How to handle multiple keypresses at once with SDL?

Question

been getting myself familiar with OpenGL programming using SDL on Ubuntu using c++. After some looking around and experimenting I am starting to understand. I need advice on keyboard event handling with SDL.

I have a 1st person camera, and can walk fwd, back, strafe left and right and use the mouse to look around which is great. Here is my processEvents function:

void processEvents()
{
    int mid_x = screen_width  >> 1;
int mid_y = screen_height >> 1;
int mpx = event.motion.x;
int mpy = event.motion.y;
float angle_y  = 0.0f;
float angle_z  = 0.0f;

while(SDL_PollEvent(&event))
{
    switch(event.type)
    {
        case SDL_KEYDOWN:
            switch(event.key.keysym.sym)
            {
                case SDLK_ESCAPE:
                    quit = true;
                    break;
                case SDLK_w:
                    objCamera.Move_Camera( CAMERASPEED);
                    break;
                case SDLK_s:
                    objCamera.Move_Camera(-CAMERASPEED);
                    break;
                case SDLK_d:
                    objCamera.Strafe_Camera( CAMERASPEED);
                    break;
                case SDLK_a:
                    objCamera.Strafe_Camera(-CAMERASPEED);
                    break;
                default:
                    break;
            }
            break;

        case SDL_MOUSEMOTION:
            if( (mpx == mid_x) && (mpy == mid_y) ) return;

            SDL_WarpMouse(mid_x, mid_y);

            // Get the direction from the mouse cursor, set a resonable maneuvering speed
            angle_y = (float)( (mid_x - mpx) ) / 1000;      
            angle_z = (float)( (mid_y - mpy) ) / 1000;

            // The higher the value is the faster the camera looks around.
            objCamera.mView.y += angle_z * 2;

            // limit the rotation around the x-axis
            if((objCamera.mView.y - objCamera.mPos.y) > 8)  objCamera.mView.y = objCamera.mPos.y + 8;
            if((objCamera.mView.y - objCamera.mPos.y) <-8)  objCamera.mView.y = objCamera.mPos.y - 8;

            objCamera.Rotate_View(-angle_y);

            break;

        case SDL_QUIT:
            quit = true;
            break;

        case SDL_VIDEORESIZE:
            screen = SDL_SetVideoMode( event.resize.w, event.resize.h, screen_bpp, SDL_OPENGL | SDL_HWSURFACE | SDL_RESIZABLE | SDL_GL_DOUBLEBUFFER | SDL_HWPALETTE );
            screen_width = event.resize.w;
            screen_height = event.resize.h;
            init_opengl();
            std::cout << "Resized to width: " << event.resize.w << " height: " << event.resize.h << std::endl;
            break;

        default:
            break;
    }
}
}

now while this is working, it has some limitations. The biggest one and the purpose of my question is that it seems to only process the latest key that was pressed. So if I am holding 's' to walk backwards and I press 'd' to strafe right, I end up strafing right but not going backwards.

Can someone point me in the right direction for better keyboard handling with SDL, support for multiple keypresses at once, etc?

Thanks

Answer 1

SDL keeps track of the current state of all keys. You can access this state via:

SDL_GetKeyState()

So, each iteration you can update the movements based on the key state. To make the movement smooth you should update the movement magnitude based on the time elapsed between updates.