diff --git a/src/app/calibration-ui.ts b/src/app/calibration-ui.ts
index b78cf07..12b99af 100644
--- a/src/app/calibration-ui.ts
+++ b/src/app/calibration-ui.ts
@@ -3,6 +3,7 @@ import { estimateHomography, applyHomography } from '../geometry/homography';
 
 /** Per-point reprojection error in pixels: |H·machine − image|. */
 export function residuals(H: Mat3, machine: Vec2[], image: Vec2[]): number[] {
+  if (machine.length !== image.length) throw new Error('residuals: machine/image length mismatch');
   return machine.map((m, i) => {
     const p = applyHomography(H, m);
     return Math.hypot(p[0] - image[i]![0], p[1] - image[i]![1]);
@@ -23,6 +24,10 @@ export interface CalibrationDeps {
 /**
  * Renders the calibration panel. Flow: enter machine X/Y → click "Click point in video" →
  * click the spindle tip in the video → repeat ≥4× → "Compute".
+ *
+ * Captured image points and the resulting homography are in normalized [0,1] camera-frame
+ * coordinates (machine-mm → normalized image), so they are display-size independent.
+ * The JSON output structure stays the same (imagePoints now hold normalized values).
  */
 export function mountCalibration(deps: CalibrationDeps): void {
   const { panel, overlay } = deps;
@@ -53,7 +58,7 @@ export function mountCalibration(deps: CalibrationDeps): void {
 
   const renderList = (errs?: number[]) => {
     list.innerHTML = points
-      .map((p, i) => `<li>(${p.machine[0]}, ${p.machine[1]}) → px(${p.image[0].toFixed(0)}, ${p.image[1].toFixed(0)})${errs ? ` <small>err ${errs[i]!.toFixed(1)}px</small>` : ''}</li>`)
+      .map((p, i) => `<li>(${p.machine[0]}, ${p.machine[1]}) → img(${p.image[0].toFixed(3)}, ${p.image[1].toFixed(3)})${errs ? ` <small>err ${(errs[i]! * overlay.width).toFixed(1)}px</small>` : ''}</li>`)
       .join('');
   };
 
@@ -69,10 +74,11 @@ export function mountCalibration(deps: CalibrationDeps): void {
   overlay.addEventListener('click', (ev) => {
     if (!pendingMachine || !overlay.classList.contains('interactive')) return;
     const rect = overlay.getBoundingClientRect();
-    // Map click to native image pixels: canvas backing store == displayed media box.
+    // Store normalized [0,1] image coords (fraction of the displayed camera frame).
+    // This makes the calibration independent of display/canvas size.
     const px: Vec2 = [
-      ((ev.clientX - rect.left) / rect.width) * overlay.width,
-      ((ev.clientY - rect.top) / rect.height) * overlay.height,
+      (ev.clientX - rect.left) / rect.width,
+      (ev.clientY - rect.top) / rect.height,
     ];
     points.push({ machine: pendingMachine, image: px });
     pendingMachine = null;
diff --git a/src/main.ts b/src/main.ts
index f3fadbb..1fd429e 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -18,7 +18,13 @@ let style: RenderStyle = { cutColor: '#00e5ff', rapidColor: '#ff9800', lineWidth
 
 function render(): void {
   clearCanvas(ctx, overlay.width, overlay.height);
-  const proj = state.projected();
+  const w = overlay.width;
+  const h = overlay.height;
+  // projected() yields normalized [0,1] image coords; scale to canvas pixels.
+  const proj = state.projected().map((seg) => ({
+    kind: seg.kind,
+    points: seg.points.map((p): [number, number] => [p[0] * w, p[1] * h]),
+  }));
   drawOverlay(ctx, proj, style);
 }
 
diff --git a/src/types.ts b/src/types.ts
index 5224bca..666b06a 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -19,11 +19,11 @@ export interface Alignment {
 }
 
 export interface Calibration {
-  /** Clicked points in the camera image, pixels. */
+  /** Calibration points in normalized [0,1] camera-frame coordinates. */
   imagePoints: Vec2[];
   /** Corresponding machine coordinates, mm. */
   machinePoints: Vec2[];
-  /** machine-mm → image-px. */
+  /** machine-mm → normalized [0,1] image coords. */
   homography: Mat3;
 }