diff --git a/src/shared/dmxUtil.ts b/src/shared/dmxUtil.ts
index ef00951e..c4cfc920 100644
--- a/src/shared/dmxUtil.ts
+++ b/src/shared/dmxUtil.ts
@@ -1,4 +1,4 @@
-import { Window, Window2D_t } from '../shared/window'
+import { Window, Window2D_t, window2DToParentCoords } from '../shared/window'
 import {
   DmxValue,
   DMX_MAX_VALUE,
@@ -291,7 +291,7 @@ export function flatten_fixture(
   let flattened: FlattenedFixture[] = fixture_type.subFixtures.map((sub) => {
     return {
       intensity: sub.intensity ?? fixture_type.intensity,
-      window: sub.relative_window ?? fixture.window,
+      window: sub.relative_window ? window2DToParentCoords(sub.relative_window, fixture.window) : fixture.window,
       channels: sub.channels.map((ch_index) => {
         subfixture_ch_indexes.add(ch_index)
         return [base_channel + ch_index, fixture_type.channels[ch_index]]
diff --git a/src/shared/window.ts b/src/shared/window.ts
index 37fcfcec..aa7d4a88 100644
--- a/src/shared/window.ts
+++ b/src/shared/window.ts
@@ -9,3 +9,17 @@ export type Window2D_t = {
   x?: Window
   y?: Window
 }
+
+export function windowToParentCoords(relative: Window, parent: Window): Window {
+  return {
+    pos: parent.pos + (relative.pos - 0.5) * parent.width,
+    width: relative.width * parent.width
+  }
+}
+
+export function window2DToParentCoords(relative: Window2D_t, parent: Window2D_t): Window2D_t {
+  return {
+    x: relative.x && parent.x ? windowToParentCoords(relative.x, parent.x) : undefined,
+    y: relative.y && parent.y ? windowToParentCoords(relative.y, parent.y) : undefined
+  }
+}