SDL_CreateRGBSurfaceFrom / SDL_BlitSurface - I see old frames on my emulator - c++

Question

I'm working on a Space Invaders emulator and for display output I'm using SDL2.
The problem is that on the output window I see all the frames since emulation starts!
Basically the important piece of code is this:
Intel8080 mainObject; // My Intel 8080 CPU emulator
mainObject.loadROM();
//Main loop flag
bool quit = false;
//Event handler
SDL_Event e;
//While application is running
while (!quit)
{
//Handle events on queue
while (SDL_PollEvent(&e) != 0)
{
//User requests quit
if (e.type == SDL_QUIT)
{
quit = true;
}
}
if (mainObject.frameReady)
{
mainObject.frameReady = false;
gHelloWorld = SDL_CreateRGBSurfaceFrom(&mainObject.frameBuffer32, 256, 224, 32, 4 * 256, 0xff000000, 0x00ff0000, 0x0000ff00, 0x000000ff);
//Apply the image
SDL_BlitSurface(gHelloWorld, NULL, gScreenSurface, NULL);
//Update the surface
SDL_UpdateWindowSurface(gWindow);
}
mainObject.executeROM();
}
where Intel8080 is my CPU emulator code and mainObject.frameBuffer32 is the Space Invaders' video RAM that I converted from 1bpp to 32bpp in order to use SDL_CreateRGBSurfaceFrom function.
Emulation's working fine but I see all the frames generated since emulator starts!
I tried to change Alpha value in the 4 bytes of each RGBA pixel but nothing changes

Answer 1

This is happening because it looks like you're rendering the game without first clearing the window. Basically, you should fill the entire window with a color and then render on top of it constantly. The idea is that filling the window with a specific color before rendering over it is the equivalent of erasing the previous frame (most modern computers are powerful enough to handle this).
You might want to read-up on SDL's SDL_FillRect function, it'll allow you to fill the entire screen with a specific color.
Rendering pseudocode:
while(someCondition)
{
[...]
// Note, I'm not sure if "gScreenSurface" is the proper variable to use here.
// I got it from reading your code.
SDL_FillRect(gScreenSurface, NULL, SDL_MapRGB(gScreenSurface->format, 0, 0, 0));
SDL_BlitSurface(gHelloWorld, NULL, gScreenSurface, NULL);
SDL_UpdateWindowSurface(gWindow);
[...]
}