SDL2: Why a figure "blinks"? - c

Question

I'm trying to move a square in a rectangle in SDL, but when i launch it, the rectangle and the square "blinks" until I move the square with up, left, right or down. When the square moves, the rectangle stop blinking and only the square keeps blinking. How can i move it without the blink?
Edit: The problem was that i put SDL_RenderPresent twice after the main loop. Thanks for your help.
#include <SDL2/SDL.h>
#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include <SDL2/SDL_image.h>
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 500;
int main() {
SDL_Init(SDL_INIT_VIDEO);
SDL_Window* window = SDL_CreateWindow("", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_OPENGL);
SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
bool out = false;
SDL_Event event;
SDL_Rect grid;
grid.x = 25;
grid.y = 25;
grid.w = 300;
grid.h = 450;
SDL_Rect square;
square.x = 175;
square.y = 25;
square.w = 25;
square.h = 25;
//Main loop
while (out==false){
while (SDL_PollEvent(&event)){
if (event.type==SDL_QUIT){out=true;}
else if (event.type==SDL_KEYDOWN){
// Keys
switch (event.key.keysym.sym){
case SDLK_m:
out=true;
break;
case SDLK_LEFT:
square.x-=25;
SDL_RenderPresent(renderer);
break;
case SDLK_RIGHT:
square.x+=25;
SDL_RenderPresent(renderer);
break;
default:
break;
}
}
}
SDL_SetRenderDrawColor(renderer,0,255,0,0);
SDL_RenderFillRect(renderer, &grid);
SDL_RenderPresent(renderer);
SDL_SetRenderDrawColor(renderer,255,0,0,255);
SDL_RenderFillRect(renderer,&square);
SDL_RenderPresent(renderer);
}
SDL_DestroyRenderer(renderer);
SDL_DestroyWindow(window);
SDL_Quit();
return 0;
}

Answer 1

As #HolyBlackCat pointed out, you should only call SDL_RenderPresent() once per frame, not once per shape or keypress:
#include <SDL.h>
#include <SDL_image.h>
#include <stdbool.h>
int main()
{
SDL_Init( SDL_INIT_VIDEO );
SDL_Window* window = SDL_CreateWindow(
"",
SDL_WINDOWPOS_UNDEFINED,
SDL_WINDOWPOS_UNDEFINED,
640,
500,
SDL_WINDOW_OPENGL );
SDL_Renderer* renderer = SDL_CreateRenderer(
window,
-1,
SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC );
bool out = false;
SDL_Event event;
SDL_Rect grid;
grid.x = 25;
grid.y = 25;
grid.w = 300;
grid.h = 450;
SDL_Rect square;
square.x = 175;
square.y = 25;
square.w = 25;
square.h = 25;
//Main loop
while( out == false )
{
while( SDL_PollEvent( &event ) )
{
if( event.type == SDL_QUIT )
{
out = true;
}
else if( event.type == SDL_KEYDOWN )
{
// Keys
switch( event.key.keysym.sym )
{
case SDLK_m:
out = true;
break;
case SDLK_LEFT:
square.x -= 25;
break;
case SDLK_RIGHT:
square.x += 25;
break;
default:
break;
}
}
}
SDL_SetRenderDrawColor( renderer, 0, 255, 0, 0 );
SDL_RenderFillRect( renderer, &grid );
SDL_SetRenderDrawColor( renderer, 255, 0, 0, 255 );
SDL_RenderFillRect( renderer, &square );
SDL_RenderPresent( renderer );
}
SDL_DestroyRenderer( renderer );
SDL_DestroyWindow( window );
SDL_Quit();
return 0;
}