frame_count=1 pos.x=508 pos.y=-16 v_x=-6 v_y=-21 loop();//pos.x+=v_x;pos.y+=v_y; frame_count=2 pos.x=502 pos.y=-21 ///??????? v_x=-6 v_y=-21 i draw a point at (pos.x,pos.y),and in the loop:pos.x+=v_x;pos.y+=v_y; then ,i redraw the point ,why the pos.y == -21? is the SDL take a number that below the zero for example -16 as the 0?(so that 0+(-21)=-21) anybody can make an explanation for me ?

Could you clarify a little bit of what your wanting to figure out (not very clear English *no offense!* :p) and tell us what your wanting to figure out, and you can put code like this:

[ source ]
code
[ /source ]

No spaces

It would seem very odd that -16 would suddenly change to 0. Are you absolutely sure that you do "pos.y += v_y" and not "pos.y = v_y" in your loop?

If there are no typos, can you tell us what kind of a structure/class pos is?

SDL_Rect pos;
int v_x,v_y;

of course i'm sure:pos.x+=v_x;pos.y+=v_y;
i just want to know:
when pos.y=-16 and v_y=-21
after pos.y+=v_y;
why pos.y = -21 ?

is the SDL take a number that below the zero for example -16 as the 0?

no answer?

Wow, you have been really patient!
As already said above, post your COMPLETE source code so we can help you.

By the way keep in mind that adding a negative number to another negative number doesn't make it positive (or zero).
For example:

tmp = -21;
tmp += -21;

tmp now is -42 and not 0.

Maybe that's the problem?

sorry,now ,i'll post the code ,
ps: i used it in DEV-C, and there's 2 balls in the screen ,and after you quit ,you could find the ball's positin

please give me the answer!!!

best regards!!!!!

code::

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <SDL/SDL.h>
#include <windows.h>
#include <SDL/SDL_image.h>
#define frame_max 10
#define g 8 //重力加速度

SDL_Surface *load_image(char *filename)
{
SDL_Surface *loadimage = NULL;
SDL_Surface *formatedimage = NULL;
Uint32 colorkey;

loadimage = IMG_Load( filename );

if(loadimage != NULL)
{
formatedimage = SDL_DisplayFormat(loadimage);
if(formatedimage !=NULL)
{
colorkey = SDL_MapRGB(formatedimage->format,0,0,0);
SDL_SetColorKey( formatedimage, SDL_RLEACCEL | SDL_SRCCOLORKEY, colorkey );
}


SDL_FreeSurface(loadimage);
}

return formatedimage;
}

int main(int argc,char *argv[])
{
int draw_x=500,draw_y=400;
SDL_Rect pos[3]; //5个球的坐标
int v_x[3],v_y[3]; //初速度
int frame_count=0; //表示第几帧
srand( (unsigned)time( NULL ) );

SDL_Surface *image[3];
SDL_Surface *screen = NULL;
SDL_Event event;

SDL_Init( SDL_INIT_EVERYTHING );

atexit(SDL_Quit);

screen = SDL_SetVideoMode(1024,768,0,SDL_HWSURFACE|SDL_DOUBLEBUF|SDL_FULLSCREEN);

Uint32 colorkey = SDL_MapRGB(screen->format,0,0,0);
Uint8 *keystates = SDL_GetKeyState( NULL );

int i,j,k;

image[1] = load_image("./resourse/1.png"); //导入每个球的图
image[2] = load_image("./resourse/2.png");
//image[3] = load_image("./resourse/3.png");
//image[4] = load_image("./resourse/4.png");
//image[5] = load_image("./resourse/5.png");

/*for(i=6;i<51;i++)
{
j = i%5;
if(!j)image = image[5];
else if(j==1)image = image[1];
else if(j==2)image = image[2];
else if(j==3)image = image[3];
else if(j==4)image = image[4];
}

//image[6] = image[1];//load_image("./resourse/1.png");
//image[7] = image[2];//load_image("./resourse/2.png");
//image[8] = image[3];//load_image("./resourse/3.png");
//image[9] = image[4];//load_image("./resourse/4.png");
//image[10] = image[5];//load_image("./resourse/5.png");
//image[11] = image[1];//load_image("./resourse/1.png");
//image[12] = image[2];//load_image("./resourse/2.png");
//image[13] = image[3];//load_image("./resourse/3.png");
//image[14] = image[4];//load_image("./resourse/4.png");
//image[15] = image[5];//load_image("./resourse/5.png");
//image[16] = image[1];//load_image("./resourse/1.png");
//image[17] = image[2];//load_image("./resourse/2.png");
//image[18] = image[3];//load_image("./resourse/3.png");
//image[19] = image[4];//load_image("./resourse/4.png");
//image[20] = image[5];//load_image("./resourse/5.png");*/

for(i=1;i<3;i++) //初始化烟花起爆的坐标和初速度
{
pos.x=draw_x;
pos.y=draw_y;
if(i%2)
v_x = rand()%40 ;
else v_x = -rand()%40;
v_y = -rand()%90;
}

void play()
{
SDL_FillRect(screen,NULL,colorkey);
frame_count++;
if(frame_count<=frame_max)
{
for(i=1;i<3;i++)
{ //烟花起爆的坐标和初速度
SDL_BlitSurface(image,NULL,screen,&pos);
pos.x+=v_x;
pos.y+=v_y;
printf("frame_count=%d\n",frame_count);
printf("pos.x=%d\n",pos.x);
printf("pos.y=%d\n",pos.y);
v_y+=g;
printf("v_x=%d\n",v_x);
printf("v_y=%d\n",v_y);
printf("pos.x=%d\n",pos.x);
printf("pos.y=%d\n\n",pos.y);
}
SDL_Flip(screen);
}
else
{
frame_count=0;
//draw_x=rand()%650; //随机产生爆炸点
//draw_y=rand()%350;
for(i=1;i<3;i++) //重新初始化烟花起爆的坐标和Y轴初速度
{
pos.x=draw_x;
pos.y=draw_y;
if(i%2)
v_x = rand()%40 ;
else v_x = -rand()%40;
v_y = -rand()%90;
printf("frame_count=%d\n",frame_count);
printf("pos.x=%d\n",pos.x);
printf("pos.y=%d\n",pos.y);
printf("v_x=%d\n",v_x);
printf("v_y=%d\n",v_y);
printf("pos.x=%d\n",pos.x);
printf("pos.y=%d\n\n",pos.y);
}
}

SDL_Delay(300);
}

int display = 1;

while(display)
{
while(SDL_PollEvent(&event))
{
switch(event.type)
{
case SDL_KEYDOWN:
break;
case SDL_MOUSEMOTION : //用鼠标位置作为烟花爆炸点
draw_x = event.motion.x-25;
draw_y = event.motion.y-25;
break;
case SDL_QUIT:
display = 0;
break;
}
}

if( keystates[SDLK_ESCAPE] ) break;//{display = 0;continue;}

play();

}
for(i=1;i<3;i++)
SDL_FreeSurface(image);
SDL_FreeSurface(screen);

SDL_Quit();

exit(0);
}

please attention: when the y = -2 or some other values blew the zero, and then y+=vy;
in the next frame, it will : y==vy, just as :in the last frame,y = 0(int fact y = -2)
why is it ??????

you can motion the mouse near the edge of the window,and check the stdout.txt