to all data.
It is also unclear to me if you have to allocate colorMat.data on your own or not.
If this is the case, once colorMat.data is allocated and sized 1920 * 1080 * 3, you can do something like the following:
uchar * data = colorMat.data;
for (int i = 0; i < 1920 * 1080; ++i)
{
*data++ = (uchar)colorPtr->red;
*data++ = (uchar)colorPtr->green;
*data++ = (uchar)colorPtr->.blue;
}

Answer 2

The following answer is not technically portable but will work on the vast majority of platforms you will encounter in real life.
It is extremely likely that your Color3 struct has no padding. You can veryify this by using a static_assert:
static_assert(sizeof(Color3) == sizeof(uint8_t) * 3);
With this confirmed you can cast an array of Color3 to an array of uint8_t and pass it directly to colorMat.data (assuming that member actually accepts uint8_t).
Your code therefore becomes:
cv::Mat colorMat = cv::Mat::zeros(cv::Size(1920, 1080), CV_8UC3)
const Color3* colorPtr = colorFrame->getData(); // Get Frame from Library
colorMat.data = reinterpret_cast<const uint8_t*>(colorPtr);
Bear in mind I have never used the cv library and know nothing about the ownership requirements of the data pointer. The above just replicates what you're doing without the unnecessary std::vector.