diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..2b31f87
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.mimocode/
diff --git a/.mimocode/plans/1782265108554-gentle-rocket.md b/.mimocode/plans/1782265108554-gentle-rocket.md
deleted file mode 100644
index 27d5d3f..0000000
--- a/.mimocode/plans/1782265108554-gentle-rocket.md
+++ /dev/null
@@ -1,92 +0,0 @@
-# Plan: Background Image Upload for CSS Matrix Calculator
-
-## Overview
-Add ability to load a background image onto the main `#field` element for visual reference when positioning corners. The image is stored as a data URL in memory (no server). Three upload methods: Ctrl+V paste, file picker, drag & drop onto the field. Adjustable via `object-fit` options.
-
-## Files to Modify
-
-### 1. `js/state.js` — Add background state
-- Add `bgImage: null` (data URL string) and `objectFit: 'contain'` to `defaultState` and `state`
-- Reset these in `resetState()`
-
-### 2. `index.html` — New "Background" panel + hidden file input
-Add a new panel in `right-col` (between Field and Widget panels, after line 78):
-```html
-<div class="panel">
-  <h2>Background</h2>
-  <div class="bg-controls">
-    <input type="file" id="bgFileInput" accept="image/*" hidden />
-    <button class="btn btn-edit-field" id="btnBgLoad">Load Image</button>
-    <button class="btn btn-cancel-field" id="btnBgRemove" style="display:none">Remove</button>
-    <select id="bgObjectFit">
-      <option value="contain">contain</option>
-      <option value="cover">cover</option>
-      <option value="fill">fill</option>
-      <option value="none">none</option>
-      <option value="scale-down">scale-down</option>
-    </select>
-  </div>
-</div>
-```
-
-Add drag & drop hint overlay on `#fieldContainer` (drop zone indicator).
-
-### 3. `style.css` — New styles
-- `.bg-controls` — flex row layout for controls
-- `#bgObjectFit` — styled select matching existing dark theme
-- `#fieldContainer.drag-over` — dashed border highlight for drop feedback
-- `#field` — add `background-size`, `background-position`, `background-repeat` properties
-- `.bg-hint` — overlay text on field when no image loaded ("Drop image or Ctrl+V")
-
-### 4. `js/ui.js` — Event handlers
-- `btnBgLoad` click → trigger hidden file input
-- `bgFileInput` change → read file as data URL via `FileReader` → `state.bgImage = dataUrl`
-- `document` paste event → detect image in `clipboardData.files` → read as data URL
-- `#fieldContainer` dragover/drop → read dropped file as data URL
-- `btnBgRemove` click → clear `state.bgImage`
-- `bgObjectFit` change → `state.objectFit = value`
-- Toggle remove button visibility based on whether image is loaded
-- On state change: call `updateAll()`
-
-### 5. `js/preview.js` — Apply background to field
-- If `state.bgImage`: set `field.style.backgroundImage = url(...)`, `field.style.backgroundSize`, `field.style.backgroundPosition`, `field.style.backgroundRepeat` based on objectFit
-- If no image: clear background properties
-
-### 6. `js/svg-renderer.js` — Show background in SVG editor
-- If `state.bgImage`: add an `<image>` element as first child of SVG (behind grid) with `href`, `width`, `height`, and `preserveAspectRatio` matching the objectFit mapping
-
-### 7. `js/main.js` — No changes needed (updateAll already propagates)
-
-## Implementation Details
-
-### Image loading flow
-1. User triggers upload (paste/file/drop) → get `File` object
-2. `FileReader.readAsDataURL(file)` → get data URL string
-3. Store in `state.bgImage`
-4. Call `updateAll()` to propagate
-
-### object-fit mapping to SVG preserveAspectRatio
-| objectFit     | SVG preserveAspectRatio |
-|---------------|------------------------|
-| contain       | xMidYMid meet          |
-| cover         | xMidYMid slice         |
-| fill          | none                   |
-| none          | xMinYMin meet          |
-| scale-down    | xMidYMid meet          |
-
-### Keyboard shortcut
-- Ctrl+V paste listener on `document` — check for image in clipboard items
-- Only active when no input/textarea is focused (to avoid conflicts)
-
-### Drag & drop
-- Listen on `#fieldContainer` for `dragover` (prevent default, show visual hint) and `drop` (read file)
-
-## Verification
-1. Open `index.html` in browser
-2. **File picker**: Click "Load Image" → select image → verify it appears as background on the field in both edit (SVG) and preview modes
-3. **Ctrl+V**: Copy an image to clipboard → press Ctrl+V → verify it loads
-4. **Drag & drop**: Drag an image file onto the field area → verify it loads
-5. **object-fit**: Change dropdown → verify background resizes correctly (contain fits, cover fills, etc.)
-6. **Remove**: Click "Remove" → verify background clears
-7. **Reset**: Click "Reset" → verify background clears along with other state
-8. **Corner drag**: Load background → drag corners → verify background stays in place, corners move independently
diff --git a/js/constants.js b/js/constants.js
new file mode 100644
index 0000000..464935a
--- /dev/null
+++ b/js/constants.js
@@ -0,0 +1,15 @@
+export const COLORS = {
+  fieldBg: '#0a0e14',
+  accent: '#58a6ff',
+  widgetFill: 'rgba(88,166,255,0.12)',
+  widgetStroke: 'rgba(88,166,255,0.4)',
+  polygonFill: 'rgba(88,166,255,0.1)',
+  gridLine: 'rgba(255,255,255,0.06)'
+};
+
+export const LABELS = ['TL', 'TR', 'BR', 'BL'];
+
+export const FIELD_PADDING = 140;
+
+export const ROUND_DIGITS = 6;
+export const round = v => parseFloat(v.toFixed(ROUND_DIGITS));
diff --git a/js/dom.js b/js/dom.js
new file mode 100644
index 0000000..40000d4
--- /dev/null
+++ b/js/dom.js
@@ -0,0 +1 @@
+export const $ = id => document.getElementById(id);
diff --git a/js/drop-zone.js b/js/drop-zone.js
new file mode 100644
index 0000000..3b02ad4
--- /dev/null
+++ b/js/drop-zone.js
@@ -0,0 +1,40 @@
+import { $ } from './dom.js';
+import { loadBgFile, loadWidgetFile } from './file-loader.js';
+
+function setupDropZone(el, onDrop) {
+  el.addEventListener('dragover', (e) => {
+    e.preventDefault();
+    el.classList.add('drag-over');
+  });
+
+  el.addEventListener('dragleave', (e) => {
+    if (!el.contains(e.relatedTarget)) {
+      el.classList.remove('drag-over');
+    }
+  });
+
+  el.addEventListener('drop', (e) => {
+    e.preventDefault();
+    document.body.classList.remove('file-dragging');
+    el.classList.remove('drag-over');
+    if (e.dataTransfer.files[0]) onDrop(e.dataTransfer.files[0]);
+  });
+}
+
+export function initDropZones(state) {
+  const lc = document.querySelector('.left-col');
+
+  document.addEventListener('dragover', (e) => {
+    e.preventDefault();
+    document.body.classList.add('file-dragging');
+  });
+
+  document.addEventListener('drop', () => {
+    document.body.classList.remove('file-dragging');
+    lc.classList.remove('drag-over');
+  });
+
+  setupDropZone(lc, (file) => loadBgFile(file, state));
+  setupDropZone($('bgPanel'), (file) => loadBgFile(file, state));
+  setupDropZone($('widgetPanel'), (file) => loadWidgetFile(file, state));
+}
diff --git a/js/file-loader.js b/js/file-loader.js
new file mode 100644
index 0000000..398c096
--- /dev/null
+++ b/js/file-loader.js
@@ -0,0 +1,49 @@
+import { $ } from './dom.js';
+
+let updateAllRef;
+
+export function setUpdateAllRef(fn) {
+  updateAllRef = fn;
+}
+
+export function loadBgFile(file, state) {
+  if (!file || !file.type.startsWith('image/')) return;
+  const reader = new FileReader();
+  reader.onload = () => {
+    state.bgImage = reader.result;
+    $('btnBgRemove').style.display = '';
+    updateAllRef();
+  };
+  reader.readAsDataURL(file);
+}
+
+export function clearBg(state) {
+  state.bgImage = null;
+  $('btnBgRemove').style.display = 'none';
+  updateAllRef();
+}
+
+export function setWidgetImageLock(locked) {
+  $('inpWidgetW').disabled = locked;
+  $('inpWidgetH').disabled = locked;
+  $('btnWidgetRemove').style.display = locked ? '' : 'none';
+}
+
+export function loadWidgetFile(file, state) {
+  if (!file || !file.type.startsWith('image/')) return;
+  const reader = new FileReader();
+  reader.onload = () => {
+    state.widgetImage = reader.result;
+    const img = new Image();
+    img.onload = () => {
+      state.widgetW = img.naturalWidth;
+      state.widgetH = img.naturalHeight;
+      $('inpWidgetW').value = state.widgetW;
+      $('inpWidgetH').value = state.widgetH;
+      setWidgetImageLock(true);
+      updateAllRef();
+    };
+    img.src = reader.result;
+  };
+  reader.readAsDataURL(file);
+}
diff --git a/js/homography.js b/js/homography.js
index c5e4c54..13d3363 100644
--- a/js/homography.js
+++ b/js/homography.js
@@ -1,3 +1,5 @@
+import { round } from './constants.js';
+
 export function computeHomography(state) {
   const w = state.widgetW, h = state.widgetH;
   if (w <= 0 || h <= 0) return null;
@@ -29,14 +31,13 @@ export function computeHomography(state) {
 
 export function toMatrix3dCSS(H) {
   if (!H) return '/* degenerate configuration */';
-  const n = v => parseFloat(v.toFixed(8));
   return [
     'transform-origin: 0 0;',
     'transform: matrix3d(',
-    `  ${n(H.h00)}, ${n(H.h10)}, 0, ${n(H.h20)},`,
-    `  ${n(H.h01)}, ${n(H.h11)}, 0, ${n(H.h21)},`,
+    `  ${round(H.h00)}, ${round(H.h10)}, 0, ${round(H.h20)},`,
+    `  ${round(H.h01)}, ${round(H.h11)}, 0, ${round(H.h21)},`,
     `  0, 0, 1, 0,`,
-    `  ${n(H.h02)}, ${n(H.h12)}, 0, ${n(H.h22)}`,
+    `  ${round(H.h02)}, ${round(H.h12)}, 0, ${round(H.h22)}`,
     ');'
   ].join('\n');
 }
diff --git a/js/inputs.js b/js/inputs.js
index c0fdf93..a4351f0 100644
--- a/js/inputs.js
+++ b/js/inputs.js
@@ -1,6 +1,5 @@
-const LABELS = ['TL', 'TR', 'BR', 'BL'];
-
-function $(id) { return document.getElementById(id); }
+import { $ } from './dom.js';
+import { LABELS } from './constants.js';
 
 export function buildCornerInputs(state) {
   const container = $('cornerInputs');
@@ -28,8 +27,14 @@ export function syncCornerInputs(state) {
 
 export function readCornerInputs(state) {
   state.corners.forEach((c, i) => {
-    const x = parseInt($(`c${i}x`).value);
-    const y = parseInt($(`c${i}y`).value);
+    const xi = $(`c${i}x`);
+    const yi = $(`c${i}y`);
+    const x = parseInt(xi.value);
+    const y = parseInt(yi.value);
+
+    xi.classList.toggle('input-error', isNaN(x));
+    yi.classList.toggle('input-error', isNaN(y));
+
     if (!isNaN(x)) c.x = x;
     if (!isNaN(y)) c.y = y;
   });
diff --git a/js/mode.js b/js/mode.js
new file mode 100644
index 0000000..c495a9b
--- /dev/null
+++ b/js/mode.js
@@ -0,0 +1,79 @@
+import { $ } from './dom.js';
+
+let fieldEditing = false;
+let fieldEditW, fieldEditH;
+let keepAspect = true;
+
+export function isFieldEditing() {
+  return fieldEditing;
+}
+
+export function getFieldEditW() {
+  return fieldEditW;
+}
+
+export function getFieldEditH() {
+  return fieldEditH;
+}
+
+export function setFieldEditW(v) {
+  fieldEditW = v;
+}
+
+export function setFieldEditH(v) {
+  fieldEditH = v;
+}
+
+export function isKeepAspect() {
+  return keepAspect;
+}
+
+export function toggleKeepAspect() {
+  keepAspect = !keepAspect;
+  updateChainIcon();
+}
+
+function updateChainIcon() {
+  const locked = $('chainLink').querySelectorAll('.chain-locked');
+  const unlocked = $('chainLink').querySelectorAll('.chain-unlocked');
+  locked.forEach(el => el.style.display = keepAspect ? '' : 'none');
+  unlocked.forEach(el => el.style.display = keepAspect ? 'none' : '');
+  $('chainLink').classList.toggle('unlocked', !keepAspect);
+}
+
+export function setFieldEditing(editing, state) {
+  fieldEditing = editing;
+  const inpW = $('inpFieldW');
+  const inpH = $('inpFieldH');
+  const options = $('fieldEditOptions');
+  const btnEdit = $('btnEditField');
+  const chain = $('chainLink');
+
+  if (editing) {
+    fieldEditW = state.fieldW;
+    fieldEditH = state.fieldH;
+    inpW.disabled = false;
+    inpH.disabled = false;
+    options.classList.add('visible');
+    btnEdit.style.display = 'none';
+    chain.classList.add('visible');
+    updateChainIcon();
+  } else {
+    inpW.value = state.fieldW;
+    inpW.disabled = true;
+    inpH.value = state.fieldH;
+    inpH.disabled = true;
+    options.classList.remove('visible');
+    btnEdit.style.display = '';
+    chain.classList.remove('visible');
+  }
+}
+
+export function setMode(mode) {
+  document.body.className = mode === 'preview' ? 'mode-preview' : 'mode-edit';
+  $('btnEdit').classList.toggle('active', mode === 'edit');
+  $('btnPreview').classList.toggle('active', mode === 'preview');
+  $('modeHint').textContent = mode === 'edit'
+    ? 'Drag corners on the field'
+    : 'matrix3d() transform result';
+}
diff --git a/js/preview.js b/js/preview.js
index 16840cf..abfdcea 100644
--- a/js/preview.js
+++ b/js/preview.js
@@ -1,6 +1,6 @@
+import { $ } from './dom.js';
 import { computeHomography } from './homography.js';
-
-function $(id) { return document.getElementById(id); }
+import { round, FIELD_PADDING } from './constants.js';
 
 function applyBgToField(field, state) {
   if (state.bgImage) {
@@ -33,7 +33,7 @@ export function updateLivePreview(state) {
   const cw = container.clientWidth;
   if (cw <= 0) return;
 
-  const maxH = window.innerHeight - 140;
+  const maxH = window.innerHeight - FIELD_PADDING;
   const scaleW = cw / state.fieldW;
   const scaleH = maxH / state.fieldH;
   const scale = Math.min(scaleW, scaleH);
@@ -54,8 +54,7 @@ export function updateLivePreview(state) {
 
   const H = computeHomography(state);
   if (H) {
-    const n = v => parseFloat(v.toFixed(6));
-    widget.style.transform = `matrix3d(${n(H.h00)},${n(H.h10)},0,${n(H.h20)},${n(H.h01)},${n(H.h11)},0,${n(H.h21)},0,0,1,0,${n(H.h02)},${n(H.h12)},0,${n(H.h22)})`;
+    widget.style.transform = `matrix3d(${round(H.h00)},${round(H.h10)},0,${round(H.h20)},${round(H.h01)},${round(H.h11)},0,${round(H.h21)},0,0,1,0,${round(H.h02)},${round(H.h12)},0,${round(H.h22)})`;
     widget.style.opacity = '1';
   } else {
     widget.style.transform = 'none';
diff --git a/js/state.js b/js/state.js
index c888eee..ca8e37d 100644
--- a/js/state.js
+++ b/js/state.js
@@ -12,29 +12,13 @@ export const defaultState = {
   widgetImage: null
 };
 
-export const state = {
-  fieldW: 1920, fieldH: 1080,
-  widgetW: 400, widgetH: 300,
-  corners: [
-    { x: 300, y: 200 },
-    { x: 1620, y: 150 },
-    { x: 1580, y: 920 },
-    { x: 340, y: 950 }
-  ],
-  bgImage: null,
-  objectFit: 'contain',
-  widgetImage: null
-};
+function deepClone(obj) {
+  return JSON.parse(JSON.stringify(obj));
+}
+
+export const state = deepClone(defaultState);
 
 export function resetState() {
-  state.fieldW = defaultState.fieldW;
-  state.fieldH = defaultState.fieldH;
-  state.widgetW = defaultState.widgetW;
-  state.widgetH = defaultState.widgetH;
-  state.bgImage = null;
-  state.objectFit = defaultState.objectFit;
-  state.widgetImage = null;
-  defaultState.corners.forEach((c, i) => {
-    state.corners[i] = { x: c.x, y: c.y };
-  });
+  const clone = deepClone(defaultState);
+  Object.assign(state, clone);
 }
diff --git a/js/svg-renderer.js b/js/svg-renderer.js
index 8ed7028..ee089b7 100644
--- a/js/svg-renderer.js
+++ b/js/svg-renderer.js
@@ -1,4 +1,5 @@
-const LABELS = ['TL', 'TR', 'BR', 'BL'];
+import { COLORS, LABELS } from './constants.js';
+
 const ns = 'http://www.w3.org/2000/svg';
 
 const OBJECT_FIT_MAP = {
@@ -22,15 +23,12 @@ function setAttrs(el, attrs) {
   }
 }
 
-export function renderSVG(svg, state) {
-  svg.setAttribute('viewBox', `0 0 ${state.fieldW} ${state.fieldH}`);
-  svg.innerHTML = '';
-
+function renderBackground(svg, state) {
   const rect = document.createElementNS(ns, 'rect');
   setAttrs(rect, {
     width: state.fieldW,
     height: state.fieldH,
-    fill: '#0a0e14'
+    fill: COLORS.fieldBg
   });
   svg.appendChild(rect);
 
@@ -44,21 +42,28 @@ export function renderSVG(svg, state) {
     });
     svg.appendChild(img);
   }
+}
 
+function renderGrid(svg, state) {
   const step = gridInterval(Math.max(state.fieldW, state.fieldH));
   const gridG = document.createElementNS(ns, 'g');
+
   for (let x = step; x < state.fieldW; x += step) {
     const line = document.createElementNS(ns, 'line');
-    setAttrs(line, { x1: x, y1: 0, x2: x, y2: state.fieldH, stroke: 'rgba(255,255,255,0.06)', 'stroke-width': 1 });
+    setAttrs(line, { x1: x, y1: 0, x2: x, y2: state.fieldH, stroke: COLORS.gridLine, 'stroke-width': 1 });
     gridG.appendChild(line);
   }
+
   for (let y = step; y < state.fieldH; y += step) {
     const line = document.createElementNS(ns, 'line');
-    setAttrs(line, { x1: 0, y1: y, x2: state.fieldW, y2: y, stroke: 'rgba(255,255,255,0.06)', 'stroke-width': 1 });
+    setAttrs(line, { x1: 0, y1: y, x2: state.fieldW, y2: y, stroke: COLORS.gridLine, 'stroke-width': 1 });
     gridG.appendChild(line);
   }
-  svg.appendChild(gridG);
 
+  svg.appendChild(gridG);
+}
+
+function renderWidgetRect(svg, state) {
   const cx = state.corners.reduce((s, c) => s + c.x, 0) / 4;
   const cy = state.corners.reduce((s, c) => s + c.y, 0) / 4;
 
@@ -68,31 +73,35 @@ export function renderSVG(svg, state) {
     y: cy - state.widgetH / 2,
     width: state.widgetW,
     height: state.widgetH,
-    fill: 'rgba(88,166,255,0.12)',
-    stroke: 'rgba(88,166,255,0.4)',
+    fill: COLORS.widgetFill,
+    stroke: COLORS.widgetStroke,
     'stroke-width': 2,
     'stroke-dasharray': '12,8'
   });
   svg.appendChild(widgetRect);
+}
 
+function renderPolygon(svg, state) {
   const poly = document.createElementNS(ns, 'polygon');
   const pts = state.corners.map(c => `${c.x},${c.y}`).join(' ');
   setAttrs(poly, {
     points: pts,
-    fill: 'rgba(88,166,255,0.1)',
-    stroke: '#58a6ff',
+    fill: COLORS.polygonFill,
+    stroke: COLORS.accent,
     'stroke-width': 3,
     'stroke-linejoin': 'round'
   });
   svg.appendChild(poly);
+}
 
+function renderCorners(svg, state) {
   state.corners.forEach((c, i) => {
     const g = document.createElementNS(ns, 'g');
 
     const circle = document.createElementNS(ns, 'circle');
     setAttrs(circle, {
       cx: c.x, cy: c.y, r: 10,
-      fill: '#58a6ff',
+      fill: COLORS.accent,
       stroke: '#ffffff',
       'stroke-width': 2.5,
       class: 'corner-circle'
@@ -116,3 +125,14 @@ export function renderSVG(svg, state) {
     svg.appendChild(g);
   });
 }
+
+export function renderSVG(svg, state) {
+  svg.setAttribute('viewBox', `0 0 ${state.fieldW} ${state.fieldH}`);
+  svg.innerHTML = '';
+
+  renderBackground(svg, state);
+  renderGrid(svg, state);
+  renderWidgetRect(svg, state);
+  renderPolygon(svg, state);
+  renderCorners(svg, state);
+}
diff --git a/js/ui.js b/js/ui.js
index e97e34f..ce57e46 100644
--- a/js/ui.js
+++ b/js/ui.js
@@ -1,77 +1,11 @@
+import { $ } from './dom.js';
 import { resetState } from './state.js';
 import { computeHomography, toMatrix3dCSS } from './homography.js';
 import { buildCornerInputs, syncCornerInputs, readCornerInputs, syncFieldInputs, syncWidgetInputs } from './inputs.js';
 import { updateLivePreview } from './preview.js';
-
-function $(id) { return document.getElementById(id); }
-
-let fieldEditing = false;
-let fieldEditW, fieldEditH;
-let keepAspect = true;
-
-function updateChainIcon() {
-  const locked = $('chainLink').querySelectorAll('.chain-locked');
-  const unlocked = $('chainLink').querySelectorAll('.chain-unlocked');
-  locked.forEach(el => el.style.display = keepAspect ? '' : 'none');
-  unlocked.forEach(el => el.style.display = keepAspect ? 'none' : '');
-  $('chainLink').classList.toggle('unlocked', !keepAspect);
-}
-
-function setFieldEditing(editing, state) {
-  fieldEditing = editing;
-  const inpW = $('inpFieldW');
-  const inpH = $('inpFieldH');
-  const options = $('fieldEditOptions');
-  const btnEdit = $('btnEditField');
-  const chain = $('chainLink');
-
-  if (editing) {
-    fieldEditW = state.fieldW;
-    fieldEditH = state.fieldH;
-    inpW.disabled = false;
-    inpH.disabled = false;
-    options.classList.add('visible');
-    btnEdit.style.display = 'none';
-    chain.classList.add('visible');
-    updateChainIcon();
-  } else {
-    inpW.value = state.fieldW;
-    inpW.disabled = true;
-    inpH.value = state.fieldH;
-    inpH.disabled = true;
-    options.classList.remove('visible');
-    btnEdit.style.display = '';
-    chain.classList.remove('visible');
-  }
-}
-
-function setMode(mode) {
-  document.body.className = mode === 'preview' ? 'mode-preview' : 'mode-edit';
-  $('btnEdit').classList.toggle('active', mode === 'edit');
-  $('btnPreview').classList.toggle('active', mode === 'preview');
-  $('modeHint').textContent = mode === 'edit'
-    ? 'Drag corners on the field'
-    : 'matrix3d() transform result';
-}
-
-let updateAllRef;
-
-function loadBgFile(file, state) {
-  if (!file || !file.type.startsWith('image/')) return;
-  const reader = new FileReader();
-  reader.onload = () => {
-    state.bgImage = reader.result;
-    $('btnBgRemove').style.display = '';
-    updateAllRef();
-  };
-  reader.readAsDataURL(file);
-}
-
-function clearBg(state) {
-  state.bgImage = null;
-  $('btnBgRemove').style.display = 'none';
-  updateAllRef();
-}
+import { setFieldEditing, setMode, toggleKeepAspect, isFieldEditing, getFieldEditW, getFieldEditH, setFieldEditW, setFieldEditH, isKeepAspect } from './mode.js';
+import { setUpdateAllRef, loadBgFile, clearBg, loadWidgetFile, setWidgetImageLock } from './file-loader.js';
+import { initDropZones } from './drop-zone.js';
 
 export function updateCSSOutput(state) {
   const H = computeHomography(state);
@@ -79,11 +13,10 @@ export function updateCSSOutput(state) {
 }
 
 export function initUI(state, updateAll) {
-  updateAllRef = updateAll;
+  setUpdateAllRef(updateAll);
 
   $('chainLink').addEventListener('click', () => {
-    keepAspect = !keepAspect;
-    updateChainIcon();
+    toggleKeepAspect();
   });
 
   $('btnEditField').addEventListener('click', () => {
@@ -96,25 +29,27 @@ export function initUI(state, updateAll) {
   });
 
   $('inpFieldW').addEventListener('input', () => {
-    if (!fieldEditing) return;
+    if (!isFieldEditing()) return;
     const newW = parseInt($('inpFieldW').value);
     if (newW <= 0) return;
-    if (keepAspect) {
-      fieldEditH = Math.round(newW * state.fieldH / state.fieldW);
-      $('inpFieldH').value = fieldEditH;
+    if (isKeepAspect()) {
+      const newH = Math.round(newW * state.fieldH / state.fieldW);
+      $('inpFieldH').value = newH;
+      setFieldEditH(newH);
     }
-    fieldEditW = newW;
+    setFieldEditW(newW);
   });
 
   $('inpFieldH').addEventListener('input', () => {
-    if (!fieldEditing) return;
+    if (!isFieldEditing()) return;
     const newH = parseInt($('inpFieldH').value);
     if (newH <= 0) return;
-    if (keepAspect) {
-      fieldEditW = Math.round(newH * state.fieldW / state.fieldH);
-      $('inpFieldW').value = fieldEditW;
+    if (isKeepAspect()) {
+      const newW = Math.round(newH * state.fieldW / state.fieldH);
+      $('inpFieldW').value = newW;
+      setFieldEditW(newW);
     }
-    fieldEditH = newH;
+    setFieldEditH(newH);
   });
 
   $('btnSaveField').addEventListener('click', () => {
@@ -207,35 +142,10 @@ export function initUI(state, updateAll) {
     updateAll();
   });
 
-  function setWidgetImageLock(locked) {
-    $('inpWidgetW').disabled = locked;
-    $('inpWidgetH').disabled = locked;
-    $('btnWidgetRemove').style.display = locked ? '' : 'none';
-  }
-
-  function loadWidgetFile(file) {
-    if (!file || !file.type.startsWith('image/')) return;
-    const reader = new FileReader();
-    reader.onload = () => {
-      state.widgetImage = reader.result;
-      const img = new Image();
-      img.onload = () => {
-        state.widgetW = img.naturalWidth;
-        state.widgetH = img.naturalHeight;
-        $('inpWidgetW').value = state.widgetW;
-        $('inpWidgetH').value = state.widgetH;
-        setWidgetImageLock(true);
-        updateAll();
-      };
-      img.src = reader.result;
-    };
-    reader.readAsDataURL(file);
-  }
-
   $('btnWidgetLoad').addEventListener('click', () => $('widgetFileInput').click());
 
   $('widgetFileInput').addEventListener('change', (e) => {
-    if (e.target.files[0]) loadWidgetFile(e.target.files[0]);
+    if (e.target.files[0]) loadWidgetFile(e.target.files[0], state);
     e.target.value = '';
   });
 
@@ -251,7 +161,7 @@ export function initUI(state, updateAll) {
     for (const item of items) {
       if (item.type.startsWith('image/')) {
         const file = item.getAsFile();
-        if (file) loadWidgetFile(file);
+        if (file) loadWidgetFile(file, state);
         break;
       }
     }
@@ -272,67 +182,5 @@ export function initUI(state, updateAll) {
     }
   });
 
-  const lc = document.querySelector('.left-col');
-
-  document.addEventListener('dragover', (e) => {
-    e.preventDefault();
-    document.body.classList.add('file-dragging');
-  });
-
-  document.addEventListener('drop', () => {
-    document.body.classList.remove('file-dragging');
-    lc.classList.remove('drag-over');
-  });
-
-  lc.addEventListener('dragover', (e) => {
-    e.preventDefault();
-    lc.classList.add('drag-over');
-  });
-
-  lc.addEventListener('dragleave', (e) => {
-    if (!lc.contains(e.relatedTarget)) {
-      lc.classList.remove('drag-over');
-    }
-  });
-
-  lc.addEventListener('drop', (e) => {
-    e.preventDefault();
-    document.body.classList.remove('file-dragging');
-    lc.classList.remove('drag-over');
-    if (e.dataTransfer.files[0]) loadBgFile(e.dataTransfer.files[0], state);
-  });
-
-  const bp = $('bgPanel');
-  bp.addEventListener('dragover', (e) => {
-    e.preventDefault();
-    bp.classList.add('drag-over');
-  });
-  bp.addEventListener('dragleave', (e) => {
-    if (!bp.contains(e.relatedTarget)) {
-      bp.classList.remove('drag-over');
-    }
-  });
-  bp.addEventListener('drop', (e) => {
-    e.preventDefault();
-    document.body.classList.remove('file-dragging');
-    bp.classList.remove('drag-over');
-    if (e.dataTransfer.files[0]) loadBgFile(e.dataTransfer.files[0], state);
-  });
-
-  const wp = $('widgetPanel');
-  wp.addEventListener('dragover', (e) => {
-    e.preventDefault();
-    wp.classList.add('drag-over');
-  });
-  wp.addEventListener('dragleave', (e) => {
-    if (!wp.contains(e.relatedTarget)) {
-      wp.classList.remove('drag-over');
-    }
-  });
-  wp.addEventListener('drop', (e) => {
-    e.preventDefault();
-    document.body.classList.remove('file-dragging');
-    wp.classList.remove('drag-over');
-    if (e.dataTransfer.files[0]) loadWidgetFile(e.dataTransfer.files[0]);
-  });
+  initDropZones(state);
 }
diff --git a/style.css b/style.css
index 94407ab..2649eb3 100644
--- a/style.css
+++ b/style.css
@@ -246,6 +246,10 @@ main {
 .corner-input-row input:focus {
   border-color: var(--accent);
 }
+.corner-input-row input.input-error {
+  border-color: #f85149;
+  background: rgba(248, 81, 73, 0.1);
+}
 
 #cssOutput {
   width: 100%;