Coordinate system of VTK points

Hello, I am experimenting with image registration, namely with elastix which also have an option to transform/warp some points (see manual, sec 4.2). They say that the points are from VTK, but so far I was not able to warp my points properly probably due to different coordinate system. Mine has origin [0, 0] in the top left corner of the image and increasing in direction to the bottom right corner… What is the VTK coordinate frame? Thx