How to plot graphics and images . png in the same window with openGL and SDL

Question

I have done some search, in manuals and such on google however I would like to know how to adapt my code below to plot a chart (line for example) along with a file image .png. I do not know where I am missing below:

#include </usr/include/SDL/SDL.h> //compilar com gcc -o exSDLopengl exSDLopengl.c -lSDL -lGL -lSDL_image
#include </usr/include/GL/gl.h>
#include </usr/include/SDL/SDL_image.h>
#include <stdio.h>
#include <stdlib.h>

    const int SCREEN_WIDTH = 640;
    const int SCREEN_HEIGHT = 480;
    const int SCREEN_BPP = 32;

    int tex;

 
    void init(){
        glClearColor(0.0,0.0,0.0,0.0);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        glOrtho(0.0,800,600,1.0,-1.0,1.0);
        glEnable(GL_BLEND);
        glEnable(GL_TEXTURE_2D);
        glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
        
    }
  
    void draw2() {
      
      //init();
      glDisable(GL_TEXTURE_2D); // isto é necessário quando se deseja desenhar SEM texturas
      glColor3f(1,1,0);
      glLineWidth(3);
      glBegin(GL_LINES);
        glVertex2i(0,0);
        glVertex2i(100,100);
      glEnd();
      glColor3f(1,1,1);
    
    
     }
     
     void mostra_imagem() {
     
              SDL_Surface* screen1;
              SDL_Surface* imagemPNG;
              SDL_Rect dest;
              
                            
              
              screen1=SDL_SetVideoMode(800,600,16,SDL_SWSURFACE | SDL_OPENGL);
              
              
              
     
              imagemPNG = IMG_Load("mar.png");
              
              dest.x=10;
              dest.y=10;  
              
              SDL_FillRect(screen1, NULL, 0x3); // Pinta de preto todo o screen
              SDL_BlitSurface(imagemPNG, NULL, screen1, &dest); // mostra imagem do mar
              SDL_UpdateRect(screen1,0,0,0,0); // Atualiza o screen com a imagem blitada
              SDL_Delay(1000);  
        } 
    
    int main(int argc,char** argv){
       
        SDL_Init(SDL_INIT_VIDEO);
        //SDL_Surface* screen=SDL_SetVideoMode(800,600,32,SDL_SWSURFACE|SDL_OPENGL);
        
               
         int running=1;
        Uint32 start;
        SDL_Event event;
        //init();
        mostra_imagem();
        draw2();
        
    
        SDL_Quit();
        return 0;
    }

Answer 1

When Opengl is activated we have to move the image to a texture. Then we need to call the Sdl_gl_swapbuffers() function to update the screen.

#include </usr/include/SDL/SDL.h> //compilar com gcc -o exSDLopengl exSDLopengl.c -lSDL -lGL -lSDL_image
#include </usr/include/GL/gl.h>
#include </usr/include/SDL/SDL_image.h>
#include <stdio.h>
#include <stdlib.h>

const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;
const int SCREEN_BPP = 32;

int tex;

void init(){
    glClearColor(0.0,0.0,0.0,0.0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0.0,640,480,1.0,-1.0,1.0);
    glViewport(0, 0, 640, 480);
    glEnable(GL_BLEND);
    glEnable(GL_TEXTURE_2D);
    glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
    
}

void draw2() {
    glDisable(GL_TEXTURE_2D); // isto é necessário quando se deseja desenhar SEM texturas
    glColor3f(1,1,0);
    glLineWidth(3);
    glBegin(GL_LINES);
    glVertex2i(0,0);
    glVertex2i(100,100);
    glEnd();
    glColor3f(1,1,1);
}

void mostra_imagem(SDL_Surface* screen, SDL_Surface* image) {
    GLuint texture = 0;
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, image->w, image->h, 0, GL_RGB, GL_UNSIGNED_BYTE, image->pixels);
    
    glColor3f(1.0f, 1.0f, 1.0f);

    glBegin(GL_QUADS);
    glTexCoord2f(0.0f, 0.0f);
    glVertex2f(0.0f, 0.0f);
    glTexCoord2f(1.0f, 0.0f);
    glVertex2f(image->w, 0.0f);
    glTexCoord2f(1.0f, 1.0f);
    glVertex2f(image->w, image->h);
    glTexCoord2f(0.0f, 1.0f);
    glVertex2f(0.0f, image->h);
    glEnd();
}

int main(int argc,char** argv){
    
    SDL_Init(SDL_INIT_VIDEO);
    int running=1;
    Uint32 start;
    SDL_Event event;
    SDL_Surface* screen1;
    screen1 = SDL_SetVideoMode(800,600,16,SDL_SWSURFACE | SDL_OPENGL);
    SDL_Surface* imagemPNG;
    imagemPNG = IMG_Load("mar.png");
    if(!imagemPNG) {
        printf("IMG_Load: %s\n", IMG_GetError());
    }

    init();
    mostra_imagem(screen1, imagemPNG);
    draw2();
    SDL_GL_SwapBuffers();

    while ( SDL_WaitEvent(&event) >= 0 ) {
        switch (event.type) {
            case SDL_QUIT: {
                printf("Quit.\n");
                exit(0);
            }
            break;
        }
    }

    SDL_Quit();
    return 0;
}