SDL_BlitSurface() blackscreen - c++

Question

I'm trying to put an imagine on surface.
So it's showed a black screen when I'm load BMP and display bmp
here's my code:
#define SDL_MAIN_HANDLED
#include <iostream>
#include "SDL/include/SDL2/SDL.h"
const int screenWidth=640;
const int screenHeight=480;
SDL_Window* window=NULL;
SDL_Surface* screenSurface=NULL;
SDL_Surface* gHelloWorld = NULL;
bool init(){
bool good = true;
if(SDL_Init(SDL_INIT_VIDEO)<0){
std::cout<<"ERROR: Failed to initialize SDL " << SDL_GetError()<<"\n";
good = false;
}else{
//Create Window
window =SDL_CreateWindow("SDL Tutorial",SDL_WINDOWPOS_UNDEFINED,SDL_WINDOWPOS_UNDEFINED,screenWidth,screenHeight,SDL_WINDOW_SHOWN);
if (window ==NULL){
std::cout<<"ERROR: Failed to create "<<SDL_GetError() <<"\n";
good = false;
}else{
//Get window surface
screenSurface = SDL_GetWindowSurface( window );
}
}
return good;
}
void close(){
SDL_FreeSurface(gHelloWorld);
gHelloWorld = NULL;
SDL_DestroyWindow(window);
window=NULL;
SDL_Quit();
}
bool loadMedia(){
bool ok = true;
gHelloWorld=SDL_LoadBMP("hello_world.bmp");
if(gHelloWorld==NULL){
std::cout<<"ERROR: Failed to load "<<SDL_GetError()<<"\n";
ok = false;
}else{
std::cout<<"Loaded successfully "<<'\n';
}
return ok;
}
int main(){
SDL_Window* window=NULL;
SDL_Surface* screenSurface=NULL;
bool Init = init();
if(Init){
bool loadMed = loadMedia();
if(loadMed){
SDL_Rect area ={30,30,250,250};
SDL_BlitSurface(gHelloWorld,&area,screenSurface,&area);
SDL_UpdateWindowSurface(window); //
SDL_Event e; bool quit = false; while( quit == false ){ while( SDL_PollEvent( &e ) ){ if( e.type == SDL_QUIT ) quit = true; } }
}
}
close();
return 0;
}
And I'm using Cmake to link lib (dynamic linking)
I tried to write SDL_Rect , and its not work ;/

Answer 1

Your problem is that you are nulling your sufaces in the main function.
Here is a version of your code without that mistake:
#include <iostream>
#include "SDL/include/SDL2/SDL.h"
const int screenWidth=640;
const int screenHeight=480;
SDL_Window* window=NULL;
SDL_Surface* screenSurface=NULL;
SDL_Surface* gHelloWorld = NULL;
bool init(){
bool good = true;
if(SDL_Init(SDL_INIT_VIDEO)<0){
std::cout<<"ERROR: Failed to initialize SDL " << SDL_GetError()<<"\n";
good = false;
}else{
//Create Window
window =SDL_CreateWindow("SDL Tutorial",SDL_WINDOWPOS_UNDEFINED,SDL_WINDOWPOS_UNDEFINED,screenWidth,screenHeight,SDL_WINDOW_SHOWN);
if (window ==NULL){
std::cout<<"ERROR: Failed to create "<<SDL_GetError() <<"\n";
good = false;
}else{
//Get window surface
screenSurface = SDL_GetWindowSurface( window );
}
}
return good;
}
void close(){
SDL_FreeSurface(gHelloWorld);
gHelloWorld = NULL;
SDL_DestroyWindow(window);
window=NULL;
SDL_Quit();
}
bool loadMedia(){
bool ok = true;
gHelloWorld=SDL_LoadBMP("/tmp/image.bmp");
if(gHelloWorld==NULL){
std::cout<<"ERROR: Failed to load "<<SDL_GetError()<<"\n";
ok = false;
}else{
std::cout<<"Loaded successfully "<<'\n';
}
return ok;
}
int main(){
// your error was nulling your surfaces.
// SDL_Window* window=NULL;
// SDL_Surface* screenSurface=NULL;
bool Init = init();
if(Init){
bool loadMed = loadMedia();
if(loadMed){
SDL_Rect area ={30,30,250,250};
SDL_BlitSurface(gHelloWorld,&area,screenSurface,&area);
SDL_UpdateWindowSurface(window); //
SDL_Event e; bool quit = false; while( quit == false ){ while( SDL_PollEvent( &e ) ){ if( e.type == SDL_QUIT ) quit = true; } }
}
}
close();
return 0;
}
Also, I recommend that you make your surface updates inside your while loop because somethimes SDL don't render the first surface and you obta a false window.
Here is a good resource for learning SDL: https://lazyfoo.net/tutorials/SDL/
and on my GitHub I have a tool to give you an SDL example: https://github.com/Monsterduty/mktest.