Which is the better way to draw?

This is a discussion on Which is the better way to draw? within the Game Programming forums, part of the General Programming Boards category; Hi, I've got a simple question. I have 2 ways to draw objects that I can think of but I ...

  1. #1
    In the Land of Diddly-Doo g4j31a5's Avatar
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    Which is the better way to draw?

    Hi, I've got a simple question. I have 2 ways to draw objects that I can think of but I didn't know which one is better.

    1. Use an abstract class Renderable for the parent of the objects and have an std::vector of the Renderable class and call an overloaded virtual function to draw.

    E.g:
    Code:
    class Renderable
    {
      public:
           virtual void draw()=0;
    }
    Code:
    class MovableObject:public Renderable
    {
     public:
            void draw()
            {
            }
     .........
    }
    
    class TerrainObject:public Renderable
    {
     public:
            void draw()
            {
            }
      ........
    }
    
    class OverlayObject:public Renderable
    {
     public:
            void draw()
            {
            }
      ...........
    }
    etc...
    Code:
    std::vector <Renderable *> mRenderable
    ...
    mRenderable.push_back(new MovableObject() );
    mRenderable.push_back(new TerrainObject() );
    mRenderable.push_back(new OverlayObject() );
    
    //Main loop
      while (!done)
      {
          ..............
          for (std::vector <Renderable *>::iterator itr=mRenderable.begin();
              itr!=mRenderable.end();
              itr++) 
           {
                 (*itr)->draw();
           }
           
       }
    ........
    2. Not using any inheritance and making the respective objects's vectors for drawing
    e.g.:

    Code:
    class MovableObject
    {
     public:
            void draw()
            {
            }
     .........
    }
    
    class TerrainObject
    {
     public:
            void draw()
            {
            }
      ........
    }
    
    class OverlayObject
    {
     public:
            void draw()
            {
            }
      ...........
    }
    etc...
    Code:
    std::vector <MovableObject *> mMovables
    std::vector <TerrainObject *> mTerrains
    std::vector <OverlayObject *> mOverlays
    ...
    mMovables.push_back(new MovableObject() );
    mTerrains.push_back(new TerrainObject() );
    mOverlays.push_back(new OverlayObject() );
    
    //Main loop
      while (!done)
      {
          ..............
          //Movables
          for (std::vector <MovableObject *>::iterator itr=mMovables.begin();
              itr!=mMovables.end();
              itr++) 
           {
                 (*itr)->draw();
           }
    
          //Terrains
          for (std::vector <TerrainObject *>::iterator itr=mTerrains.begin();
              itr!=mTerrains.end();
              itr++) 
           {
                 (*itr)->draw();
           }
    
          //Overlays
          for (std::vector <OverlayObject *>::iterator itr=mOverlays.begin();
              itr!=mOverlays.end();
              itr++) 
           {
                 (*itr)->draw();
           }
           
       }
    ........
    The question is which is better? I understand that using virtual calls can very much affect the performance. But how slow will it be?

    Thanks in advance.
    ERROR: Brain not found. Please insert a new brain!

    “Do nothing which is of no use.” - Miyamoto Musashi.

  2. #2
    Super Moderator VirtualAce's Avatar
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    I really have no idea what you are talking about.

  3. #3
    The Right Honourable psychopath's Avatar
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    Quote Originally Posted by g4j31a5
    I understand that using virtual calls can very much affect the performance.
    Really? I don't see how. I use them all the time, and I've yet to see a noticable performace hit.
    Memorial University of Newfoundland
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  4. #4
    Crazy Fool Perspective's Avatar
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    I'd take good, extensible design over a minor increase in performance.. but thats just me.

  5. #5
    Insane Game Developer Nodtveidt's Avatar
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    Depends on your plans for the game engine. If you plan to reuse the engine for other games, go for a more flexible method. But if the various data in your game is specific to this one game, and you have no need for flexibility, the seemingly "minor" performance increases will add up and you'll get better results. Remember, the players don't care how the game was coded, they care how the game plays.
    Code:
    cout << "Language comparisons are dumb";
    echo("Language comparisons are dumb");
    PRINT "Language comparisons are dumb"
    alert ("Language comparisons are dumb")

  6. #6
    In the Land of Diddly-Doo g4j31a5's Avatar
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    @Bubba

    Well, basically the question is whether I should use inheritance or not. Because I understand that virtual calls have a slight performance decrease. So I was just wondering whether it's appropriate to use virtual calls for a method that always be called every loop (e.g. drawing) in lots of objects.

    @All

    Actually my current method is the second one. I created lots of category of renderable objects independently. And in the class that handles the scene (I called it SceneManager), there's a vector for each category (e.g. MovableObjects vector, TerrainObjects vector, etc) and iterated each vector for drawing every loop. But after I give it some more thought, wouldn't it be better to use inheritance and virtual calls to draw.

    I still hesitated to change the design though because I was afraid that the performance drawback of the virtual calls will be quite huge. FYI, I'm developing for a very low spec machine so I don't need any more performance drop because it's quite slow as it is right now.
    ERROR: Brain not found. Please insert a new brain!

    “Do nothing which is of no use.” - Miyamoto Musashi.

  7. #7
    Super Moderator VirtualAce's Avatar
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    At current speeds I doubt even a 1000 virtual calls will cost you 2 frames.

  8. #8
    In the Land of Diddly-Doo g4j31a5's Avatar
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    Quote Originally Posted by Bubba
    At current speeds I doubt even a 1000 virtual calls will cost you 2 frames.
    It will if the processor is 400 MHz. BTW, that's the machine I'm developing to.
    ERROR: Brain not found. Please insert a new brain!

    “Do nothing which is of no use.” - Miyamoto Musashi.

  9. #9
    Supermassive black hole cboard_member's Avatar
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    Eww.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  10. #10
    S Sang-drax's Avatar
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    Go for the first alternative. Virtual calls are neglible.

    Also, you should use a std::list instead of a vector, because you'd probably want to remove objects not being used anymore.
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

  11. #11
    Crazy Fool Perspective's Avatar
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    >the seemingly "minor" performance increases will add up and you'll get better results.

    I don't mean to sound rude but, no, no it won't. You won't see any (statistically significant) measurable performance increase that can be attributed specifically to using virtual functions.

  12. #12
    Registered User Tonto's Avatar
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    I think he was suggesting that you'd get better performance not using virtual functions (he said if you don't need flexibility [polymorphic design] don't have it). I would venture to say that function pointer + call from vtbl cost of the virtual function is inconsequential in the grand scheme of things, but maybe the little things do add up, but I honestly don't know.

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  13. #13
    Crazy Fool Perspective's Avatar
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    With all the other stuff going on in a game engine, a series of constant time lookups are the least of your worries.

  14. #14
    The larch
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    You can do some testing with both?

    But honestly, are you sure that the rest of the program will perform optimally? Are you sure that you don't waste time detecting collision between objects that have no way of colliding, for example? Are you sure that your pathfinding algorithm doesn't consider some really crooked ways?

    If a program is slow, it is usually because a) it does enormous amounts of work and it's unreasonable to expect it to be any faster; b) the algorithms are implemented poorly - all your efforts to gain microseconds are useless if you use bubble-sort on a large array or do something that is algorithmically equally suboptimal.

    I see that questions about micro-optimisations are asked often, but never is any real code provided, so we could see if there's even any point discussing them.

  15. #15
    Registered User Tonto's Avatar
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    I think it's just more of a general question that would come up while designing base code, anon. I mean, if you implemented awesome algorithms and such, but then found that polymorphism killed performance and you wanted to redesign things, it would take forever. But luckily, the consensus seems to be that such will not be the case for the OP.

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