SDL2 OpengGL Anti-aliasing has no effect

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 .