[webkit-dev] Can someone explain tx/ty?

I believe int tx, int ty -- which we see sprinkled around the rendering tree -- are the offset from the top left corner of the current renderer's parent to the containing block. Is that correct? In SVG we use IntSize containingBlockOffset in the rare places we have to deal with this RenderBoxModelObject-only concept. I've long considered fixing renders to use IntPoint and IntSize instead of x, y, tx, ty, but to do that, I need to make sure I understand what tx, ty are, and what name they should have as an IntSize. -eric