I am following the LearnOpenGL tutorials and have been tinkering with shadows casting. So far everything is working correctly but there is this very specific problem where I can't cast shadows that from a purely vertical directional light. Let's add some code. My light space matrix looks like this:
glm::mat4 view = glm::lookAt(-direction, glm::vec3(0.0f), glm::vec3(0.0f, 1.0f, 0.0f));
return glm::ortho(-10.0f, 10.0f, -10.0f, 10.0f, -10.0f, 10.0f) * view;
direction is a vector with as direction the direction of the directional light, of course. Everything works well until that direction vector is set to (0,-1,0), because it is parallel to the up vector (0,1,0). To construct the lookAt matrix, glm is performing the cross product between the up vector and the difference between the center and the eye (so in that case it's basically the direction), but that cross product won't give any result since the two vectors are parallel.
Knowing all of this, my question would be : how should my lookAt view matrix when the up vector and the direction of the light are parallel ?
Edit : Thank you for your answer, I changed my code to this :
if(abs(direction.x) < FLT_EPSILON && abs(direction.z) < FLT_EPSILON)
view = glm::lookAt(-direction, glm::vec3(0.0f), glm::vec3(0.0f, 0.0f, 1.0f));
else
view = glm::lookAt(-direction, glm::vec3(0.0f), glm::vec3(0.0f, 1.0f, 0.0f));
return glm::ortho(-10.0f, 10.0f, -10.0f, 10.0f, -10.0f, 10.0f) * view;
and now everything works fine !
