SDL2 function that erases lines? - sdl-2

Question

I am making a program that creates lines, how can I erase a line? Is there a function for it?
I have already tried using SDL_RenderClear() but it did not work.
(please note that i am a beginner and that im NOT native american, also i only am 10 years old so explain in a simple way.)
Here is my line creating code:
#include <iostream>
#include <SDL.h>
using namespace std;
int main( int argc, char * argv[] )
{
SDL_Init(SDL_INIT_EVERYTHING);
SDL_Window *janela = NULL;
SDL_Renderer *renderer = NULL;
bool roda = true;
SDL_Event evento;
int x;
int y;
int x2;
int y2;
janela = SDL_CreateWindow( "janela" , SDL_WINDOWPOS_CENTERED , SDL_WINDOWPOS_CENTERED ,500 , 500 , SDL_WINDOW_RESIZABLE);
renderer = SDL_CreateRenderer(janela , -1 , SDL_RENDERER_ACCELERATED);
while (roda) {
SDL_Event evento;
while (SDL_PollEvent(&evento)) {
switch(evento.type){
case SDL_QUIT:
roda = false;
break;
case SDL_MOUSEBUTTONDOWN:{
x = evento.motion.x;
y = evento.motion.y;
break;}
case SDL_MOUSEBUTTONUP:
while(roda){
x2 = evento.motion.x;
y2 = evento.motion.y;
SDL_SetRenderDrawColor(renderer , 255 , 0 , 0 , 255);
SDL_SetRenderDrawColor(renderer , 125 , 234 , 253 , 255);
SDL_RenderDrawLine(renderer , x , y , x2 , y2);
SDL_RenderPresent(renderer);
break;}
case SDL_KEYDOWN:{
switch(evento.key.keysym.sym){
case SDLK_b:{
SDL_RenderClear(renderer);
}
}
}
}
}
}
SDL_DestroyWindow(janela);
janela = NULL;
SDL_DestroyRenderer(renderer);
renderer = NULL;
SDL_Quit();
return 1;
}
What I expect is the lines to be erased but it did not happen.

Answer 1

Typical main loop is basically this:
while(!quit) {
while(pool_event()) {
// change state to react to event
}
clear_previous_frame();
for each line {
draw_line();
}
present();
}
So you should always clear and redraw your lines, adding new line state if events says so (but not drawing it yet - event processing shouldn't do that, or it becomes very compilcated). It is problemmatic to keep previous frame's contents and just add more lines; short explaination is because SDL_RenderPresent documentation says The backbuffer should be considered invalidated after each present (and long explaination is basically a list of reasons why it says so).
To keep track of lines to draw you need to save them somewhere. std::vector could be a simple option, if your requirements aren't against it.
To sum it up, your question's modified code with a bit of comments:
#include <vector>
#include <SDL.h>
struct line {
int x0, y0, x1, y1;
};
int main( int argc, char * argv[] )
{
SDL_Init(SDL_INIT_EVERYTHING);
SDL_Window *janela = NULL;
SDL_Renderer *renderer = NULL;
bool roda = true;
int x;
int y;
janela = SDL_CreateWindow( "janela" , SDL_WINDOWPOS_CENTERED , SDL_WINDOWPOS_CENTERED ,500 , 500 , SDL_WINDOW_RESIZABLE);
renderer = SDL_CreateRenderer(janela , -1 , SDL_RENDERER_ACCELERATED);
std::vector<line> lines;
while (roda) {
SDL_Event evento;
while (SDL_PollEvent(&evento)) {
switch(evento.type){
case SDL_QUIT:
roda = false;
break;
case SDL_MOUSEBUTTONDOWN:
// save starting coordinates
x = evento.motion.x;
y = evento.motion.y;
break;
case SDL_MOUSEBUTTONUP: {
// add new line to draw
line l = { x, y, evento.motion.x, evento.motion.y };
lines.push_back(l);
} break;
case SDL_KEYDOWN:
if(evento.key.keysym.sym == SDLK_b) {
// drop lines
lines.resize(0);
}
break;
}
}
// clear previous contents - in most cases, screen content is
// invalidated after RenderPresent and you need to draw again
SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255); // your 'clear' colour
SDL_RenderClear(renderer);
// draw all accumulated lines
SDL_SetRenderDrawColor(renderer, 125, 234, 253, 255); // your lines colour
for(const line &l : lines) {
SDL_RenderDrawLine(renderer, l.x0, l.y0, l.x1, l.y1);
}
// all drawn - present
SDL_RenderPresent(renderer);
}
SDL_DestroyWindow(janela);
SDL_DestroyRenderer(renderer);
SDL_Quit();
return 0; // 0 is 'success' return code, non-0 is failure
}