I'm trying to make a Draggable view in a 3D world.

Currently, I've made it using mouse position on the screen, but, when I move the distance traveled by my mouse is not equal to the distance traveled in the 3D world.

So, I've tried to do that :

1. Compute a ray from mouse position to 3D world.
2. Calculate intersection with the ground.
3. Check intersection difference old position <-> new position.
4. Translate camera with the difference.

I've got a problem with this method:

1. The ray is computed with the current camera's position
2. I move the camera
3. I compute the new ray with new camera position.
4. The difference between old ray and new ray is now invalid.

So, graphically my camera don't stop to move to previous/new position everytime.

How can I do a draggable camera with another solution ?

Thanks!