SDL movement

Now I am trying to get the image to just move off center a little bit by using differant x and y values. But when I run this program the image move off screen in increments. Ironically this is the effect I wanted but, I didn't think I was ready so i was just trying to move image a little bit off center not have it keep moving. So here is my code the problem is that the images keeps moving rather than just blitting onto the screen a bit off center. Not any where in the code do I modify xpos or ypos, that I can tell, so I Dont see why when ever I redraw the screen the image moves.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <iostream>
#include "SDL.h"
using namespace std;

SDL_Surface *scrn1, *back;
int misc;
int xpos=2;
int ypos=1;


int InitSDL();
int Setscreenvidmode();
int Blitscreen(SDL_Surface *scrn, SDL_Surface *scrnn);
int SwitchLoadImg(SDL_Surface *scrn,int imgnum);



int main(int argc, char*argv[]){
    
    int imgnum=1,done=0;
    
    InitSDL();
    atexit(SDL_Quit);
    Setscreenvidmode();
    SwitchLoadImg(scrn1,imgnum);
while(done == 0)
{
    Blitscreen(scrn1,back);
    
    if (imgnum>4){imgnum=1;}
    
    SDL_Event event;

    while ( SDL_PollEvent(&event) )
    {
      if ( event.type == SDL_QUIT )  {  done = 1;  }

      if ( event.type == SDL_KEYDOWN )
      {
        if ( event.key.keysym.sym == SDLK_ESCAPE ) { done = 1; }
        if ( event.key.keysym.sym == SDLK_n) {SwitchLoadImg(scrn1,++imgnum);}
      }
    }
    
  


}}//////////////////////////////////////////END OF MAIN//////////////////////

int InitSDL() {
    if ((SDL_Init(SDL_INIT_VIDEO||SDL_INIT_AUDIO))<0){
        cout<<SDL_GetError();
    }
};

int Setscreenvidmode(){
    scrn1 = SDL_SetVideoMode(500,380,32,SDL_SWSURFACE||SDL_DOUBLEBUF);
    back = SDL_SetVideoMode(500,380,32,SDL_SWSURFACE||SDL_DOUBLEBUF);    
};

int Blitscreen(SDL_Surface *scrnn, SDL_Surface *scrn){
    SDL_Rect trect;
    trect.x=xpos;
    trect.y=ypos;
    SDL_BlitSurface(scrnn,NULL,scrn,&trect);
    SDL_Flip(scrn);
};    
    
int SwitchLoadImg(SDL_Surface *scrn,int imgnum){
    SDL_Surface *temp;
    switch (imgnum){
        case 1:
            temp = SDL_LoadBMP("Pic1.bmp");
            break;
        case 2:
            temp = SDL_LoadBMP("Pic2.bmp");
            break;
        case 3:
            temp = SDL_LoadBMP("Pic3.bmp");
            break;
        case 4:
            temp = SDL_LoadBMP("Pic4.bmp");
            break;
        default:
            temp = SDL_LoadBMP("Pic1.bmp");
            break;
    }
    SDL_BlitSurface(temp,NULL,scrn,NULL);
};

Im confused what exactally are you trying to do if I am reading your code correctly your just swapping different images at the same point.

I am trying to get the. The image I have to be placed a little off center when the program starts instead it is continually moving to the upper left. Leave whatever color trail it is behind it. The original code I had just displayed and image then when u hit the n key it switched to the next image. Then I decided to play around with the SDL_Rect structures and this started to happen. I am pretty sure the problem is around here:

Code:

int Blitscreen(SDL_Surface *scrnn, SDL_Surface *scrn){
    SDL_Rect trect;
    trect.x=xpos;
    trect.y=ypos;
    SDL_BlitSurface(scrnn,NULL,scrn,&trect);
    SDL_Flip(scrn);
};

The thing is that, that piece of code should just disp the image of center as long as the xpos and ypos do not change, right? So somewhere else they must be changing inadvertantly.

I get a seg fault when I try to run your code im not exactly sure why you have two screen modes with the back and the scrn1 but you dont need to setting 2 different video modes is not a good idea. I re-wrote your code to do what you wanted.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <iostream>
#include "SDL/SDL.h"
using namespace std;

SDL_Surface *scrn1, *back;
int misc;
int xpos=2;
int ypos=1;


int InitSDL();
int Setscreenvidmode();
int Blitscreen(SDL_Surface *scrn, SDL_Surface *scrnn);
int SwitchLoadImg(SDL_Surface *scrn,int imgnum);



int main(int argc, char*argv[]){
    
    int imgnum=1,done=0;
    
    InitSDL();
    atexit(SDL_Quit);
    Setscreenvidmode();
    SwitchLoadImg(scrn1,imgnum);
while(done == 0)
{
    //BAD idea you were blitting two different video modes to one another
    //Blitscreen(scrn1,back);
    
    
    f (imgnum>4){imgnum=1;}
    SDL_Event event;

    while ( SDL_PollEvent(&event) )
    {
      i
      if ( event.type == SDL_QUIT )  {  done = 1;  }

      if ( event.type == SDL_KEYDOWN )
      {
        if ( event.key.keysym.sym == SDLK_ESCAPE ) { done = 1; }
        if ( event.key.keysym.sym == SDLK_n) {SwitchLoadImg(scrn1,++imgnum);}
      }
      
    }
    
  


}
}//////////////////////////////////////////END OF MAIN//////////////////////

int InitSDL() {
    if ((SDL_Init(SDL_INIT_VIDEO||SDL_INIT_AUDIO))<0){
        cout<<SDL_GetError();
    }
};

int Setscreenvidmode(){
    scrn1 = SDL_SetVideoMode(500,380,32,SDL_SWSURFACE||SDL_DOUBLEBUF);
    //you didn't need this you were setting 2 different video modes
    //back = SDL_SetVideoMode(500,380,32,SDL_SWSURFACE||SDL_DOUBLEBUF);    
};

int Blitscreen(SDL_Surface *scrnn, SDL_Surface *scrn){
    SDL_Rect trect;
    trect.x=xpos;
    trect.y=ypos;
    SDL_BlitSurface(scrnn,NULL,scrn,&trect);
    SDL_Flip(scrn);
};    
    
int SwitchLoadImg(SDL_Surface *scrn,int imgnum){
    SDL_Surface *temp;
    switch (imgnum){
        case 1:
            temp = SDL_LoadBMP("Pic1.bmp");
            break;
        case 2:
            temp = SDL_LoadBMP("Pic2.bmp");
            break;
        case 3:
            temp = SDL_LoadBMP("Pic3.bmp");
            break;
        case 4:
            temp = SDL_LoadBMP("Pic4.bmp");
            break;
        default:
            temp = SDL_LoadBMP("Pic1.bmp");
            break;
    }
    Blitscreen(temp,scrn);//changed this so that your blitting your temp to the screen
};

Wow thanks that fixxed it. Int the other one the image was moving approx 1 or 2 pixels at a time toward the upper left. Do You know why that fixed it?

I want to make a pong game (but dont think I am ready yet), so could someone explain why I dont want 2 SetVideoMode's? I was under the impression that that was what sets the width and heigth of each screen. So lets say I have a 5 by 5 pixel ball bmp. How would I set a screen just around that ball so I can disp it im the main game/paddle are?

The way you want to do thing is just blit the bitmap to the screen at its specific location. so you just create an x,y coord I personally like to do this with a struct if iI don't need member functions.

Code:

struct ball
{
  int x;
  int y;
};

Then you just create an instance of the ball lets say pongBall.

Code:

int main()
{
  SDL_Surface *screen= SDL_SetVideoMode(500,380,32,SDL_SWSURFACE||SDL_DOUBLEBUF)
  SDl_Surface *ball = SDL_LoadBitmap("ball.bmp");
  ball pongBall;
  pongBall.x = 200;
  pongBall.y = 200;
  SDL_Rect trect;
  trect.x = pongBall.x;
  trext.y = pongBall.y;
  SDL_BlitSurface(screen,NULL,ball,&trect);
  SDL_Flip(screen);
  return 0;
}

As for the two screen modes well Im not absoluty sure if its bad in the sense that it is going to crash etc. I just know you really shouldn't need two different screen modes for a pong game. What was bad with your code is that you were blitting a screen mode to a screen mode so Im pretty sure thats bad your program just opened and closed on my comp and the was SDL deploy parachute in stderr.txt.

Wait in that last bit of Pong Code aren't you blitting, screen onto the ball. Dont you want to be blitting ball onto screen?

Ya im sorry i was typing all the code in the quick reply

In dont care, I just wanted to make sure I understood what you said. Thx for ur help by the way.