Program crashes, because of std::rotate_copy - c++

Question

I am trying to display two cams next to each other, rotated by 90°. Displaying both cams works fine, but if I want to rotate the cams, the program crashes.
The camera is read with a QByteArray and shown with the QCamera variable.
You can choose which camera is displayed in which viewfinder, so it has a code like this:
QActionGroup *videoDevicesGroup = new QActionGroup(this);
videoDevicesGroup->setExclusive(true);
foreach(const QByteArray &deviceName, QCamera::availableDevices()) {
QString description = camera->deviceDescription(deviceName);
QAction *videoDeviceAction = new QAction(description, videoDevicesGroup);
videoDeviceAction->setCheckable(true);
videoDeviceAction->setData(QVariant(deviceName));
if (cameraDevice.isEmpty()) {
cameraDevice = deviceName;
videoDeviceAction->setChecked(true);
}
ui->menuDevices->addAction(videoDeviceAction);
}
connect(videoDevicesGroup, SIGNAL(triggered(QAction*)), SLOT(updateCameraDevice(QAction*)));
if (cameraDevice.isEmpty())
{
camera = new QCamera;
}
else
{
camera = new QCamera(cameraDevice);
}
connect(camera, SIGNAL(stateChanged(QCamera::State)), this, SLOT(updateCameraState(QCamera::State)));
connect(camera, SIGNAL(error(QCamera::Error)), this, SLOT(displayCameraError()));
camera->setViewfinder(ui->viewfinder);
updateCameraState(camera->state());
camera->start();
Now I'm trying to rotate this cam with the command:
std::roate_copy(cameraDevice.constBegin(), cameraDevice.constEnd(), cameraDevice.constEnd, reverse.begin());
camera = new QCamera(reverse);
But when I try to start the program the program crashes, without any errors.
How can I fix this?

Answer 1

I think you have a misunderstanding on what std::rotate_copy does.
std::rotate_copy takes a range of data and shifts it as it copies into the location pointed to by the result iterator.
This won't rotate a camera. It just shifts and copies ranges: http://www.cplusplus.com/reference/algorithm/rotate_copy
EDIT:
Think about it this way say I have: std::string("wroybgivb");
Now say I do str::rotate_copy and I pick the "y" as my middle the std::string that I copied into will contain "ybgivbwro".
Now think about that like I was working with a 3X3 image and each character represented a color:
wro ybg
ybg => ivb
ivb wro
Note that this is doing a linear array rotation (position shifting). I can never pick a middle such that rows will become columns and columns will become rows.
PS:
OK so say that you knew the width of the image, and assigned it to the variable width. You can do something like this to rotate 90° clockwise:
for(int x = 0; x < size; ++x){
output[width - 1 - i / width + (i % width) * width] = input[i];
}
To understand this you need to understand indexing a linear array as though it's a 2D array.
Use this to access the x coordinate: i % width
Use this to access the y coordinate: (i / width) * width
Now you need to take those indices and rotate them still inside a linear array.
Use this to access the x coordinate: width - 1 - i / width
Use this to access the y coordinate: (i % width) * width