camera.go

package main

// Camera defines the visible viewport in world pixels (screen = world*zoom - offset).
type Camera struct {
	X, Y float64 // top-left of the view in world pixel space
	Zoom float64
}

func NewCamera(x, y, zoom float64) Camera {
	return Camera{X: x, Y: y, Zoom: zoom}
}

// ScreenToWorld converts screen pixel coordinates into world pixel coordinates.
func (c *Camera) ScreenToWorld(sx, sy float64) (float64, float64) {
	return c.X + sx/c.Zoom, c.Y + sy/c.Zoom
}

// Move pans the camera by world-space deltas.
func (c *Camera) Move(dx, dy float64) {
	c.X += dx
	c.Y += dy
}

// ZoomAt zooms the camera by factor, keeping the world point under the given
// screen coordinate fixed (zoom toward the cursor). There is no zoom limit:
// the renderer swaps to a pre-rendered overview sprite when zoomed out far
// enough, so arbitrary zoom-out stays fast. Only non-positive zoom is guarded
// against, since it would break the projection math.
func (c *Camera) ZoomAt(factor float64, sx, sy float64) {
	wx, wy := c.ScreenToWorld(sx, sy)
	c.Zoom *= factor
	if c.Zoom < 1e-6 {
		c.Zoom = 1e-6
	}
	c.X = wx - sx/c.Zoom
	c.Y = wy - sy/c.Zoom
}