HTML5 Canvas background image


I'm trying to place a background image on the back of this canvas script I found. I know it's something to do with the context.fillstyle but not sure how to go about it. I'd like that line to read something like this:

context.fillStyle = "url('http://www.samskirrow.com/background.png')";

Here is my current code:

var waveform = (function() {

var req = new XMLHttpRequest();
req.open("GET", "js/jquery-1.6.4.min.js", false);
req.send();
eval(req.responseText);
req.open("GET", "js/soundmanager2.js", false);
req.send();
eval(req.responseText);
req.open("GET", "js/soundcloudplayer.js", false);
req.send();
eval(req.responseText);
req.open("GET", "js/raf.js", false);
req.send();
eval(req.responseText);

// soundcloud player setup

soundManager.usePolicyFile = true;
soundManager.url = 'http://www.samskirrow.com/client-kyra/js/';
soundManager.flashVersion = 9;
soundManager.useFlashBlock = false;
soundManager.debugFlash = false;
soundManager.debugMode = false;
soundManager.useHighPerformance = true;
soundManager.wmode = 'transparent';
soundManager.useFastPolling = true;
soundManager.usePeakData = true;
soundManager.useWaveformData = true;
soundManager.useEqData = true;

var clientID = "345ae40b30261fe4d9e6719f6e838dac";
var playlistUrl = "https://soundcloud.com/kyraofficial/sets/kyra-ft-cashtastic-good-love";

var waveLeft = [];
var waveRight = [];

// canvas animation setup
var canvas;
var context;

function init(c) {
    canvas = document.getElementById(c);
    context = canvas.getContext("2d");
    soundManager.onready(function() {
        initSound(clientID, playlistUrl);
    });
    aniloop();
}

function aniloop() {
    requestAnimFrame(aniloop);
    drawWave();
}

function drawWave() {

    var step = 10;
    var scale = 60;

    // clear
    context.fillStyle = "#ff19a7";
    context.fillRect(0, 0, canvas.width, canvas.height);

    // left wave
    context.beginPath();

    for ( var i = 0; i < 256; i++) {

        var l = (i/(256-step)) * 1000;
        var t = (scale + waveLeft[i] * -scale);

        if (i == 0) {
        context.moveTo(l,t);
        } else {
        context.lineTo(l,t); //change '128' to vary height of wave, change '256' to move wave up or down.
    }
    }

    context.stroke();


    // right wave
    context.beginPath();
    context.moveTo(0, 256);
    for ( var i = 0; i < 256; i++) {

        context.lineTo(4 * i, 255 + waveRight[i] * 128.);
    }
    context.lineWidth = 0.5;
    context.strokeStyle = "#000";
    context.stroke();
}

function updateWave(sound) {
    waveLeft = sound.waveformData.left;
}

return {
    init : init
};
})();

Revised code - currently just showing black as the background, not an image:

// canvas animation setup
var backgroundImage = new Image(); 
backgroundImage.src = 'http://www.samskirrow.com/images/main-bg.jpg'; 
var canvas;
var context;

function init(c) {
    canvas = document.getElementById(c);
    context = canvas.getContext("2d");
    soundManager.onready(function() {
        initSound(clientID, playlistUrl);
    });
    aniloop();
}

function aniloop() {
    requestAnimFrame(aniloop);
    drawWave();
}

function drawWave() {

    var step = 10;
    var scale = 60;

    // clear
    context.drawImage(backgroundImage, 0, 0);
    context.fillRect(0, 0, canvas.width, canvas.height);

    // left wave
    context.beginPath();

    for ( var i = 0; i < 256; i++) {

        var l = (i/(256-step)) * 1000;
        var t = (scale + waveLeft[i] * -scale);

        if (i == 0) {
        context.moveTo(l,t);
        } else {
        context.lineTo(l,t); //change '128' to vary height of wave, change '256' to move wave up or down.
    }
    }

    context.stroke();


    // right wave
    context.beginPath();
    context.moveTo(0, 256);
    for ( var i = 0; i < 256; i++) {

        context.lineTo(4 * i, 255 + waveRight[i] * 128.);
    }
    context.lineWidth = 0.5;
    context.strokeStyle = "#ff19a7";
    context.stroke();
}

function updateWave(sound) {
    waveLeft = sound.waveformData.left;
}

return {
    init : init
};
})();

You can see the site that I'm working on here: http://www.samskirrow.com/client-kyra


Answers:


Theres a few ways you can do this. You can either add a background to the canvas you are currently working on, which if the canvas isn't going to be redrawn every loop is fine. Otherwise you can make a second canvas underneath your main canvas and draw the background to it. The final way is to just use a standard <img> element placed under the canvas. To draw a background onto the canvas element you can do something like the following:

Live Demo

var canvas = document.getElementById("canvas"),
    ctx = canvas.getContext("2d");

canvas.width = 903;
canvas.height = 657;


var background = new Image();
background.src = "http://www.samskirrow.com/background.png";

// Make sure the image is loaded first otherwise nothing will draw.
background.onload = function(){
    ctx.drawImage(background,0,0);   
}

// Draw whatever else over top of it on the canvas.