man SDL_ConvertSurface (3): Converts a surface to the same format as another surface.

SYNOPSIS

#include "SDL/SDL.h"

SDL_Surface *SDL_ConvertSurface(SDL_Surface *src, SDL_PixelFormat *fmt, Uint32 flags);

Creates a new surface of the specified format, and then copies and maps the given surface to it. If this function fails, it returns NULL.

The flags parameter is passed to SDL_CreateRGBSurface and has those semantics.

This function is used internally by SDL_DisplayFormat.

This function can only be called after SDL_Init.

Returns either a pointer to the new surface, or NULL on error.