Question

I am saving my driven X/Y coordinates, and then using a function that convert the coordinates to meters, and add 1280 to each point (so it will fit nicely into a 2560×2560 image), and then draw a polygon between the ‘points’, resulting in a some sort of racing line. But once I have generated the polygon and saved it as an image, it is vertically flipped somehow. Flipping the image vertically will make it match the track bitmaps perfectly. I was told this is due to DirectX internally has the Y axis flipped. Why does DirectX use a flipped Y axis?

Answer 1

Well, the question is, does DirectX have a flipped Y-axis or does the image?

DirectX uses a 3D/4D coordinate system where the X-axis points to the right and Y-axis points upwards when no transformation is applied. This is because the screen (where Y-axis points downwards) is the last instance that has to process the image. Every step before that uses the coordinate system with the upward Y-axis. Since Direct3D is designed for 3D worlds, a coordinate system that is aligned like the world and like most coordinate system in maths is much more convenient for the programmer and designer. Imagine, you would create a 3D model. Wouldn’t it be kind of weird, if you design it so that the Y-axis is pointing downwards?

When you have no transformation at all that would allow perspective and so on, you have the same coordinate system. Ignoring the Z-axis, the top left corner is (-1 | 1), the bottom right corner is (1, -1). This is equal to the coordinate systems used in e.g. maths. In the end, this coordinate system is transformed with the viewport which will result in the top left corner to be (0 | 0) and the bottom right corner to be (ResolutionX | ResolutionY).

So all in all, the reason why the Y-axis points upwards is that Direct3D’s main purpose is to describe worlds in a convenient way independently of the screen’s physical attributes.