glm::lookAt gives unexpected result for 2D axis

user_11108133 0
Viewed 54

Basically I have a local space where y points down and x points right, like

       |
       |
  ---------> +x
       |
       | +y

and the center is at (320, 240), so the upper left corner is (0,0). Some windowing system uses it.

So I have this code

    auto const proj = glm::ortho(0.0f, 640.0f, 0.0f, 480.0f);
    auto const view = glm::lookAt(
        glm::vec3(320.0f, 240.0f, -1.0f),   //eye position
        glm::vec3(320.0f, 240.0f, 0.0f),    //center
        glm::vec3(0.0f, -1.0f, 0.0f)        //up
    );

I imagine I need to look at the (320,240,0) from the negative position of z towards positive z, and the "up" direction is negative y.

However it doesn't seems to provide the right result

    auto const v = glm::vec2(320.0f, 240.0f);   
    auto const v2 = glm::vec2(0.0f, 0.0f);      
    auto const v3 = glm::vec2(640.0f, 480.0f);  
    auto const result = proj * view * glm::vec4(v, 0.0f, 1.0f);   //expects: (0,0), gives (-1,-1)
    auto const result2 = proj * view * glm::vec4(v2, 0.0f, 1.0f); //expects: (-1,1), gives (-2,0)
    auto const result3 = proj * view * glm::vec4(v3, 0.0f, 1.0f); //expects: (1,-1), gives (0,-2)
glm-math game-development opengl c++
1 Answers

That's not how view and projection work. The view matrix tells you which point should be mapped to [0,0] in view space. It seems you try to map the center of the visible area to [0,0], but then you use a projection matrix which assumes that [0,0] is the top-left corner.

Since you first apply the view matrix, that gives a result of view * glm::vec4(v, 0.0f, 1.0f) = [0,0]. Then the projection matrix get's applied where you defined that 0,0 as the top-left corner, thus proj * [0,0] will result in [-1,-1].

I'm not 100% sure what you want to achieve, but if you want to use the given projection matrix, then the view matrix has to transform the scene in a way that the top-left point of the visible area get's mapped to [0,0].

You can also adjust the project to use the range [-320, 320] (and respectively [-240, 240]) and keep mapping the center to [0,0] with the view matrix.

user_25797380
Related