no-wrap option; pivoted exponential zoom

client/world/pivot_zoom.go

@@ -0,0 +1,44 @@
+package world
+
+// PivotZoomCameraNoWrap adjusts camera center so that the world point under the cursor remains fixed
+// when zoom changes from oldZoomFp to newZoomFp.
+//
+// Coordinate conventions:
+// - CameraXWorldFp/YWorldFp is the center of the viewport in world-fixed units.
+// - cursorXPx/cursorYPx are pixel coordinates relative to the top-left of the viewport.
+// - viewportW/H are viewport size in pixels.
+//
+// This function does not clamp the result; caller should clamp for no-wrap mode using ClampCameraNoWrapViewport.
+func PivotZoomCameraNoWrap(
+	cameraXWorldFp, cameraYWorldFp int,
+	viewportW, viewportH int,
+	cursorXPx, cursorYPx int,
+	oldZoomFp, newZoomFp int,
+) (newCamX, newCamY int) {
+	if oldZoomFp <= 0 || newZoomFp <= 0 {
+		panic("PivotZoomCameraNoWrap: invalid zoom")
+	}
+	if viewportW <= 0 || viewportH <= 0 {
+		panic("PivotZoomCameraNoWrap: invalid viewport")
+	}
+
+	// Offset of cursor from viewport center in pixels.
+	offXPx := cursorXPx - viewportW/2
+	offYPx := cursorYPx - viewportH/2
+
+	// World-fixed per 1 pixel at each zoom.
+	// (Conservative: integer arithmetic, consistent with PixelSpanToWorldFixed.)
+	oldWorldPerPx := PixelSpanToWorldFixed(1, oldZoomFp)
+	newWorldPerPx := PixelSpanToWorldFixed(1, newZoomFp)
+
+	// World point under cursor before zoom:
+	// world = camera + offsetPx * worldPerPx
+	worldX := cameraXWorldFp + offXPx*oldWorldPerPx
+	worldY := cameraYWorldFp + offYPx*oldWorldPerPx
+
+	// Choose new camera so that the same world point stays under cursor:
+	// camera' = world - offsetPx * newWorldPerPx
+	newCamX = worldX - offXPx*newWorldPerPx
+	newCamY = worldY - offYPx*newWorldPerPx
+	return
+}