Why is SDL2 rectangle and line drawing not pixel perfect?

Question

When using SDL2 (2.0.8), drawing a 2D rectangle or a line with SDL_RenderDrawRect or SDL_RenderDrawLine is not pixel perfect. There are some artifacts. Why is that? And is there a way to prevent that?

Code example:

#ifdef WIN32
#include "SDL.h"
#else
#include "SDL2/SDL.h"
#endif

int main(int argc, char* args[])
{
    SDL_Init(SDL_INIT_EVERYTHING);
    SDL_Window*   window = SDL_CreateWindow("test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 1000, 800, SDL_WINDOW_RESIZABLE | SDL_WINDOW_SHOWN);
    SDL_Renderer* renderer = SDL_CreateRenderer(window, 0,SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
    SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
    SDL_Event event;
    bool quit = false;
    while (!quit)
    {
        SDL_WaitEvent(&event);
        switch (event.type)
        {
            case SDL_QUIT:
            {
                quit = true;
                break;
            }
        }
        SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
        SDL_RenderClear(renderer);
        SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
        SDL_RenderDrawLine(renderer, 40, 10, 60, 10);
        SDL_Rect rect{10, 10, 20, 20};
        SDL_RenderDrawRect(renderer, &rect);
        SDL_RenderPresent(renderer);
    }
    return 0;
}

Environment: Acer Laptop, Intel HD Graphics, NVIDIA GEFORCE 940m, Windows 10

Answer 1

this dude is right. But SetProcessDpiAwareness is undefined in my case.
SetProcessDPIAware() work for me.

#ifdef _WIN32
#include <windows.h>
#endif

int main(int, char**)
{
#ifdef _WIN32
    SetProcessDPIAware();// Fix DPI settings
#endif
    //some code
}

I hope this code helps.

Answer 2

Take a look at this:

https://wiki.libsdl.org/SDL_HINT_RENDER_SCALE_QUALITY

Make sure to disable this and set it to 0. You may also wish to ensure anti-aliasting is disabled in your video drivers/NVIDIA Control Panel.

There are many upstream factors that may cause this.

Answer 3

In my Windows 10 display settings "Scale and Layout" was set to "125%". After Setting it to "100%" the rendering was pixel perfect.

Windows 10 display settings

Though this is just a workaraound. The real solution is to call the Windows API function "SetProcessDpiAwareness". I found the answer and code to do that on different Windows versions in this link:

SetProcessDpiAwareness

Setting "Scale and Layout" back to "125%" and calling that function at the begin of my application gives the expected result.