Using OpenGL for 2D

Question

I have started with OpenGL and learned about model,view,and the projection matrix. From my understanding the projection matrix is only needed to project a 3D entity onto a 2D surface(the screen). So if I want to create a 2D game would I even need to mess around with the projection matrix?

Answer 1

It can still be nice to use a projection matrix for defining your coordinate system. By default a window will be defined between [-1,1] for both x and y no matter what resolution and aspect ratio. If you don't fix this using a projection matrix, you'll have to compensate in other ways. You want a square to render as a square, not a rectangle.

Depending on your GL version you can call glOrtho, construct it manually or use glm::ortho.

In my experience, working on the default [-1,1] system is extremely unpractical. For example : You don't want rotations around the z axis to deform your geometry.

Answer 2

No. When dealing purely with two dimensions, you can leave the projection matrix as the identity matrix.