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This function takes in mouse coordinates and picks the model from the 3d scene by shooting a ray through it. Btw the screen size in world coordinates is 2x2 stretching from -1 to +1 on both the x and y axis. The screen size in pixels would be mClientWidth and mClientHeight. You'll see both those in "Compute picking ray in view space"

void Pick(int sx, int sy)
{
    float dist = MathHelper::Infinity;
    float tmin = 0.0f;
    XMMATRIX P = cam->GetProj();

    // Compute picking ray in view space.
    float vx = (+2.0f*sx / mClientWidth - 1.0f) / P(0, 0);
    float vy = (-2.0f*sy / mClientHeight + 1.0f) / P(1, 1);

    // Ray definition in view space.
    XMVECTOR rayOrigin = XMVectorSet(0.0f, 0.0f, 0.0f, 1.0f);
    XMVECTOR rayDir = XMVectorSet(vx, vy, 1.0f, 0.0f);

    // Tranform ray to local space of Mesh.
    XMMATRIX V = cam->GetView();
    XMMATRIX invView = XMMatrixInverse(&XMMatrixDeterminant(V), V);

    // Make the ray direction unit length for the intersection tests.
    rayDir = XMVector3Normalize(rayDir);

    XMMATRIX W = XMMatrixIdentity();
    XMMATRIX invWorld = XMMatrixInverse(&XMMatrixDeterminant(W), W);
    XMMATRIX toLocal = XMMatrixMultiply(invView, invWorld);
    rayOrigin = XMVector3TransformCoord(rayOrigin, toLocal);
    rayDir = XMVector3TransformNormal(rayDir, toLocal);

    if (XNA::IntersectRayOrientedBox(rayOrigin, rayDir, scene["Main_Scene"]->instance[i].boundBox, &tmin))
        if (tmin < dist)
        {
            // The model of Main_Scene and instance i was hit.
            dist = tmin;
            tmin = MathHelper::Infinity;
        }
}