LayoutTests/fast/canvas/canvas-getImageData.html

This test ensures that getImageData works correctly.
<div id="log"></div>
<script>
if (window.layoutTestController)
    layoutTestController.dumpAsText();

var canvas = document.createElement("canvas");
canvas.width = 200;
canvas.height = 200;
var context = canvas.getContext("2d");

function log(msg){
    document.getElementById("log").innerHTML += msg + "<br/>";
}

function dataToArray(data) {
    var result = new Array(data.length)
    for (var i = 0; i < data.length; i++)
        result[i] = data[i];
    return result;
}

function getPixel(ctx, x, y) {
    var data = ctx.getImageData(x,y,1,1);
    if (!data) // getImageData failed, which should never happen
        return [-1,-1,-1,-1];
    return dataToArray(data.data);
}

function pixelShouldBe(ctx, x, y, colour) {
    var ctxColour = getPixel(ctx, x, y);
    var correct = true;
    for (var i = 0; i < 4; i++)
        if (colour[i] != ctxColour[i]) {
            correct = false;
            break;
        }
    if (correct)
        log("PASS: pixel at ("+[x,y]+") was ["+colour+"]");
    else
        log("FAIL: pixel at ("+[x,y]+") was ["+ctxColour+"], expected ["+colour+"]");
}

if (!context.setFillColor)
    context.setFillColor = function(r,g,b,a) {
        this.fillStyle = "rgba("+[Math.round(r*255),Math.round(g*255),Math.round(b*255),Math.round(a*255)]+")"
    }

// Check that getImageData is return the data for the right portion of the image
for(var x = 0; x < 100; x+=4) {
    context.setFillColor(0, x/96, 0, 1);
    context.fillRect(x,0,1,1);
    pixelShouldBe(context, x, 0, [0, Math.round(255*x/96), 0, 255]);
}

// Check rgba ordering
context.clearRect(0,0,100,100);
context.setFillColor(0.25, 0.5, 0.75, 1);
context.fillRect(5,5,1,1);
pixelShouldBe(context, 5, 5, [Math.round(0.25*255), Math.round(0.5*255), Math.round(0.75*255), 255]);

// Make sure we return correct values for the row
for (var i = 0; i < 100; i++) {
    context.fillStyle = "rgba("+[0, i, 0, 1]+")";
    context.fillRect(i, 10, 1, 1);
}

var rowImageData = context.getImageData(0, 10, 100, 1).data;
var rowCheck = true;
for (var i = 0; i < 100; i++) {
    if (rowImageData[i * 4 + 1] != i) {
        rowCheck = false;
        break;
    }
}
if (!rowCheck)
    log("FAIL: Did not correctly retrieve every pixel in a row");
else
    log("PASS: Correctly retrieved every pixel in a row");

// Check that we return transparent black for regions outside the canvas proper
context.fillStyle = "rgba(255,255,255,255)";
context.fillRect(198, 5, 4, 1); // final 2 pixels horizontally should be clipped
var content = dataToArray(context.getImageData(198, 5, 4, 1).data);
var expected = [255,255,255,255,255,255,255,255,
                0,0,0,0,0,0,0,0];
var matched = true;
for (var i = 0; i < 16; i++)
    if (content[i] != expected[i]) {
        matched = false;
        break;
    }
if (matched)
    log("PASS: Correct data for content outside canvas bounds");
else
    log("FAIL: Did not get correct data for content outside canvas bounds: "+content);

// Ensure returned values are not premultiplied
var values = [0,1,2,16,32,64,96,128,192,254,255];

// this function simply accounts for truncation due to premultiplication in the canvas implementation
function premult(value, alpha) {
    return alpha ? Math.floor(Math.round(value*alpha/255)*255/alpha) : 0;
}

for (var i = 0; i < values.length; i++) {
    for (var a = 0; a < values.length; a++) {
        context.fillStyle = "rgba(" + [values[i], 0, 0, values[a]/255] +")";
        context.fillRect(50+i*4,50+a*4,2,2);
        pixelShouldBe(context, 50+i*4, 50+a*4, [premult(values[i], values[a]), 0, 0, values[a]]);
    }
}

</script>