SDL image keep flashing, why?

[ccmccm]

Anybody can help? How can I modify my code to not keeping the text and images flashing?

// Screen surface
SDL_Surface *gScreen;
SDL_Surface* fontSurface;
SDL_Color fColor;
SDL_Rect fontRect;
TTF_Font* font;
int score=0;
char *rank="";
char buffer [50];
char *question[] = {"1. The Boston matrix is used to help a firm with its:","2. The Cash cow in the Boston matrix has:","3. Which of these is not a common objective of a startup?","4. Which source of finance is a new sole trader business likely to use?","5. Which of these types of businesses are in the public sector?","XXX","XXX"};
char *choiceA[]= {"A. Cash flow","A. High market share, high growth","A. Celebrity status","A. Friends and family","A. Private limited company",""};
char *choiceB[]= {"B. Product portfolio","B. Low market hare, high growth","B. Growth","B. Bank loan","B. Public limited company"," "};
char *choiceC[]= {"C. Share price","C. High market share, low growth","C. Survival","C. Retained profit","C. Sole trader"," "};
char *choiceD[]= {"D. Organizational structure","D. Low market share, low growth","D. Celebrity status","D. All of the above","D. Government organizations"," "};
int score_q[] = {0,110,150,130,200,200};

char *correct_ans[] = {"","B","C","A","A","D"};
int index=0;

//Initialize the font of all text in the game, set to green
void fontInit(){
        TTF_Init();
        font = TTF_OpenFont("arial.ttf", 18);
        fColor.r = 0;
        fColor.g = 255;
        fColor.b = 0;
}

//Function for loading bmp files and displaying on the screen. 
void ShowBMP(char *file, SDL_Surface *screen, int x, int y)
{
    SDL_Surface *image;
	SDL_Surface *optimized_image;
    SDL_Rect dest;

	
	
    /* Load the BMP file into a surface */

    //image = SDL_LoadBMP(file);
	image = IMG_Load(file);
	optimized_image = SDL_DisplayFormat(image);
	SDL_FreeSurface(image);
	
    if ( optimized_image == NULL ) {
        fprintf(stderr, "Couldn't load %s: %s\n", file, SDL_GetError());
        return;    
	}

    /*   onto the screen surface.
       The surfaces should not be locked at this point.
     */
    dest.x = x;
    dest.y = y;
    dest.w = image->w;
    dest.h = image->h ;
    
    SDL_SetColorKey(optimized_image, SDL_SRCCOLORKEY, SDL_MapRGB(optimized_image->format, 255, 255, 255));
	SDL_BlitSurface(optimized_image, NULL, screen, &dest);
    /* Update the changed portion of the screen */
    SDL_UpdateRects(screen, 1, &dest);
	
	//SDL_FreeSurface(optimized_image);
}


//Print the designated string at the specified coordinates
void printF(char *c, int x, int y){
        fontSurface = TTF_RenderText_Solid(font, c, fColor);
        fontRect.x = x;
        fontRect.y = y;
        SDL_BlitSurface(fontSurface, NULL, gScreen, &fontRect);
}

struct UIState
{
  int mousex;
  int mousey;
  int mousedown;

  int hotitem;
  int activeitem;
} 
uistate = {0,0,0,0,0};

// Simplified interface to SDL's fillrect call
void drawrect(int x, int y, int w, int h, int color)
{
  SDL_Rect r;
  r.x = x;
  r.y = y;
  r.w = w;
  r.h = h;
  SDL_FillRect(gScreen, &r, color);
}

// Check whether current mouse position is within a rectangle
int regionhit(int x, int y, int w, int h)
{
  if (uistate.mousex < x ||
    uistate.mousey < y ||
    uistate.mousex >= x + w ||
    uistate.mousey >= y + h)
    return 0;
  return 1;
}

// Create a button
int button(int id, int x, int y)
{
  // Check whether the button should be hot
  if (regionhit(x, y, 282, 48))
  {
    uistate.hotitem = id;
    if (uistate.activeitem == 0 && uistate.mousedown)
      uistate.activeitem = id;
  }

  // Render button 
  drawrect(x+8, y+8, 282, 48, 0);
  if (uistate.hotitem == id)
  {
    if (uistate.activeitem == id)
    {
      // Shadow for the button 
      drawrect(x+2, y+2, 282, 48, 0xffffff);
    }
    else
    {
      // Button is merely 'hot'
      drawrect(x, y, 282, 48, 0xffffff);
    }
  }
  else
  {
    // button is not hot, but it may be active    
    drawrect(x, y, 282, 48, 0xaaaaaa);
  }

  // If button is hot and active, but mouse button is not
  // down, the user must have clicked the button.
  if (uistate.mousedown == 0 && 
    uistate.hotitem == id && 
    uistate.activeitem == id)
    return 1;

  // Otherwise, no clicky.
  return 0;
}

// Prepare for IMGUI code
void imgui_prepare()
{
  uistate.hotitem = 0;
}

// Finish up after IMGUI code
void imgui_finish()
{
  if (uistate.mousedown == 0)
  {
    uistate.activeitem = 0;
  }
  else
  {
    if (uistate.activeitem == 0)
      uistate.activeitem = -1;
  }
}

// Rendering function
void render()
{ 
		  
  imgui_prepare();
      
  //printF(question[index], 100, 100);  
  //button(10,25,90);
  printF(question[index], 30, 100);
  
  
  //Button A
  if (button(1,30,170)) {
	  if (correct_ans[index] == "A") {
	     score =score + score_q[index];
      }
	  else { 
         score =score - score_q[index];		  
	  }
	  //return 0;
  }
  printF(choiceA[index], 35, 180);
  
  //Button B
  if (button(2,330,170)){
	  if (correct_ans[index] == "B") {
	     score =score + score_q[index];
	  }
	  else { 
         score =score - score_q[index];		  
	  }
  }
  printF(choiceB[index], 335, 180);
  
  //Button C
  if (button(3,30,250)){
	  if (correct_ans[index] == "C") {
	     score =score + score_q[index];
	  }
	  else {
         score =score - score_q[index];		  
	  }
  }
  printF(choiceC[index], 35, 260);
    
  //Button D
  if (button(4,330,250)){
	  if (correct_ans[index] == "D") {
	     score =score + score_q[index];
      }		 
	  else { 
         score =score - score_q[index];		  
	  }
  }
  printF(choiceD[index], 335, 260);
  
  //convert the score to char and use base 10 to display
  printF("Your score is", 50, 320);
  printF(itoa(score,buffer,10), 230, 320);
  
  printF("Your rank is",50, 350);
  printF(rank,230,350);
 

 //Load image depending on points
  if (score <= -50) {     
	 ShowBMP("desktop_01.bmp", gScreen, 400,360);  
	 //SDL_Delay(1000);
	 //SDL_Quit();
  }	 
  if (score > -50 && score <100) {
	 ShowBMP("Hobo_0.bmp", gScreen, 400,360);   
	 rank = "Hobo";
  }
  if (score >=100 && score < 250) {
	 ShowBMP("Normal_150.bmp", gScreen, 400,360);   
	 rank = "Normal";
  }	  
  if (score >=250 && score < 500) {
     ShowBMP("Businessman_250.bmp",	gScreen, 400,360);   
	 rank = "Businessman";
  }
  if (score >=500) {
     ShowBMP("Wealthy_500.bmp",	gScreen, 400,360);  
     rank = "Wealthy";	 
  }	
 
   imgui_finish();  
  // update the screen
  SDL_UpdateRect(gScreen, 0, 0, 640, 480);    
     
  // don't take all the cpu time
  SDL_Delay(10); 
  
}

// Entry point
int main(int argc, char *argv[])
{
  // Initialize SDL's subsystems - in this case, only video.
    if (SDL_Init(SDL_INIT_VIDEO) < 0) 
  {
        fprintf(stderr, "Unable to init SDL: %s\n", SDL_GetError());
        exit(1);
    }

  // Register SDL_Quit to be called at exit; makes sure things are
  // cleaned up when we quit.
    atexit(SDL_Quit);
    
  // Attempt to create a 640x480 window with 32bit pixels.
    gScreen = SDL_SetVideoMode(640, 480, 32, SDL_SWSURFACE);
  
  //Initialize fonts
    fontInit();

  // If we fail, return error.
    if (gScreen == NULL) 
  {
        fprintf(stderr, "Unable to set up video: %s\n", SDL_GetError());
        exit(1);
    }
	
  static int bgcolor = 0x77 ;

  // clear screen
  drawrect(0,0,640,480,bgcolor);
  
  ShowBMP("background_640x480.png",gScreen,0,0);
  
  // Main loop: loop forever.
	index=0;
    while (index < sizeof(*question)+1 )
    {
	
        ShowBMP("background_640x480.png",gScreen,0,0);

        // Render stuff
		render();
		
					
    // Poll for events, and handle the ones we care about.
        SDL_Event event;
        while (SDL_PollEvent(&event)) 
        {
            switch (event.type) 
            {
			  case SDL_MOUSEMOTION:
				// update mouse position
				uistate.mousex = event.motion.x;
				uistate.mousey = event.motion.y;
				break;
			  case SDL_MOUSEBUTTONDOWN:
				// update button down state if left-clicking
				if (event.button.button == 1) {
				  uistate.mousedown = 1;
					  index++;
}								
				break;
			  case SDL_MOUSEBUTTONUP:
				// update button down state if left-clicking
				if (event.button.button == 1)
				  uistate.mousedown = 0;
				break;
			  
			  case SDL_KEYUP:                  
				switch (event.key.keysym.sym)
				{
				case SDLK_ESCAPE:
				  // If escape is pressed, return (and thus, quit)
				  return 0;
				}
                break;
              case SDL_QUIT:
                return(0);
            }
	
			
        }
		
    }


    return 0;
}

[Peter87]

You draw an image and then you make what you draw visible right away by calling SDL_UpdateRects, so between you draw the background and a button you will only see the background at that position and that is what causes the flashing.

It's much better to draw everything that you want to draw to the screen, and then, when you have drawn everything want to draw, you make it visible by calling SDL_UpdateRects, SDL_UpdateRect or SDL_Flip.