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Ray-cast onto plane

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I need to cast a ray from the position of a user's touch on a iOS device onto a plane on the XZ axis in OpenGL ES. I have code to build the ray points from the touch position and viewport size, and code to generate the inverse view*proj. Also, I have some code to calculate the position on the plane where it does intersect, but that part isn't working so well. Any help with it is much appreciated!

CGPoint p = [[touches anyObject] locationInView:self.view];
cml::vector4f r1(p.x / windowSize.width / windowSize.height, p.y, 0.0f, 1.0f);
cml::vector4f r2(p.x / windowSize.width, p.y / windowSize.height, 1.0f, 1.0f);

// unproject the ray points
cml::matrix44f_c invViewProj = orbitCam.viewMat * ((OpenGLView*)self.view).projMat;
invViewProj = cml::inverse(invViewProj);

r1 = invViewProj * r1;
r2 = invViewProj * r2;

cml::vector3f R1(r1[0], r1[1], r1[2]);
cml::vector3f R2(r2[0], r2[1], r2[2]);
cml::vector3f dir = cml::normalize(R2 - R1);

cml::vector3f V1(1.0f, 0.0f, 0.0f);
cml::vector3f V2(0.0f, 0.0f, 1.0f);
cml::vector3f V3 = cml::cross(V1, V2);

cml::vector3f vRotRay1(cml::dot(V1, R1 - V1), cml::dot(V2, R1 - V1), cml::dot(V3, R1 - V1));
cml::vector3f vRotRay2(cml::dot(V1, R2 - V1), cml::dot(V2, R2 - V1), cml::dot(V3, R2 - V1));

float percent = vRotRay1[2] / (vRotRay2[2] - vRotRay1[2]);
cml::vector3f intersect = vRotRay1 + (vRotRay1 - vRotRay2) * percent;

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