LWJGL cursor trouble when resizing

Question

I have probably a simple math problem that I'm having surprisingly hard time figuring out. I have a resizable LWJGL (OpenGL) window. The game I'm making is in 2D, but I use glFrustum() to get depth for my sprites. Here's a (little bit simplified) snippet from my projection code:

double divider = 64.0;
double left = -vw / divider;
double right = vw / divider;
double bottom = vh / divider;
double top = -vh / divider;

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(left * aspect, right * aspect, bottom, top, 8, 65536);

float zModifier = -256;
float vRatio = Game.HEIGHT / Display.getHeight();

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(-vw / 2.0f, -vh / 2.0f, zModifier * vRatio);

(I must admit that I don't understand the frustum bit very well, I found this piece of code somewhere in the Internet and it has served me well...)

However, I am rendering my own cursor with a sprite, and switch to native cursor when the cursor comes in contact with the window border. This worked all well until I made the window resizable.

Now my problem is, when I resize the window, the native cursor is "out of sync" with the sprite cursor relative to the vRatio. It "jumps" a bit, or a bit more, depending of the resized window size.

When the sprite cursor comes in contact with border, I do the following:

Hide the sprite cursor
Show the native cursor
Set the native cursor's location to sprite cursor's location (this does not work correctly anymore)

I feel like there's a simple math solution to this but I cannot figure it out (I've tried things... but..).

I hope my question isn't too vague, and that I provided enough information!

Nope I am not, I use that bit of code to fit the game area to the resized window. — n00bster, Jul 04 '12 at 22:53
I see. So, is any concern given to the width of the window and how that scales appropriately? It would seem you really need two ratios, unless you maintain that your game window always has the same w:h ratio, — corsiKa, Jul 04 '12 at 22:55
No I'm not taking the width in account anywhere, my game area also happens to be a 512x512 area. Here's hopefully a better explanation of the problem. When the window isn't resized at all, the sprite cursor's position is in range 0,0 to 512x512, with top-left corner being 0,0. When I resize the window to be smaller than the original size, the top left corner actually becomes negative for the sprite cursor. So when I move the sprite cursor to the top-left corner of a resized window, it could be at -50, -50, for example (depending of the resize ratio). — n00bster, Jul 06 '12 at 16:04
So, I need to convert the sprite cursor's position to "screen position", so that the native cursor "snaps" at correct position when the sprite cursor comes in contact with the edge. I'm attempting to write a method getLocationOnScreen() for my sprite cursor class that takes the resize ratio in account... but my brains are failing me. :( — n00bster, Jul 06 '12 at 16:05

score 1 · Accepted Answer · answered Sep 16 '12 at 13:11

At first your window size matches that of OpenGLs. But when you resize the window, the projection must change. So at least a scaling factor is added. This explains why the cursors are not in sync.

If you want to get them in sync all the time, you have to map window coordinates into world coordinates. This can be accomplished with gluUnProject OpenGL: gluUnProject. You are already doing that by hand but without considering the changes in resolution which leads to your problem.

For completeness, the reverse, going from world coordinates to window coordinates, is also possible OpenGL: gluProject

When using these functions, you have to get your hands on model matrix, projection matrix and the viewport. Use the functions of the glGet* family for that purpose. Be sure to load these matrices at a point where they are set correctly!

Thank you very much for your response! I just got back to this, and got it working perfectly with gluUnProject! I see if I'll post the code a bit later... in case someone else is struggling with this same problem. — n00bster, Oct 27 '12 at 16:27

LWJGL cursor trouble when resizing

1 Answers1