How to Transform the Model Matrix with glm::lookAt for Accurate Object Placement?

Transform the modelMatrix

When using the glm::lookAt function to set the model matrix, unexpected results can arise. To achieve the desired transformation, an understanding of the model matrix and its role in the graphics pipeline is necessary.

Model Matrix

The model matrix defines the location, orientation, and scale of an object in the world space. It transforms vertex positions from model space to world space.

View Matrix and Projection Matrix

The view matrix transforms vertices from world space to view space, relative to the camera's position and orientation. The projection matrix transforms vertices from view space to clip space, which is where the clipping occurs.

Transforming the Model

To set an object's position and orientation similarly to setting the camera, the model matrix can be modified following these steps:

1. Use glm::lookAt to specify the position, direction, and up vector.

2. Adjust the third column (z-axis) by negating its elements:

   m_Orientation[3][0] = -m_Orientation[3][0];
   m_Orientation[3][1] = -m_Orientation[3][1];
   m_Orientation[3][2] = -m_Orientation[3][2];

3. In the vertex shader, apply the transformations in this order:

   gl_Position = CameraMatrix * ModelMatrix * Pos;

Vertex Coordinates

To correctly position vertices in the scene, the Pos variable should represent the vertex position in object space. This requires transforming vertex coordinates from model space to object space.

By following these instructions, you can effectively use the glm::lookAt function to set the model matrix and correctly position and orient objects in your scene.

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