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Graphics matrices give odd render results

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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:

KjDRt8q.jpg

 

45 degrees rotation around the z-axis.

Z1rjnak.jpg

 

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.

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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.

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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

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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

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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...

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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}

}

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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?

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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;

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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

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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:

KjDRt8q.jpg

 

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

Q2ixg1D.png

Edited by assainator

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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

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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.

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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

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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.

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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

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