C++ SDL Movement Player "duplication"? - c++

Question

I'm a beginner in SDL and I am simply experimenting with movement of sprites in SDL. I have a rectangle at the bottom of the screen that you can move left and right. However, when you move the sprite left and right, it "duplicates", creating a sort of trace/snake-game-like effect when you move the player left and right. Any ideas on how to fix this?
///the headers
#include "stdafx.h"
#include <SDL.h>
#include <SDL_image.h>
#include <string>
using namespace std;
//screen attributes
const int SCREEN_WIDTH = 800;
const int SCREEN_HEIGHT = 480;
const int SPRITE_SIZE = 32;
//the surfaces
SDL_Surface *image = NULL;
SDL_Surface *screen = NULL;
int main( int argc, char* argv[] )
{
SDL_Surface *screen, *temp,/* *temp2*/ *sprite/*, *sprite2*/;
SDL_Rect rcSprite/*, rcSprite2*/;
SDL_Event event;
Uint8 *keystate;
int gameover;
//initiailise sdl
SDL_Init(SDL_INIT_VIDEO);
SDL_WM_SetCaption("Pong Move Test", "Pong Move Test");
screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, 0, 0);
//load the sprite (paddle)
temp = SDL_LoadBMP("pongpaddle1.bmp");
sprite = SDL_DisplayFormat(temp);
SDL_FreeSurface(temp);
//set positions of sprite
rcSprite.x = 400;
rcSprite.y = 450;
gameover = 0;
//message pump
while (!gameover)
{
//look for an event
if (SDL_PollEvent(&event))
{
switch (event.type)
{
case SDL_QUIT:
gameover = 1;
break;
case SDL_KEYDOWN:
switch (event.key.keysym.sym)
{
case SDLK_ESCAPE:
case SDLK_q:
gameover = 1;
break;
}
break;
}
}
//handle sprite movement
keystate = SDL_GetKeyState(NULL);
if (keystate[SDLK_LEFT])
{
rcSprite.x -= 1;
}
if (keystate[SDLK_RIGHT])
{
rcSprite.x += 1;
}
//colide with edges of screen
if (rcSprite.x < 0)
{
rcSprite.x =0;
}
else if (rcSprite.x > (SCREEN_WIDTH - SPRITE_SIZE))
{
rcSprite.x = (SCREEN_WIDTH - SPRITE_SIZE);
}
//draw the sprite
SDL_BlitSurface(sprite, NULL, screen, &rcSprite);
//update the screen
SDL_UpdateRect(screen, 0, 0, 0, 0);
}
//clean up area
SDL_FreeSurface(sprite);
SDL_Quit();
return 0;
}

Answer 1

You aren't clearing the screen before drawing each frame, so you're actually drawing over the previous frame, which is why you get a 'trailing' effect.
You can solve this by using SDL_FillRect to fill the entire screen with a background color before drawing each frame:
SDL_FillRect(screen, NULL, SDL_MapRGB(screen->format, 0, 0, 0));