OpenGL Math - Projecting Screen space to World space coords
Solution 1
RESOLVED
Here's how to do it exactly, step by step.
- Obtain your mouse coordinates within the client area
- Get your Projection matrix and View matrix if no Model matrix required.
- Multiply Projection * View
- Inverse the results of multiplication
Construct a vector4 consisting of
x =
mouseposition.x
within a range of window x- transform to values between -1 and 1
y =
mouseposition.y
within a range of window y- transform to values between -1 and 1
- remember to invert mouseposition.y if needed
z =
the depth value
( this can be obtained with glReadPixel)- you can manually go from -1 to 1 ( zNear, zFar )
w =
1.0
Multiply the vector by inversed matrix created before
Divide result vector by it's w component after matrix multiplication ( perspective division )
POINT mousePos; GetCursorPos(&mousePos); ScreenToClient( this->GetWindowHWND(), &mousePos ); CMatrix4x4 matProjection = m_pCamera->getViewMatrix() * m_pCamera->getProjectionMatrix() ; CMatrix4x4 matInverse = matProjection.inverse(); float in[4]; float winZ = 1.0; in[0]=(2.0f*((float)(mousePos.x-0)/(this->GetResolution().x-0)))-1.0f, in[1]=1.0f-(2.0f*((float)(mousePos.y-0)/(this->GetResolution().y-0))); in[2]=2.0* winZ -1.0; in[3]=1.0; CVector4 vIn = CVector4(in[0],in[1],in[2],in[3]); pos = vIn * matInverse; pos.w = 1.0 / pos.w; pos.x *= pos.w; pos.y *= pos.w; pos.z *= pos.w; sprintf(strTitle,"%f %f %f / %f,%f,%f ",m_pCamera->m_vPosition.x,m_pCamera->m_vPosition.y,m_pCamera->m_vPosition.z,pos.x,pos.y,pos.z); SetWindowText(this->GetWindowHWND(),strTitle);
Solution 2
I had to make some adjustments to the answers provided here. But here's the code I ended up with (Note I'm using GLM, that could affect multiplication order). nearResult is the projected point on the near plane and farResult is the projected point on the far plane. I want to perform a ray cast to see what my mouse is hovering over so I convert them to a direction vector which will then originate from my camera's position.
vec3 getRayFromScreenSpace(const vec2 & pos)
{
mat4 invMat= inverse(m_glData.getPerspective()*m_glData.getView());
vec4 near = vec4((pos.x - Constants::m_halfScreenWidth) / Constants::m_halfScreenWidth, -1*(pos.y - Constants::m_halfScreenHeight) / Constants::m_halfScreenHeight, -1, 1.0);
vec4 far = vec4((pos.x - Constants::m_halfScreenWidth) / Constants::m_halfScreenWidth, -1*(pos.y - Constants::m_halfScreenHeight) / Constants::m_halfScreenHeight, 1, 1.0);
vec4 nearResult = invMat*near;
vec4 farResult = invMat*far;
nearResult /= nearResult.w;
farResult /= farResult.w;
vec3 dir = vec3(farResult - nearResult );
return normalize(dir);
}
Solution 3
Multiply all your matrices. Then invert the result. Point after projection are always in the -1,1. So the four corner screen points are -1,-1; -1,1; 1,-1;1,1. But you still need to choose th z value. If you are in OpenGL, z is between -1 and 1. For directx, the range is 0 to 1. Finally take your points and transform them with the matrix
Solution 4
If you have access to the glu libraries, use gluUnProject(winX, winY, winZ, model, projection, viewport, &objX, &objY, &objZ);
winX
and winY
will be the corners of your screen in pixels. winZ
is a number in [0,1] which will specify where between zNear
and zFar
(clipping planes) the points should fall. objX-Z
will hold the results. The middle variables are the relevant matrices. They can be queried if needed.
Comments
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PeeS almost 4 years
Time for a little bit of math for the end of the day..
I need to project 4 points of the window size:
<0,0> <1024,768>
Into a world space coordinates so it will form a quadrilateral shape that will later be used for terrain culling - without GluUnproject
For test only, I use mouse coordinates - and try to project them onto the world coords
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PeeS over 12 yearsYeah sorry for that, i haven't mentioned i don't want to use gluunproject - i need to operate on matrices only.
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Christian Rau over 12 years@PeeS But the linked documentation explains the implementation of gluUnProject, and as it's extremely easy, you can just write it yourself. See crazyjul's answer.
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PeeS over 12 yearsThis is what i do actually, I do a MVP matrix multiplying projection*(View * model) Then i inverse that, have a look above ( updated first post ) - maybe there's something i'm missing within the Z value..?
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Christian Rau over 12 years@PeeS At the moment your z is -3, I'm sure this isn't intended, as z has to be in [-1,1] (maybe you just want -1?). The rest seems reasonable, but don't forget to invert
source.y
if your window coordinate system has y going from top to bottom. And I hopeVectorMatrixMultiply
implicitly assumes a 1 for the w component, and not a 0. -
PeeS over 12 yearsChristian, thanks i am now patching what you have pointed out, and also i have updated the main post with
CMatrix4x4 operator*
method, so you can have a look if it's ok - as this class is not mine, and i don't know if i can trust it in this situation. -
crazyjul over 12 yearsYour * miss the translation part.
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PeeS over 12 yearscrazyjul, fixed by implementing new mathlib.
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RobotRock almost 8 yearsYou can just do pos.x /= pos.w
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Teng Long over 7 yearswhy you calculate the projection by viewMatrix * projectMatrix instead of projectMatrix * viewMatrix?
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washcloth over 5 yearsDo not divide by w.
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Chelendez over 3 yearsThis answer is long, but I was able to understand the math instead of just copy and paste, thanks