Hello,
First, thank you to all for your answars! I’m so glad 
Second, I made this example to show you how I see the results (I build
it on Windows, my main computer where I work is broken…).
http://img73.imageshack.us/img73/2516/demo01lf8.png
http://img490.imageshack.us/img490/3441/demo02js5.png
Bye and have a nice day! 
xEsk.---------
Source code:
#include <math.h>
#include <SDL/SDL.h>
float pre_cos[360];
float pre_sin[360];
bool is_pre_cos_sin_init = false;
const float PI = 3.14159265358979323846;
float RadToDeg(float rad) { return rad * 180 / PI; }
float DegToRad(float deg) { return deg * PI / 180; }
void init_pre_math()
{
for(int i = 0; i <= 360; i++)
{
pre_cos[i] = cos(DegToRad(i));
pre_sin[i] = sin(DegToRad(i));
}
is_pre_cos_sin_init = true;
}
SDL_Rect advance(SDL_Rect pos, float angle, int units)
{
if (!is_pre_cos_sin_init) init_pre_math();
int ang = static_cast(-angle);
ang = (ang < 0) ? (ang % 360) + 360: ang % 360;
pos.x = pos.x + static_cast(units * pre_cos[ang]);
pos.y = pos.y + static_cast(units * pre_sin[ang]);
return pos;
}
/*
This function is a modification of
"int _putPixelAlpha(SDL_Surface * surface, Sint16 x, Sint16 y,
Uint32 color, Uint8 alpha);"
extracted from:
SDL_gfxPrimitives.h - Graphics primitives for SDL surfaces
LGPL (c) A. Schiffler
*/
/* Defines for pixel clipping tests (SDL_gfxPrimitives) */
#define clip_xmin(surface) surface->clip_rect.x
#define clip_xmax(surface) surface->clip_rect.x+surface->clip_rect.w-1
#define clip_ymin(surface) surface->clip_rect.y
#define clip_ymax(surface) surface->clip_rect.y+surface->clip_rect.h-1
bool putPixel(SDL_Surface* dst, Sint16 x, Sint16 y, Uint32 color,
const Uint8 alpha)
{
/* Lock the surface */
if (SDL_MUSTLOCK(dst))
if (SDL_LockSurface(dst) < 0)
return false;
Uint32 R, G, B, A = 0;
Uint32 Rmask = dst->format->Rmask,
Gmask = dst->format->Gmask,
Bmask = dst->format->Bmask,
Amask = dst->format->Amask;
/* First Check if this new pixel is in the surface /
if (x >= clip_xmin(dst) && x <= clip_xmax(dst) && y >=
clip_ymin(dst) && y <= clip_ymax(dst))
{
if (alpha == 255)
((Uint16 )dst->pixels + y * dst->pitch / 2 + x) = color;
else
{
Uint16 pixel = (Uint16)dst->pixels + y dst->pitch / 2 + x;
Uint32 dc = *pixel;
R = ((dc & Rmask) + (((color & Rmask) - (dc & Rmask)) * alpha >>
8)) & Rmask;
G = ((dc & Gmask) + (((color & Gmask) - (dc & Gmask)) * alpha >>
8)) & Gmask;
B = ((dc & Bmask) + (((color & Bmask) - (dc & Bmask)) * alpha >>
8)) & Bmask;
if (Amask)
A = ((dc & Amask) + (((color & Amask) - (dc & Amask)) * alpha
8)) & Amask;
*pixel = R | G | B | A;
}
return true;
}
/* Unlock the surface */
if (SDL_MUSTLOCK(dst))
SDL_UnlockSurface(dst);
return true;
}
int main (int argc, char argv[]){
/ Initialize SDL */
SDL_Init (SDL_INIT_VIDEO);
atexit (SDL_Quit);
/* Set 640x480 16-bits video mode */
SDL_Surface *screen = SDL_SetVideoMode (320, 250, 16,
SDL_SWSURFACE | SDL_DOUBLEBUF);
/* white screen */
SDL_FillRect(screen, NULL, SDL_MapRGB (screen->format, 255, 255, 255));
SDL_Rect position;
position.x = 160;
position.y = 240;
/* example: */
for (int n = 0; n < 360; n++)
{
position = advance(position, n, 3);
putPixel(screen, position.x, position.y, 0, 255);
}
/* Make sure everything is displayed on screen */
SDL_Flip(screen);
/* main loop */
int done = 0;
while (!done)
{
SDL_Event event;
/* Check for events */
while (SDL_PollEvent (&event))
{
switch (event.type)
{
case SDL_KEYDOWN:
break;
case SDL_QUIT:
done = 1;
break;
default:
break;
}
}
}
return 0;
}
