Linear interpolate of image buffer

gistfile1.txt

function data_a_interpolate(data, alpha, width, height, red, green, blue)
{
    var m = 0;
    for(var n = 0; n < width * height * 4; n += 4, m++)
    {
        data[n + 0] = Math.linear_interpolate(alpha, red, data[n + 0]);
        data[n + 1] = Math.linear_interpolate(alpha, green, data[n + 1]);
        data[n + 2] = Math.linear_interpolate(alpha, blue, data[n + 2]);
        data[n + 3] = 0xff;
    }
}

function data_interpolate(data, arr, width, height, red, green, blue)
{
    var m = 0;
    for(var n = 0; n < width * height * 4; n += 4, m++)
    {
        data[n + 0] = Math.linear_interpolate(arr[m + 0], red, 0x00);
        data[n + 1] = Math.linear_interpolate(arr[m + 1], green, 0x00);
        data[n + 2] = Math.linear_interpolate(arr[m + 2], blue, 0x00);
        data[n + 3] = 0xff;
    }
}

//if alpha is 255, then all weight is val1, if alpha is 0, then all weight is val2
Math.linear_interpolate = function(alpha, val1, val2)
{
    return ((val1 + 1) * alpha >> 8) - ((val2 + 1) * alpha >> 8) + val2;
}