画布精灵表错误

Question

I found a lovely code snippet on canvas spritesheet animations. this is ts:

http://jsfiddle.net/m1erickson/h85Gq/

I tried to beautify this code, by writing an animate function that accepts Image objects, so that I can animate multiple images in my canvas simultaneously. This is my attempt at it:

    $(function(){
        var canvas=document.getElementById("canvas");
        var ctx=canvas.getContext("2d");
        var spritePosition=0;
        var spriteWidth=100;
        var spriteHeight=100;
        var spriteCount=40;
        var spritePlayCount=0;
        var maxSpritePlays=2;

        var objectS=new Image();
        objectS.src="sprites/first.png";

        var fps = 50;
        function animate(sprite) {
            setTimeout(function() {

                if(spritePlayCount<maxSpritePlays){
                    requestAnimationFrame(animate);
                }

                // Drawing code goes here
                ctx.clearRect(0,0,canvas.width,canvas.height);
                ctx.drawImage(sprite,spritePosition*spriteWidth, 0,spriteWidth, spriteHeight, 0,0,spriteWidth, spriteHeight);

                spritePosition++;
                if(spritePosition>spriteCount-1){
                    spritePosition=0;
                    spritePlayCount++;
                }
            }, 1000 / fps);
        }

        objectS.onload=function(){
            animate(objectS);
        }
    }); // end $(function(){});

I am getting a much observed error, but I cant seem to find the fix for it:

index3.html:59 Uncaught TypeError: Failed to execute 'drawImage' on 'CanvasRenderingContext2D': The provided value is not of type '(CSSImageValue or HTMLImageElement or HTMLVideoElement or HTMLCanvasElement or ImageBitmap or OffscreenCanvas)'

Can you help me out finding my bug?

Answer 1

OMDG!

quote OP "Imagine having 50 spritesheets you want to animate."

50!

Looking at the code "accepted answer version"

function animate(sprite) {
    // create a timer event to fire in 1/50th second 
    setTimeout(function() {
         if (spritePlayCount < maxSpritePlays) {
            // create a animation frame event that may fire at some time
            // between 0 and 16ms or 32ms from now
            requestAnimationFrame(function() { 
                animate(sprite);
            });
        }
        // Drawing code etc... Make canvas dirty
        // exiting with dirty canvas outside the requestAnimationFrame
        // forces DOM to swap canvas backbuffer immediately on exit.

    }, 1000 / 50);
}

This is the worst possible way to animate one, let alone more than one sprite.

1. The timing is out of sync with the display refresh.
2. Using requestAnimationFrame's callback to create a timed event that renders, completely negates the reason for using requestAnimationFrame. requestAnimationFrame tells the DOM that what you draw in the callback is part of an animation. Using a timer to draw means you don't draw anything in the requested frame making the request redundant.
3. requestAnimationFrame does its best to get all the callbacks in before the next display refresh (1/60th) but it will delay the callback if there is no time to update the DOM (swap all dirty buffers) before the next refresh. You have no control over the timing of your animation, they may fire anywhere from 20ms to 36ms or more with no consistency over time.
4. By using timers to draw to the canvas you are forcing a backbuffer swap for each sprite you draw. This is an expensive process and will severely limit the speed that you can draw sprites at, and cause sprites to flicker and shear randomly during animation.

The best practice way.

• Use a single animation loop triggered by requestAnimationFrame.
• Use an array to store all sprites and update them in the main loop if/as needed.
• Only render from within the main loop. Do not render or do anything (at a regular interval) to the DOM outside the main loop's execution.
• Don't render inside events like timers, IO, or any other rapidly firing event.

Answer 2

You'll also need to pass the sprite parameter when calling the animate function using requestAnimationFrame .

$(function() {
    var canvas = document.getElementById("canvas");
    var ctx = canvas.getContext("2d");
    var spritePosition = 0;
    var spriteWidth = 100;
    var spriteHeight = 100;
    var spriteCount = 40;
    var spritePlayCount = 0;
    var maxSpritePlays = 2;
    var objectS = new Image();
    objectS.src = "sprites/first.png";
    var fps = 50;

    function animate(sprite) {
        setTimeout(function() {
            if (spritePlayCount < maxSpritePlays) {
                requestAnimationFrame(function() {
                    animate(sprite);
                });
            }
            // Drawing code goes here
            ctx.clearRect(0, 0, canvas.width, canvas.height);
            ctx.drawImage(sprite, spritePosition * spriteWidth, 0, spriteWidth, spriteHeight, 0, 0, spriteWidth, spriteHeight);
            spritePosition++;
            if (spritePosition > spriteCount - 1) {
                spritePosition = 0;
                spritePlayCount++;
            }
        }, 1000 / fps);
    }
    objectS.onload = function() {
        animate(objectS);
    };
});