从外部加载的图像中生成的HTML 5画布getImageData

I know it is not possible to do it straightaway due to security reasons. However, I have heard there are some image-hosting websites that allow their images to be used in a similar manner (Google Picasa?). I could be all wrong, but I would like to know if this is true and if so, how can I get image data from such an image.I know it is not possible to do it straightaway