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Rasmadrak

OpenGL Is glRotated of a lower precision than glTranslated?

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Hi there! In my FPS game (set in a HUGE world) I recently discovered the nasty headaches of floating point imprecision... :/ But I realized that if I don't do any rotates precision is just fine!
        glRotated(ViewPitch,1,0,0);  //comment out these for jumpfree graphics.
        glRotated(ViewYaw,0,1,0);    //comment out these for jumpfree graphics.
        glTranslated(-Pos.x,-Pos.y,-Pos.z);

Is there something wrong with my camera class, or does opengl do something funky when using glRotate? Cheers people!

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I think OpenGL stores the transforms internally as floats. So try keeping your own double-precision matrix for the camera and using glLoadMatrixd to send it to OpenGL. This eliminates accumulated floating point precision errors (actually it doesn't but makes them order of 2^29 times smaller because of the mantissa being 29 bits longer double vs float).

This reasoning is however only correct if OpenGL doesn't store double precision copies of the matrix stacks and use them modifying the stacks. But try it and see if it makes a difference.

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At least I got the impression his rotations and translations were working all right unless he moved in his world so that the absolute value of the translation is REALLY big and floating point inaccuracies cause movement to get all choppy and jerky.

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Quote:
Original post by Glaiel_Gamer
i would translate first then rotate


That won't work the same way at all - the rotation axis will be different (with non-zero translation, that is).

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Ah, one more thing! When using your own matrices in OpenGL (with glMultMatrix / glLoadMatrix) be sure to remember to follow its row-column ordering. It is so that the base vectors of the 4x4 matrix are laid out contiguously in memory.

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Hmm... so you are now using a matrix of your own, which is stored as double-precision, not single-precision numbers?

What is your coordinate system like? In other words, in what range are the translation values going to be?

Really strange indeed. In normal circumstances, however large your worlds might be, doubles should give you more than enough precision. Hmm... I wonder if using long doubles and then converting them to doubles before sending to OpenGL you would get any benefit. You can always try fairly easily...

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f and d's difference is d's more precision. But he already tried building his own matrix and dropping those calls out of the equation alltogether, so that's not the problem as it seems.

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Update:

I just noticed that it's the terrain that jumps around... NOT the camera or the objects.. they're rock solid. Double's for the win!
That's the good news, hehe.

But what is causing the terrain to jump around? :S I'm not doing anything fancy except old fashioned glTranslated and glVertex3d calls.
Also, when the terrain is not compiled into a displaylist, it's more stable (but still jumpy)...

The code used for compiling the displaylist is more or less exactly the same, so could this be some graphics driver optimization and/or bug..?

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Quote:
Original post by Rasmadrak
But what is causing the terrain to jump around?

On what planet do the inhabitants have the needed information to possibly answer that question? Please post relavent code and describe more than "jumps around". Other than that: remove the display list code all together until you have ruled it out.
Also: Do you LoadIdentity after drawing your objects?

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Hehe. That made me laugh! :D

The thing that puzzled me the most is that a display list (now using the EXACT same code) is jumping a lot more than without a display list.
Anyway, I'll test to rewrite the terrain drawing later this evening, see if I made any infamous typos-of-death... :/

Yeah. cheers!

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Quote:
Original post by Rasmadrak
Update:

I just noticed that it's the terrain that jumps around... NOT the camera or the objects.. they're rock solid. Double's for the win!
That's the good news, hehe.

But what is causing the terrain to jump around? :S I'm not doing anything fancy except old fashioned glTranslated and glVertex3d calls.
Also, when the terrain is not compiled into a displaylist, it's more stable (but still jumpy)...

The code used for compiling the displaylist is more or less exactly the same, so could this be some graphics driver optimization and/or bug..?


Are you making sure to push and pop the matrix stack and load an identity matrix every time you draw the terrain? If the terrain is so big, like you said, that jittering effect could be caused by that.

Edit: Err... I mean it could be that because there's a translation somewhere in your code that might be "bleeding" into your terrain positioning code (ie. Forgetting to push the matrix stack and set an identity matrix to the top of the stack).

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Hardware GL implementations are internally float, not double.

The precision of float is not enough for a large world.

Thus, you have to save the camera position/orientation as well as the object position/orientation.

When you render, you subtract the camera position from the object position, and subtract the camera orientation from the object orientation, and then rotate/translate by THAT.

We do it this way when simulating the earth (from foot-soldier view), and it works very well.

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