Question

Hi there I have a image with a black rectangle drawn on it, and its background is transparent. This file is saved as a png (clear.png). Then I have another image which is just a solid red background saved as a jpeg (background.jpeg). What I was trying to do is make so that the black rectangle in clear.png show up on top of the solid red background image.

This is what I have done..

/*Transparent image*/
#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include <iostream>
using namespace std;
int main(int argc,char *argv[]){
    SDL_Surface *screen = NULL;
    SDL_Surface *background = NULL;
    SDL_Surface *transparentimage = NULL;

    if ( SDL_Init(SDL_INIT_EVERYTHING) == -1){
        cout <<"could not start sdl" << endl;
    }

    screen = SDL_SetVideoMode(640,480,32,SDL_SWSURFACE);
    if ( screen == NULL){
        cout<<"could not create the screen" << endl;
    }

    background = IMG_Load("background.jpeg");
    if ( background == NULL){
        cout<<"could not load background" << endl;
    }

    transparentimage = IMG_Load("clear.png");
    if ( transparentimage == NULL){
        cout<< "could not load transparentimage" << endl;
    }

    if ( SDL_BlitSurface(background,NULL,screen,NULL) == -1 ){
        cout<<"Couldnt do background blitting " << endl;
    }
    if (SDL_BlitSurface(transparentimage,NULL,background,NULL) == -1 ){
        cout<<"could not do clear image blitting "<< endl;
    }

    SDL_Flip(screen);
    SDL_Delay(5000);

    SDL_FreeSurface(background);
    SDL_FreeSurface(transparentimage);

    SDL_Quit();

    return 0;
}

The above doesn’t work and it just shows me a screen with a red background and a black footer at the bottom of the screen (this isn’t my rectangle :)). What have I done wrong here?. Also the size of the images are identical (640×480).

Answer 1

Make sure to init SDL_image and blit both bitmaps to the screen:

/*Transparent image*/
#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include <iostream>
using namespace std;
int main(int argc,char *argv[]){
    SDL_Surface *screen = NULL;
    SDL_Surface *background = NULL;
    SDL_Surface *transparentimage = NULL;

    if ( SDL_Init(SDL_INIT_EVERYTHING) == -1){
        cout <<"could not start sdl" << endl;
    }

    screen = SDL_SetVideoMode(640,480,32,SDL_SWSURFACE);
    if ( screen == NULL){
        cout<<"could not create the screen" << endl;
    }

    int flags = IMG_INIT_JPG | IMG_INIT_PNG;
    int initted=IMG_Init(flags);
    if( initted & flags != flags) {
        cout<<"could not init SDL_Image" << endl;
        cout<<"Reason: " << IMG_GetError() << endl;
    }

    background = IMG_Load("red.jpg");
    if ( background == NULL){
        cout<<"could not load background" << endl;
    }

    transparentimage = IMG_Load("green.png");
    if ( transparentimage == NULL){
        cout<< "could not load transparentimage" << endl;
    }

    if ( SDL_BlitSurface(background,NULL,screen,NULL) == -1 ){
        cout<<"Couldnt do background blitting " << endl;
    }
    if (SDL_BlitSurface(transparentimage,NULL,screen,NULL) == -1 ){
        cout<<"could not do clear image blitting "<< endl;
    }

    SDL_Flip(screen);
    SDL_Delay(5000);

    SDL_FreeSurface(background);
    SDL_FreeSurface(transparentimage);

    SDL_Quit();

    return 0;
}