Variation on Gradiant lesson produces Halide::CompileError

[crmartin781]

I have adapted lesson_02 from the tutorials, which defines a gradiant Func with an int32_t buffer, and verifies it with C code.

My variation is as follows (comes from lesson_02, but packaged in a function)

`
Buffer<uint16_t> gradient_image(uint16_t width, uint16_t height)
{
Buffer<uint16_t> im1 = Runtime::Buffer<uint16_t>(width, height);

// Next we define our Func object that represents our one pipeline stage
Func grad;

// Our Func will have three arguments, representing the position
// in the image and the color channel. Halide treats color
// channels as an extra dimension of the image.
Var x, y;

Expr img = im1(x,y);
img = x+y;

// Define the 'grad' function, as a sequence of Expr's
grad(x, y) = img;

// Remember, all we've done so far is build a representation of a
// Halide program in memory. We haven't actually processed any
// pixels yet. We haven't even compiled that Halide program yet.

// So now we'll realize the Func, with the specified schedule.
// The size of the output image
// should match the size of the input image. If we just wanted to
// manipulate a portion of the input image we could request a
// smaller size. If we request a larger size Halide will throw an
// error at runtime telling us we're trying to read out of bounds
// on the input image.

//CRM:  this invocation throws a Halide::CompileError exception...why??
Buffer<uint16_t> output = grad.realize({width, height});

return output;

}
`
My questions:

1 - why would it throw here?
2 - how do I debug this exception?

Thanks, Charles.