How to efficiently render different things at different times in a game?

Question

Sorry for the ambiguous title. What I am wondering is what is an efficient way to alternate rendering between lets say a main menu, options menu, and "in the game."

The only two ways I've come up with so far are to have 1 render function, with code for each part (menu, ...) and a variable to control what gets drawn, or to have multiple render functions, and use a function pointer to point to the appropriate one, and then just call the function pointer.

I always wonder how more professional games do it.

Answer 1

Try to use state-machine / strategy OOP pattern. Game application is in different states and renders different things and reacts on keyboard/mouse input differently when you are playing and when you are interacting with menu.

Answer 2

If your controls are represented as classes then a polymorhic API render would solve the problem. Depending on the object ( menu types) the corresponding rendering happens.

class UIObject
{
public:
  virtual bool render() = 0;
  ~UIObject(){}
};

class MainMenu : pu{
public:
  virtual bool render()
  { //rendering for main menu
  }
 };
class OptuionMenu
{
public:
  virtual bool render() { //render for option menu}
};

Answer 3

Well this is a bit more complicated if you want to do it right. First I create a CScreen class that's the base class for all the screens. It's an abstract class( use pure virtual functions) that has 2 functions: Render and Update. Then I derive it in more screens that I need such as CMainMenuScreen, COptionsScreen, CCreditsScreen, CGameScreen etc. Let each of these classes take care of their own stuff. In each of them you have the interface and then when press for instance the options button in the main menu screen then you change the screen to COptionsScreen. For that you have to just keep one variable CScreen screen somewhere and on every frame call screen->Update() and screen->Draw() remeber to adjust if you do not use pointers(tough I'd recommend this)

Answer 4

Games that I've shipped, that have sold lots of copies, have had a state machine and used switch statements to choose the appropriate functionality. While ostensibly less flexible than an "OOP" state machine, it was far easier to work with than the OOP designs I've subsequently been subjected to.

It actually may be appropriate to have only one render function, but that function shouldn't know specifics about what it's doing. It'll have 3D and 2D passes (at least, for a 3D game, since even those often have 2D UI elements), but it doesn't need to know what "mode" the game is in.

The magic happens in the UpdateMainMenu or UpdateGame or UpdateInGameMenu functions, as well as the Start and Stop functions associated with switching states. Choose which with a switch statement on an enum and use it two places, switching states (one switch to stop the old state, one more to start the new one) and updating.

As I write that my alarm bells go off that this is a perfect opportunity to use OOP, but from experience I would advise against that. You don't want to end up in the situation where you have a million little states that are coming and going; you want to constrain it to the major "run modes," and each of those modes should be able to operate on data that tells it what to display. E.g. one state for the entire in-game menu, which "loads" data (usually, "updates its pointer to the data") to indicate what the behavior of the current screen is. There is nothing worse than having a hundred micro classes and not knowing which one triggers when, not to mention the duplicated logic that often arises from such a design (game developers are very bad at reducing duplication through refactoring).