function testrectangle()
{
    var c = document.getElementById("canvas");
    var ctx = c.getContext("2d");
    var imgData = ctx.getImageData(0,0,100,100);
    var i;
    for (i = 0; i < imgData.data.length; i += 4) {
    imgData.data[i + 0] = 255;
    imgData.data[i + 1] = 0;
    imgData.data[i + 2] = 0;
    imgData.data[i + 3] = 255;
    }
    ctx.putImageData(imgData, 0, 0);



    var c = document.getElementById("canvas");
    var ctx = c.getContext("2d");
    var imgData = ctx.getImageData(0,0,100,100);
    var i;
    for (i = 0; i < imgData.data.length; i += 4) {
    imgData.data[i + 0] = 0;
    imgData.data[i + 1] = 255;
    imgData.data[i + 2] = 0;
    imgData.data[i + 3] = 25;
    }
    ctx.putImageData(imgData, 0,0);
}