How can I return a custom struct from a BOOST_COMPUTE_FUNCTION

Lets say I have a pixel struct and want to loop through two vectors of pixels add the components of the two pixels together and then put them in another vector, how can I return a new struct from transform?

```c++
struct pixel {
int r, g, b, a;
};

BOOST_COMPUTE_ADAPT_STRUCT(pixel, pixel, (r,g,b,a))

//main

vector image1( size, pixel{0,0,0,1});
vector image2( size, pixel{1,0,0,1});
vector result( size, pixel{0,0,0,1});

// setup up device, context, queue and device vectors ...

BOOST_COMPUTE_FUNCTION(pixel, add_images, (pixel a, pixel b),
{
pixel out;
out.r = a.r + b.r;
// etc....

return out;
});

compute::transform(
device_image1.begin(),
device_image1.end(),
device_image2.begin(),
device_result.begin(),
add_images,
queue
);

// get results...
```

How do I allocate memory for the result pixel inside the Compute Function?