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When using the SDL_SetRenderDrawColor() function and all 3 colors and the alpha channel is set to zero, following SDL_RenderDrawLine and SDL_RenderDrawRect functions will render in white instead of the expected black.

A workaround for this, is setting any of the 4 values to a different value.

However a SDL_RenderClear() after SDL_SetRenderDrawColor(r,0,0,0,0) works as expected, and clears with black.

Following example code demonstrates the issue:


// compile with: cc colorbug.c `pkg-config --libs --cflags sdl2` -o colorbug
#include <SDL.h>

int main(){
    SDL_Window* win;
    SDL_Renderer* ren;
    SDL_Event eve;
    SDL_Rect rec = {.x = 25, .y = 25, .w = 50, .h = 50};

    if (SDL_Init(SDL_INIT_VIDEO) < 0) return -1;
    if (SDL_CreateWindowAndRenderer(100, 100, SDL_WINDOW_RESIZABLE, &win, &ren)) return -1;
    SDL_SetRenderDrawBlendMode(ren, SDL_BLENDMODE_NONE);
        
    for(;;){
        for (;SDL_PollEvent(&eve);)
            if (SDL_QUIT == eve.type) return 0;
        // setting background color (works as expected)
        SDL_SetRenderDrawColor(ren, 0x80, 0x80, 0x80, 0x00); 
        SDL_RenderClear(ren);
        // setting color for rectangle (should be black, but is white)
        SDL_SetRenderDrawColor(ren, 0x00, 0x00, 0x00, 0x00); 
        SDL_RenderDrawLine(ren, 25, 25, 75, 75);
        SDL_RenderDrawRect(ren, &rec);
        SDL_RenderPresent(ren);
        SDL_Delay(20);
    }
}