SDL2 - How to render with one buffer instead of two? - c

Question

In SDL2 I want to be able to draw changes to one buffer rather than redraw the whole image to two different buffers as my setup seems to be doing. Below is a really quick test which shows the unwanted behavior:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <SDL.h>
// Compile: gcc test.c -I/usr/local/include/SDL2 -L/usr/local/lib -lSDL2
void putPixel(SDL_Renderer *renderer, int x, int y)
{
SDL_SetRenderDrawColor(renderer, 255,255,255,255);
SDL_RenderDrawPoint(renderer, x, y);
}
int main(int argc, char* argv[]) {
int width = 640;
int height = 480;
SDL_Window *window = SDL_CreateWindow("Test", 0,0,width,height, 0);
if (window == NULL)
{
return -1;
}
SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
if (renderer == NULL)
{
return -2;
}
SDL_SetRenderDrawBlendMode(renderer,SDL_BLENDMODE_BLEND);
SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
SDL_RenderClear(renderer);
for(int x=0;x<8;x++)
{
for(int y=0;y<10;y++)
{
putPixel(renderer,40+x*10,50+y);
}
SDL_RenderPresent(renderer);
sleep(1);
}
SDL_Quit();
return 0;
}
The output from this is two alternating screens. It is obviously using a double buffer which means I have to clear and redraw to get the output I want. After each cycle of the for...loop I wanted to add a line to the buffer - there should have been 8 lines at the end of the program running. In this case I got 4 on one buffer and 4 on another. I don't want to redraw the previous lines again either, hence the need for one buffer:

Answer 1

This uses a texture as a buffer and copies this to the screen when done.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <SDL.h>
// Compile: gcc test.c -I/usr/local/include/SDL2 -L/usr/local/lib -lSDL2
void putPixel(SDL_Renderer *renderer, int x, int y)
{
SDL_SetRenderDrawColor(renderer, 255,255,255,255);
SDL_RenderDrawPoint(renderer, x, y);
}
int main(int argc, char* argv[]) {
int width = 640;
int height = 480;
SDL_Window *window = SDL_CreateWindow("Test", 0,0,width,height, 0);
if (window == NULL)
{
return -1;
}
SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_TARGETTEXTURE);
if (renderer == NULL)
{
return -2;
}
SDL_SetRenderDrawBlendMode(renderer,SDL_BLENDMODE_BLEND);
/* Create texture for display */
SDL_Texture *display = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, width, height);
SDL_SetRenderTarget(renderer, display);
SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
SDL_RenderClear(renderer);
for(int x=0;x<8;x++)
{
SDL_SetRenderTarget(renderer, display);
for(int y=0;y<10;y++)
{
putPixel(renderer,40+x*10,50+y);
}
SDL_SetRenderTarget(renderer, NULL);
SDL_RenderCopy(renderer, display, NULL, NULL);
SDL_RenderPresent(renderer);
sleep(1);
}
SDL_Quit();
return 0;
}
The output from this is below: