2D Movement Theory - c++

Question

I have recently been getting into OpenGL/SDL and playing around with objects in 2D/3D Space and as i'm sure most newbies to this area do, have a few queries, about the 'best' way to do something. I quote best, because i'm assuming there isn't a best way, it's personal preference.
So, I have an entity, a simple GL_QUAD, which I want to move around. I have keyboard events setup, i can get the keypress/release events, not a problem.
I have an entity class, which is my GL_QUAD, pseudo implementation....
class Entity
{
void SetVelocity(float x, float y);
}
I then have this event handler code...
if theEvent.Key = UPARROW AND theEvent.State = PRESSED
Entity.SetVelocity(0.0f, -1.0f);
else if theEvent.Key = UPARROW AND theEvent.State = RELEASED
Entity.SetVelocity(0.0f, 0.0f);
My question is, is this the best way to move my entity? This has led me to thinking that we could make it a little more complex, by having a method for adjusting the X/Y Velocity, seperately. As SetVelocity would forget me X velocity if i started moving left? So i could never travel diagonally.
For Example...
class Entity
{
void SetXVelocity(float x);
void SetYVelocity(float y);
}
if theEvent.Key = UPARROW AND theEvent.State = PRESSED
Entity.SetYVelocity(-1.0f);
else if theEvent.Key = UPARROW AND theEvent.State = RELEASED
Entity.SetYVelocity(0.0f);
if theEvent.Key = LEFTARROW AND theEvent.State = PRESSED
Entity.SetXVelocity(-1.0f);
else if theEvent.Key = LEFTARROW AND theEvent.State = RELEASED
Entity.SetXVelocity(0.0f);
This way, if I have an XVelocity and I then press the UPARROW, I will still have my XVelocity, as well as a new YVelocity, thus moving diagonally.
Is there a better way? Am I missing something very simple here?
I am using SDL 1.2, OpenGL, C++. Not sure if there is something in SDL/OpenGL which would help?
Thanks in advance for your answers.

Answer 1

The question is really general since it depends on how you want to model the movement of your objects in your world.
Usually every object has its velocity which is calculated basing on an acceleration and capped to a maximum. This means that a key press alters the acceleration of the object for the specified frame which is then calculated and applied to the current velocity of the object.
This is done through an update phase which goes through all the objects and calculates the velocity change according to the object acceleration. In this way you don't bother to modify the velocity itself but you let your engine to do its calculations depending on the state of every object..

Answer 2

acceleration is applied over a period of time, so in the example by #jack, you would apply an acceleration 10m/s^2 over a time period of one second.
you should also modify your application to make it time based, not frame based.
have a look at this basic game physics introduction, and I would also really have a look at the GameDev.net Physics Tutorials

Answer 3

I assume the way you want movement to work is that you want the player to move only when a key is held.
In short: your solution is fine.
Some potential gotchas to take consideration of: What happens if both left and right is pressed?

Answer 4

Well, what you describe here is a simple finite state machine. You have the different directions in which you can move (plus no movement at all) as the states, and the key-events as transitions. This can usually be implemented quite well using the state pattern, but this is often quite painful in C++ (lots of boilerplate code), and might be over the top for your scenario.
There are of course other ways to represent speed and direction of your entity, e.g. as a 2D-vector (where the length gives the speed). This would enable you to easily represent arbitrary directions and velocities.