The ship "shudders" in scrolling Asteroids
- by Ciaran
In my Asteroids game the user can scroll through space. When scrolling, the ship is drawn in the centre of the window. I use interpolation. I scroll the window uing glOrtho, centering it around the centre of the ship. On my first machine (7 years old, Windows XP, NVIDIA), I am doing 50 updates and 76 frames per second. This is smooth. My other machine an old compaq laptop (Pentium III) with Linux and Radeon OpenGL driver delivers 50 updates and 30 frames per second. The ship regularly seems to "shudder" back and forth when at maximum thrust. When you position the mouse cursor beside the ship it is obvious that its relative position in the window changes. Also, the stars seem blurred into short "lines".
Playing the game in non-scrolling mode, the ship moves within the window, glOrtho is therefore not called repeatedly and there is no problem.
I suspect a bug in my positioning of the ship and the window but I have dumped out these values and they seem to only go forward, not forward-back-forward. The driver does support double buffering. I guess if it is my bug I need to slow the frame-rate down to debug properly. My question: is this an obvious driver bug or is the slower machine uncovering a bug in my stuff and if so, some debugging tips would be appreciated. I am drawing in world co-ordinates and letting OpenGL do the scaling and translation so if I had a quick way of verifying what pixel co-ordinates OpenGL produces for the ship centre, that would help clarify this.
