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I've noticed some weird behaviour on the Raspberry Pi. Take a look at the following little example code:

#include <SDL.h>

int main(int argc, char *argv[])
{
        SDL_Window *window;     
        SDL_Renderer *renderer;
        SDL_Texture *texture1, *texture2, *texture3;
        SDL_Event se;
        int quit = 0, k;
        int w = 295, reverse = 0;
        char *pixbuf = malloc(640 * 480 * 4);
        unsigned int *ptr;

        SDL_Init(SDL_INIT_VIDEO);
        SDL_VideoInit("RPI");

        window = SDL_CreateWindow("Foo", 0, 0, 640, 480, 0);
        renderer = SDL_CreateRenderer(window, -1, 0);
        texture1 = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_ARGB8888, SDL_TEXTUREACCESS_STREAMING, 640, 480);
        texture2 = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_ARGB8888, SDL_TEXTUREACCESS_STREAMING, 640, 480);

        memset(pixbuf, 0, 640 * 480 * 4);
        SDL_UpdateTexture(texture1, NULL, pixbuf, 640 * 4);     
        SDL_RenderCopy(renderer, texture1, NULL, NULL);
        SDL_RenderPresent(renderer);
        
        ptr = (unsigned int *) pixbuf;
        for(k = 0; k < 640 * 480; k++) *ptr++ = 0xffff0000;
        SDL_UpdateTexture(texture2, NULL, pixbuf, 640 * 4);     
        
        SDL_ShowWindow(window);
        SDL_RaiseWindow(window);

        while(!quit) {

                SDL_RenderCopy(renderer, texture1, NULL, NULL);

                texture3 = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_ARGB8888, SDL_TEXTUREACCESS_TARGET, w, w);
                SDL_SetRenderTarget(renderer, texture3);
                SDL_SetTextureBlendMode(texture2, SDL_BLENDMODE_NONE);  
                SDL_RenderCopy(renderer, texture2, NULL, NULL);
                SDL_SetTextureBlendMode(texture3, SDL_BLENDMODE_BLEND); 
                SDL_SetRenderTarget(renderer, NULL);
                SDL_RenderCopy(renderer, texture3, NULL, NULL);

                SDL_RenderPresent(renderer);
                
                while(SDL_PollEvent(&se)) {
                        if(se.type == SDL_MOUSEBUTTONDOWN) quit = 1;
                }

                SDL_DestroyTexture(texture3);

                if(reverse) {
                        w -= 10;        
                } else {
                        w += 10;
                }

                if(w >= 640) {
                        w = 640;
                        reverse = 1;
                } else if(w <= 2) {
                        w = 2;
                        reverse = 0;    
                }
        }       
        
        SDL_DestroyTexture(texture1);
        SDL_DestroyTexture(texture2);
        SDL_DestroyRenderer(renderer);
        SDL_DestroyWindow(window);

        SDL_Quit();
        
        free(pixbuf);

        return 0;
} 

On Windows, Linux & Mac this always fills the screen with red. On the Raspberry Pi, however, the screen alternates between red and black. It is mostly red, but sometimes it's also black, so it looks really weird. 

Tests have shown that the issue is related to scaling. It works correctly when disabling scaling, i.e. changing the following line

        texture3 = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_ARGB8888, SDL_TEXTUREACCESS_TARGET, w, w);

into this line

        texture3 = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_ARGB8888, SDL_TEXTUREACCESS_TARGET, 640, 480);

solves the problem and there is no alternation between red and black screens any more. The problem only seems to appear when the dimensions of texture3 and texture2 don't match, i.e. when SDL_RenderCopy() needs to scale-copy texture2 to texture3 instead of just copying it without the involvement of scaling.