vtk_pick_voxel.cpp

auto picker = vtkSmartPointer<vtkPropPicker>::New();
picker->Pick(screen_x, screen_y, 0, renderer);

// Check if pick is in bounds
vtkAssemblyPath* path = picker->GetPath();
bool validPick = false;
if (path) {
    vtkCollectionSimpleIterator sit;
    path->InitTraversal(sit);
    vtkAssemblyNode *node;
    for (int i = 0; i < path->GetNumberOfItems() && !validPick; ++i) {
        node = path->GetNextNode(sit);
        if (vtkImageSlice::SafeDownCast(node->GetViewProp()) == labels_actor) {
            validPick = true;
            break;
        }
    }
}

// Out of bounds
if (!validPick) {
    return -1;
}

// Convert from world coordinates to image coordinates
double p[3] = { 0. };
picker->GetPickPosition(p);
double origin[3] = { 0. };
labels_image->GetOrigin(origin);
double spacing[3] = { 0. };
labels_image->GetSpacing(spacing);

int x = (p[0] - origin[0])/spacing[0];
int y = (p[1] - origin[1])/spacing[1];
int z = (p[2] - origin[2])/spacing[2];

// Retrieving scalar value
return labels_image->GetScalarComponentAsFloat(x, y, z, 0);