Fix cursor offset

js/canvas-utils.mjs

@@ -1,33 +1,82 @@
 import { COLOR } from "./state/constants.mjs";
-import { getCursorCanvas } from "./dom.mjs";
+import { getCursorCanvas, getCanvas } from "./dom.mjs";
+import { getPanel } from "./dom.mjs";
+import { isCursorWithinPanelBounds } from "./ui-utils.mjs";
 
 const ctx = getCursorCanvas().getContext("2d");
-const CURSOR_SIZE = 20;
+const canvasCtx = getCanvas().getContext("2d");
+const CURSOR_SIZE = 10;
+const CURSOR_PART_SIZE = CURSOR_SIZE / 2;
+const GAP = CURSOR_SIZE / 2;
+const LINE_WIDTH = 5;
+
+/**
+ * Inverts color to avoid invisible cursor.
+ *
+ * @param {number} r - Red color.
+ * @param {number} g - Green color.
+ * @param {number} b - Blue color.
+ *
+ * @returns {number[]} - Inverted color in r, g,b order.
+ */
+function invertColor(r, g, b, a) {
+  const rgb = [r, g, b, a];
+  for (var i = 0; i < rgb.length; i++) rgb[i] = (i === 3 ? 1 : 255) - rgb[i];
+  return rgb;
+}
+
+/**
+ * Returns color on given canvas area in RGBA format.
+ *
+ * @param {number} x - X coordinate.
+ * @param {number} y - Y coordinate.
+ *
+ * @returns {string} - RGBA color.
+ */
+function getPixelRGBAColor(x, y) {
+  const panel = getPanel();
+  const rect = panel.getBoundingClientRect();
+
+  if (isCursorWithinPanelBounds(x, y, rect)) {
+    return COLOR.BLACK;
+  }
+
+  const pixel = canvasCtx.getImageData(x, y, GAP, GAP);
+  const [r, g, b, a] = invertColor(...pixel.data);
+
+  return `rgb(${r}, ${g}, ${b}, ${Math.abs(a)})`;
+}
 
 export function drawCursor(x, y) {
   ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
-  const half = CURSOR_SIZE / 2;
-
-  ctx.lineWidth = 5;
+  ctx.lineWidth = LINE_WIDTH;
   ctx.lineCap = "round";
-  ctx.strokeStyle = COLOR.BLACK;
+  ctx.strokeStyle = getPixelRGBAColor(x, y);
 
+  /* --- Cursor --- */
+  // Draw top line
   ctx.beginPath();
-  ctx.moveTo(x, y);
-  ctx.lineTo(x + CURSOR_SIZE, y);
+  ctx.moveTo(x, y - GAP);
+  ctx.lineTo(x, y - CURSOR_PART_SIZE - GAP - CURSOR_SIZE);
   ctx.stroke();
 
+  // Draw bottom line
   ctx.beginPath();
-  ctx.moveTo(x + half, y - half);
-  ctx.lineTo(x + half, y + half);
+  ctx.moveTo(x, y + GAP);
+  ctx.lineTo(x, y + CURSOR_PART_SIZE + GAP + CURSOR_SIZE);
   ctx.stroke();
 
-  ctx.closePath();
+  // Draw left line
+  ctx.beginPath();
+  ctx.moveTo(x - GAP, y);
+  ctx.lineTo(x - CURSOR_PART_SIZE - GAP - CURSOR_SIZE, y);
+  ctx.stroke();
 
-  ctx.moveTo(x + half, y + half);
-  ctx.strokeStyle = COLOR.WHITE;
-  ctx.fillStyle = COLOR.WHITE;
+  // Draw right line
   ctx.beginPath();
-  ctx.arc(x + half, y, 3, 0, Math.PI * 2);
-  ctx.fill();
+  ctx.moveTo(x + GAP, y);
+  ctx.lineTo(x + CURSOR_PART_SIZE + GAP + CURSOR_SIZE, y);
+  ctx.stroke();
+
+  ctx.closePath();
 }

js/ui-utils.mjs

@@ -0,0 +1,12 @@
+/**
+ * Is cursor located on UI panel?
+ *
+ * @param {number} x - Cursor x coordinate.
+ * @param {number} y - Cursor y coordinate.
+ * @param {DOMRect} rect - Panel bounds.
+ *
+ * @returns {boolean} - Is cursor located on UI panel?
+ */
+export function isCursorWithinPanelBounds(x, y, rect) {
+  return x >= rect.left && x <= rect.right && y >= rect.top && y <= rect.bottom;
+}

js/ui/panel.mjs

@@ -3,10 +3,7 @@ import {
   isPrimaryGamepadButtonPressed,
 } from "../controls/gamepad.mjs";
 import { getPanel } from "../dom.mjs";
-
-function isCursorWithinPanelBounds(x, y, rect) {
-  return x >= rect.left && x <= rect.right && y >= rect.top && y <= rect.bottom;
-}
+import { isCursorWithinPanelBounds } from "../ui-utils.mjs";
 
 function getPanelButtonByCoordinates(x, y, panel) {
   const buttons = panel.querySelectorAll("button");

package.json

@@ -1,6 +1,6 @@
 {
   "name": "sdraw",
-  "version": "0.1.2",
+  "version": "0.1.3",
   "description": "Simple drawing application for kids, can be controlled via mouse, keyboard or gamepad.",
   "private": true,
   "main": "index.mjs",