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A pannable, zoomable, html5 canvas in React and Typescript
// sandbox here:
import {
} from "react";
import * as React from "react";
type CanvasProps = {
canvasWidth: number;
canvasHeight: number;
type Point = {
x: number;
y: number;
const ORIGIN = Object.freeze({ x: 0, y: 0 });
// adjust to device to avoid blur
const { devicePixelRatio: ratio = 1 } = window;
function diffPoints(p1: Point, p2: Point) {
return { x: p1.x - p2.x, y: p1.y - p2.y };
function addPoints(p1: Point, p2: Point) {
return { x: p1.x + p2.x, y: p1.y + p2.y };
function scalePoint(p1: Point, scale: number) {
return { x: p1.x / scale, y: p1.y / scale };
const ZOOM_SENSITIVITY = 500; // bigger for lower zoom per scroll
export default function Canvas(props: CanvasProps) {
const canvasRef = useRef<HTMLCanvasElement>(null);
const [context, setContext] = useState<CanvasRenderingContext2D | null>(null);
const [scale, setScale] = useState<number>(1);
const [offset, setOffset] = useState<Point>(ORIGIN);
const [mousePos, setMousePos] = useState<Point>(ORIGIN);
const [viewportTopLeft, setViewportTopLeft] = useState<Point>(ORIGIN);
const isResetRef = useRef<boolean>(false);
const lastMousePosRef = useRef<Point>(ORIGIN);
const lastOffsetRef = useRef<Point>(ORIGIN);
// update last offset
useEffect(() => {
lastOffsetRef.current = offset;
}, [offset]);
// reset
const reset = useCallback(
(context: CanvasRenderingContext2D) => {
if (context && !isResetRef.current) {
// adjust for device pixel density
context.canvas.width = props.canvasWidth * ratio;
context.canvas.height = props.canvasHeight * ratio;
context.scale(ratio, ratio);
// reset state and refs
lastOffsetRef.current = ORIGIN;
lastMousePosRef.current = ORIGIN;
// this thing is so multiple resets in a row don't clear canvas
isResetRef.current = true;
[props.canvasWidth, props.canvasHeight]
// functions for panning
const mouseMove = useCallback(
(event: MouseEvent) => {
if (context) {
const lastMousePos = lastMousePosRef.current;
const currentMousePos = { x: event.pageX, y: event.pageY }; // use document so can pan off element
lastMousePosRef.current = currentMousePos;
const mouseDiff = diffPoints(currentMousePos, lastMousePos);
setOffset((prevOffset) => addPoints(prevOffset, mouseDiff));
const mouseUp = useCallback(() => {
document.removeEventListener("mousemove", mouseMove);
document.removeEventListener("mouseup", mouseUp);
}, [mouseMove]);
const startPan = useCallback(
(event: React.MouseEvent<HTMLCanvasElement, MouseEvent>) => {
document.addEventListener("mousemove", mouseMove);
document.addEventListener("mouseup", mouseUp);
lastMousePosRef.current = { x: event.pageX, y: event.pageY };
[mouseMove, mouseUp]
// setup canvas and set context
useLayoutEffect(() => {
if (canvasRef.current) {
// get new drawing context
const renderCtx = canvasRef.current.getContext("2d");
if (renderCtx) {
}, [reset, props.canvasHeight, props.canvasWidth]);
// pan when offset or scale changes
useLayoutEffect(() => {
if (context && lastOffsetRef.current) {
const offsetDiff = scalePoint(
diffPoints(offset, lastOffsetRef.current),
context.translate(offsetDiff.x, offsetDiff.y);
setViewportTopLeft((prevVal) => diffPoints(prevVal, offsetDiff));
isResetRef.current = false;
}, [context, offset, scale]);
// draw
useLayoutEffect(() => {
if (context) {
const squareSize = 20;
// clear canvas but maintain transform
const storedTransform = context.getTransform();
context.canvas.width = context.canvas.width;
props.canvasWidth / 2 - squareSize / 2,
props.canvasHeight / 2 - squareSize / 2,
context.arc(viewportTopLeft.x, viewportTopLeft.y, 5, 0, 2 * Math.PI);
context.fillStyle = "red";
}, [
// add event listener on canvas for mouse position
useEffect(() => {
const canvasElem = canvasRef.current;
if (canvasElem === null) {
function handleUpdateMouse(event: MouseEvent) {
if (canvasRef.current) {
const viewportMousePos = { x: event.clientX, y: event.clientY };
const topLeftCanvasPos = {
x: canvasRef.current.offsetLeft,
y: canvasRef.current.offsetTop
setMousePos(diffPoints(viewportMousePos, topLeftCanvasPos));
canvasElem.addEventListener("mousemove", handleUpdateMouse);
canvasElem.addEventListener("wheel", handleUpdateMouse);
return () => {
canvasElem.removeEventListener("mousemove", handleUpdateMouse);
canvasElem.removeEventListener("wheel", handleUpdateMouse);
}, []);
// add event listener on canvas for zoom
useEffect(() => {
const canvasElem = canvasRef.current;
if (canvasElem === null) {
// this is tricky. Update the viewport's "origin" such that
// the mouse doesn't move during scale - the 'zoom point' of the mouse
// before and after zoom is relatively the same position on the viewport
function handleWheel(event: WheelEvent) {
if (context) {
const zoom = 1 - event.deltaY / ZOOM_SENSITIVITY;
const viewportTopLeftDelta = {
x: (mousePos.x / scale) * (1 - 1 / zoom),
y: (mousePos.y / scale) * (1 - 1 / zoom)
const newViewportTopLeft = addPoints(
context.translate(viewportTopLeft.x, viewportTopLeft.y);
context.scale(zoom, zoom);
context.translate(-newViewportTopLeft.x, -newViewportTopLeft.y);
setScale(scale * zoom);
isResetRef.current = false;
canvasElem.addEventListener("wheel", handleWheel);
return () => canvasElem.removeEventListener("wheel", handleWheel);
}, [context, mousePos.x, mousePos.y, viewportTopLeft, scale]);
return (
<button onClick={() => context && reset(context)}>Reset</button>
<pre>scale: {scale}</pre>
<pre>offset: {JSON.stringify(offset)}</pre>
<pre>viewportTopLeft: {JSON.stringify(viewportTopLeft)}</pre>
width={props.canvasWidth * ratio}
height={props.canvasHeight * ratio}
border: "2px solid #000",
width: `${props.canvasWidth}px`,
height: `${props.canvasHeight}px`
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Do you know how can I add another box into the canvas?

The line of code that draws the box is context.fillRect. You should be able to add more stuff like that in that function and have it show up.

Thanks for the quick reply. I tried just pasting that line 3 times but I still only have one box. Should I be doing something different?

It might be the same box drawn on top of the same place multiple times :) you might be interested in this MDN tutorial, which helped me a lot when learning the fundamentals of the canvas api!

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What's the proper way to serialize and restore a view with this setup?

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Hi @gradywetherbee , good question! I implemented this myself at


Hitting "Saved View" restores the saved state, and "Overwrite Saved View" updates the stored state.

The TL;DR is you need to store the current zoom and offset in some application state and manage/overwrite it accordingly when the user performs some actions (hits buttons, presses certain keys, whatever)

The way I implemented it in my project is in this hook:

The view state is an input to the component. In the parent logic, there is the buttons for storing/reseting that state.

I won't really be able to help much with further implementation details, but hope that sets you off on the right track. Good luck!

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@robinovitch61 in my case 'canvasHeight' and 'canvasWidth" are not static ... they change depending on the resize. What I need to do to make it work right ? now it resets to prev state each time I resize parent element

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@robinovitch61 in my case 'canvasHeight' and 'canvasWidth" are not static ... they change depending on the resize. What I need to do to make it work right ? now it resets to prev state each time I resize parent element

Hi @YevhenTarashchyk , I can point you to the react component's height and width state that powers the resizability of Hopefully that helps. In general, you'll want to do something like this:

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YevhenTarashchyk commented Dec 27, 2022 via email

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@robinovitch61 Could u pls help me a little ?
When I resize element the image position in canvas is a little bit off (it is moving)... What should I do ?
Here is a vid for better understanding


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@YevhenTarashchyk I don't have time to dig in unfortunately, but you've done a good job demonstrating and reproducing your issue! I'm sure someone on stack overflow would be down to help out if you post a question there.

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