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# Graphics matrices give odd render results

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19 replies to this topic

### #1assainator  Members

Posted 30 October 2013 - 02:18 PM

Hello all,

I've been fiddling around with matrices. I think I have everything it working, however, the results seem kind of strange.

No rotation:

45 degrees rotation around the z-axis.

It seems like the proportions after rotations are incorrect, is this just me or what?

I've set the FOV to 90 degrees and the aspect ratio to 1024/768 for a 1024x768 window.

Thanks in advance for any help.

"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

### #2Brother Bob  Moderators

Posted 30 October 2013 - 02:23 PM

By far the most common error would be the way you calculate the aspect ratio. If both the dividend and the divisor are integers, then, depending on the language, you are likely getting an integer division so the aspect ratio is an integer. Solution is to cast either the dividend or the divisor, or both, to a floating point value to force a floating point division.

### #3assainator  Members

Posted 30 October 2013 - 03:27 PM

(Un)fortunately, this is not the case. Are there any other common mistakes?

"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

### #4Brother Bob  Moderators

Posted 30 October 2013 - 03:33 PM

### #5assainator  Members

Posted 30 October 2013 - 03:51 PM

http://pastebin.com/4hgiY8vr

I've tried to include everything without too much clutter.

"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

### #6Brother Bob  Moderators

Posted 30 October 2013 - 03:55 PM

Are WINDOW_WIDTH and WINDOW_HEIGHT floating point values? If not, you are doing integer division when calculating the aspect ratio.

Edited by Brother Bob, 30 October 2013 - 03:55 PM.

Posted 30 October 2013 - 03:57 PM

Camera c(1, 100, .25*TAU, WINDOW_WIDTH / WINDOW_HEIGHT);

are WINDOW_WIDTH and WINDOW_HEIGHT integers? Try this

Camera c(1, 100, .25*TAU, (float)WINDOW_WIDTH / WINDOW_HEIGHT);

EDIT: Ninja'd

Edited by Paradigm Shifter, 30 October 2013 - 03:58 PM.

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #8assainator  Members

Posted 30 October 2013 - 03:59 PM

Oh sorry, yes:

const static float WINDOW_WIDTH = 1024;
const static float WINDOW_HEIGHT = 768;


"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

Posted 30 October 2013 - 04:04 PM

void setLookat(const Vector3f& target, const Vector3f& up)

{
n = target.normalized();
u = up.normalized().cross(n);
v = n.cross(u);

computeView();
}

If n and u aren't orthogonal you need to normalise v after the cross product, try that...

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #10assainator  Members

Posted 30 October 2013 - 04:11 PM

As far as I know n and u are orthogonal for target=(0,0,1) and up=(0,1,0) but I've changed

v = n.cross(u)

to

v = n.cross(u).normalized()

but still the same image.

Maybe I should have mentioned this earlier, but the quad is a square, the vertex coordinates are:

{

{-0.5f,  0.5f, 1},
{ 0.5f,  0.5f, 1},
{-0.5f, -0.5f, 1},
{ 0.5f, -0.5f, 1}

}

"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

Posted 30 October 2013 - 04:14 PM

It doesn't look like a square when it isn't rotated. Have you got the numerator and denominator the right way around when you calculate the aspect ratio?

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #12assainator  Members

Posted 30 October 2013 - 04:17 PM

Camera c(1, 100, .25*TAU, WINDOW_WIDTH / WINDOW_HEIGHT);

where WINDOW_WIDTH / WINDOW_HEIGHT is the aspect ratio.

with:

const static float WINDOW_WIDTH = 1024;
const static float WINDOW_HEIGHT = 768;


"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

Posted 30 October 2013 - 04:18 PM

Have you tried switching them round? Because a square should look square and it doesn't unless the screenshots you posted are stretched.

EDIT: The eyes look round though!

Edited by Paradigm Shifter, 30 October 2013 - 04:22 PM.

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #14assainator  Members

Posted 30 October 2013 - 04:22 PM

Well this is strange, if I switch them around and disable the rotation, the image before looks exactly like after.

Earlier (AR = WIDTH/HEIGHT) - no rotation:

Now (AR = HEIGHT/WIDTH) - no rotation:

Edited by assainator, 30 October 2013 - 04:23 PM.

"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

Posted 30 October 2013 - 04:26 PM

Are you actually calling Camera::computePerspective? EDIT: You set the perspective matrix to the identity in the construtor and I couldn't see where you call it. Try this

Camera(float near, float far, float fov, float ar) : near(near), far(far), fov(fov), ar(ar), view(Matrix4f::identity()), perspective(Matrix4f::identity()), x(0), y(0), z(0)

{
setLookat(Vector3f(0, 0, 1), Vector3f(0, 1, 0));
computePerspective();
}

Edited by Paradigm Shifter, 30 October 2013 - 04:34 PM.

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #16Brother Bob  Moderators

Posted 30 October 2013 - 04:28 PM

Put a break point at the point where the matrices are finally calculated, and when they are finally used, and see what the values of the actual matrices. If none of the changes affect anything, then the matrices are ultimately not used. The coordinates you use and the location of the quad ultimately is consistent with an identity matrix.

### #17assainator  Members

Posted 30 October 2013 - 04:31 PM

Are you actually calling Camera::computePerspective?

That fixed it. WHY did I miss that?

EDIT: Do you have any tips for finding these small problems?

EDIT 2: My palm is inseparable from my face...

Edited by assainator, 30 October 2013 - 04:34 PM.

"What? It disintegrated. By definition, it cannot be fixed." - Gru - Dispicable me

"Dude, the world is only limited by your imagination" - Me

Posted 30 October 2013 - 04:36 PM

Brother Bob gave some good tips.

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

### #19Brother Bob  Moderators

Posted 30 October 2013 - 04:48 PM

My advice kind of assume that you know the projection matrix is wrong to begin with. If you don't know that, you don't really know what to look for in the first place.

But, my advice still stands in a more general form; you need to take advantage of the debugger. You are experiencing problems with the viewing, so use the debugger to inspect everything that has anything to do with getting something onto the screen. Break into the debugger when the values are calculated, and ensure that they are exactly as you expect, or even just reasonable. This requires knowledge of linear algebra and experience to actually know what a "reasonable" value is though, and comes with time.

And what if you finally find out that the projection matrix is not as expected? Go back and figure out where the projection matrix is calculated. This would have been a dead give-away; the debugger would not have reached the computePerspective function if it wasn't called.

Posted 30 October 2013 - 04:50 PM

Another tip to do with aspect ratios is changing them when you write and test the function so you can verify the code works. You don't want the width and height to be const anyway, assuming the window can be resized.

Edited by Paradigm Shifter, 30 October 2013 - 04:51 PM.

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

Old topic!

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