# Object's speed control against different framerates

**URL:** <https://discourse.libsdl.org/t/objects-speed-control-against-different-framerates/14497>\
**Category:** SDL Development\
**Created:** [April 27, 2007, 2:48pm UTC](https://discourse.libsdl.org/t/objects-speed-control-against-different-framerates/14497 "2007-04-27T14:48:58Z")\
**Posts on this page:** 1\
**Showing post:** 1

<div class="post-metadata">

**Author:** ![C.Alvarez](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/c/b487fb/32.png) [@C.Alvarez](https://discourse.libsdl.org/u/C.Alvarez)\
**Post date:** [April 27, 2007, 2:48pm UTC](https://discourse.libsdl.org/t/objects-speed-control-against-different-framerates/14497/1 "2007-04-27T14:48:58Z")

</div>

For time based movements, I use a basic pos = pos + speed \* lastframetime  
When I move an object with constant speed during constant time, one  
second for example, I have no problem to end the movement at the same  
position in any framerate.

When I tried to include variable speed movement, this seems to be a  
little more complicated to interpolate the speed variation.

I have made this little code to explain what I mean, it just need to  
press space to start the test, runs a desired movement, decreases the  
speed, and automatically ends ( with log in stdout.txt ).

( There is no function’s check because this is just a little test ) :

/\*\*\* code start \*\*\*/  
#include \<stdio.h\>  
#include \<SDL/SDL.h\>  
#include \<SDL/SDL\_opengl.h\>

int main( int argc, char\*\* argv )  
{  
// framerate vars  
int iCurrentTick = 0, iLastTick = 0, iTmpFPS = 0, iNextUpdate = 0;  
float fFrameDuration;

```
   // object vars
float fObjPosX = 0;
float fSpeed = 500.0f;

   // other
BOOL bLoop = TRUE, bMoving = FALSE;
SDL_Event event;

  // start
SDL_Init( SDL_INIT_VIDEO|SDL_INIT_TIMER );

SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );

SDL_SetVideoMode( 800, 600, 32, SDL_OPENGL );

glMatrixMode ( GL_PROJECTION );
glLoadIdentity ();
glOrtho( 0, 800, 0, 600, -1.0, 1.0 );

glMatrixMode ( GL_MODELVIEW );

   // main loop
while( bLoop )
{

		// EVENT SECTION
	while( SDL_PollEvent( &event ) )
	{
		switch( event.type )
		{
			case SDL_QUIT:
			{
				SDL_Quit();
				return 0;
			}

			case SDL_KEYDOWN:
				if( event.key.keysym.sym == SDLK_ESCAPE )
				{
					SDL_Quit();
					return 0;
				}

				if( event.key.keysym.sym == SDLK_SPACE )
					bMoving = TRUE;
		}
	}

		// UPDATE SECTION
	if( bMoving )
	{
		float deltaf = fFrameDuration / 1000.0f;
		fObjPosX += fSpeed * deltaf; // adjust the position
		fSpeed -= 200.0f * deltaf; // adjust the speed

		if( fSpeed <= 0.0f )
		{
			bMoving = FALSE;
			SDL_Quit();
			return 0;
		}
		
		printf( "FT=%.02f pos=%.02f speed=%.02f\n", fFrameDuration,

```

fObjPosX, fSpeed ); // logger  
}

```
		// DRAWING SECTION
	glClearColor( 0, 0, 0, 0 );
	glClear( GL_COLOR_BUFFER_BIT );

	glLoadIdentity();
	glTranslatef( fObjPosX, 300.0f, 0.0f );

	glBegin(GL_TRIANGLES);
		glColor3f( 255, 255, 0 );
		glVertex3f( 10.5f, .0f, .0f );

		glColor3f( 255, 0, 0);
		glVertex3f( -10.5f, -10.25f, 0.0f );

		glColor3f( 255, 0, 0 );
		glVertex3f( -10.5f, 10.25f, .0f );
	glEnd();
	SDL_GL_SwapBuffers();

		// FRAMERATE UPDATE
	iCurrentTick = SDL_GetTicks();
	fFrameDuration = iCurrentTick - iLastTick;
	iLastTick = iCurrentTick;

		// FPS count
	iTmpFPS++;
	if( iCurrentTick >= iNextUpdate )
	{
		iNextUpdate += 1000;
		printf("FPS=%i\n", iTmpFPS );
		iTmpFPS = 0;
	}
}

return 0;

```

}

/\*\*\* code end \*\*\*/

Running at max speed ( 360/380 fps on my system ) the object position  
ends at 625.  
Running at 75 fps ( vsync forced from drivers ) the object position ends at 628.

I tried with different framerates ( with FPSmanager from sdl\_gfx ),  
and lower is the framerate, and greater is the ending position.

How to do a correct speed adjustment ?

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_[View the full topic](https://discourse.libsdl.org/t/objects-speed-control-against-different-framerates/14497)._