C++/SDL: Images moving too slowly. Images loaded once, etc. Why?

RHLinuxGUY · 05-25-2007, 11:46 AM

I am looking at games on the Super Nintendo, NES and etc., and I noticed how smooth their backgrounds and other objects move across the screen. I'm trying to recreate that after a couple unsuccessful attempts. I have made a small program that just moves a stick man across the screen everywhere, except where some blocks are at.

http://www.yousendit.com/transfer.ph...E52A1E3A0EF883

Just execute sc_game and you can see what I mean.

I am trying to get speed variation in my objects. So far, to get that smoothness I see in 2D games, moving images one pixel at a time across the screen using a specific time frame for each image sounds like the best way to achieve this.

The problem with the above application is that it does not use a time frame. And strangely, it is running strangely slow. Why?

Am I supposed to move images across the screen by the pixels (instead of one) along with a time frame?

Thanks in advance.

dmail · 05-26-2007, 08:21 AM

Currently your object "blob" just updates positions using a defined offset

Code:

void scobject::move ()
{
	switch ( move_x )
	{
		case LEFT : xywh.x -= offsetX; break;
		case RIGHT: xywh.x += offsetY; break;
		case STOP :                    break;
...

The simplest method to use is linear interpolation with each frame having a time step.
Using a fixed time step can help in making a game run the same no matter what the machine specs are, where as a variable time step gives differing results.

Code:

struct Object
{
	float x,y;//position
	float vx,vy;//velocity

	update(float time_step)
	{
		//v = u + at
		//position = initial + velocity * time
		x += vx * time_step;
		y += vy * time_step;

	}
};

Here is two links from Glenn Fiedler that you should read:
http://www.gaffer.org/game-physics/fix-your-timestep/
http://www.gaffer.org/game-physics/integration-basics/

RHLinuxGUY · 05-26-2007, 05:11 PM

Thank you, I am implemented time based movement into my game.