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60 posts

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[openGL] unproject problem
1 year, 8 months ago Edited by erpeo93 on Sept. 15, 2018, 9:53 a.m.
Hi,

today I was trying to unproject my mouse position to see where it is in world space.
Here it is the unproject function:

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//project is the same exact matrix passed to opengl
Vec3 Unproject( Renderer* renderer, Camera* camera, matrix4x4 project, matrix4x4 backward, Vec2 screenP, r32 distanceFromCameraZ)
{
    //compute ndc X and Y based on screen coordinates
    Vec2 screenCenter = 0.5f * renderer->screenDim;

    r32 ndcX = (screenP.x - screenCenter.x) * (2.0f / renderer->screenDim.x);
    r32 ndcY = (screenP.y - screenCenter.y) * (2.0f / renderer->screenDim.y);
    
    //let's see what's the clipZ of the targetZ
    Vec4 probeZ = V4( camera->Position -worldDistanceFromCameraZ * camera->ZAxis, 1.0f );

    //for debugging, let's force the worldZ of the probe to be on the ground plane, so that our points will always be on the ground plane
    probeZ.z = 0;
    probeZ = project * probeZ;
    r32 clipZ =probeZ.z;

    //"backward" perspective divide to obtain the clip coordinates
    Vec4 clip = clipZ * V4(ndcX, ndcY, 1.0f, 1.0f);

    //"backward" projection to obtain world coordinates
    Vec3 world = (backward * clip).xyz;

    return world;
}



It seems to work fine when the camera is perfectly aligned with the world Z Axis (0, 0, 1): (you can see the red rectangle perfectly fits the screen)

https://ibb.co/iejA9z


but as soon as I start rotating the camera it doesn't work anymore, as it seems it is still using the straight up camera.

https://ibb.co/msPXNK


Assuming the problem is not in the camera matrixes (checked multiple times both the forward and back matrixes), I think the problem should be _somewhere_ in this function, but I really can't see it.

Can you guys give me a little help with this?

Thank you,
Leonardo


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60 posts

None

[openGL] unproject problem
1 year, 8 months ago
Figured it out!

I have to grab the W coordinate from the "forward" projection as well as the Z coordinate:

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    Vec4 probeZ = V4( camera->Position -worldDistanceFromCameraZ * camera->ZAxis, 1.0f );
    probeZ = project * probeZ;
    r32 clipZ =probeZ.z;
    r32 clipW = probeZ.w;

    //"backward" perspective divide to obtain the clip coordinates
    Vec4 clip = clipW * V4(ndcX, ndcY, clipZ, 1.0f);



Problem solved. :)



None
1 posts
[openGL] unproject problem
1 year, 5 months ago
I'm assuming
1
matrix4x4 project
is the camera projection matrix.

What is parameter
1
matrix4x4 backward
?
Mārtiņš Možeiko
2076 posts / 1 project
[openGL] unproject problem
1 year, 5 months ago Edited by Mārtiņš Možeiko on Dec. 29, 2018, 1:24 a.m.
Inverse camera view matrix (camera space to world space transform).