SDL2 OpengGL Anti-aliasing has no effect - c++

Question

I want to draw smooth fill elipse with SDL2.
I have following code
#include <iostream>
#include <SDL2/SDL.h>
#include <SDL2/SDL2_gfxPrimitives.h>
#include <GL/gl.h>
void init()
{
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 2);
glEnable(GL_MULTISAMPLE);
}
int main()
{
SDL_Init(SDL_INIT_EVERYTHING);
init();
SDL_Window* window = SDL_CreateWindow("Anti-aliasing test",
SDL_WINDOWPOS_CENTERED,
SDL_WINDOWPOS_CENTERED,
100, 100,
SDL_WINDOW_RESIZABLE);
SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
int Buffers, Samples;
SDL_GL_GetAttribute( SDL_GL_MULTISAMPLEBUFFERS, &Buffers );
SDL_GL_GetAttribute( SDL_GL_MULTISAMPLESAMPLES, &Samples );
std::cout << "buf = " << Buffers << ", samples = " << Samples;
std::cout.flush();
SDL_Event e;
int x, y;
while(true)
{
SDL_WaitEvent(&e);
if (e.type == SDL_QUIT)
break;
SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
SDL_RenderClear(renderer);
SDL_GetWindowSize(window, &x, &y);
filledEllipseRGBA(renderer, x / 2, y / 2, 50, 50, 255, 0, 0, 255);
SDL_RenderPresent(renderer);
}
}
I expected to draw with anti-aliasing, but actually I have no effect from OpenGL.
We can see that my circle don't use alpha channel, which is necessary for AA. But console output is "buffer = 0, samples = 4".
Any idea about how can I activate(of fix) AA in SDL2 (with or without OpenGL).
Next screenshot demonstrate what I have and what I want. Bottom picture drawed in Pinta software.

Answer 1

You are trying to use OpenGL window hints for anti-aliasing but this only works when you create a window with a OpenGL context, which would disable all SDL renderering.
The SDL2/SDL2_gfxPrimitives.h library you are using to draw a filled ellipse has several anti-aliasing functions but not for a filled eclipse, a work around could be to draw a not filled anti-aliased ellipse with a filled eclipse inside everytime you draw an anti-aliased ellipse.
void aafilledEllipseRGBA(...) {
aaellipseRGBA(...);
filledEllipseRGBA(...);
}
or to write your own anti-aliased filled ellipse rendering code. You could base this on the code of the open-source library you are using by peeking at their code here.