Form update before hefty function

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C#

I don't quite know how to solve this. My Googling obviously was obviously not using the correct terms.

I have just built a Ray Tracer (works very well if I don't say so myself). Obviously the

Image Draw() {}

function is a hefty load and can take anywhere between 1 sec -- many days depending on the scene


When I call the draw function I want the user to know it has begun processing by displaying Action: Rendering and when its complete I want it to display Rendering complete

This is all handled by a render button, which calls this when clicked

action_label.Text = "Action: Rendering...Please Wait!";
Invalidate(); //I hoped this would work
viewport.Image = Scene.Draw();
action_label.Text = "Action: Rendering Complete!";

I hoped calling invalidate just after setting the text would work. All that happens is it shows Rendering complete.

How can I get it to update the text on the form before it starts to process?
Thank you all in advance :)