SDL producing black image

Question

I'm currently following a book on using SDL in image compression. There is a piece of example code that projuces an image on the screen, the code seems to read the image ok becaues it produces a window of the correct size but the image is black.

I have tried a few examples and all the images are black

i have attached the code below

#include <stdio.h>
#include <stdlib.h>
#include <SDL.h>
#include <string.h>

//A public class is the same as a 'struct'
class CImage {
public:
  unsigned char red;
  unsigned char green;
  unsigned char blue;
};


void ppm_read_comments ( FILE *fp )
{
  int c;
  while ( (c = getc ( fp ) )  == '#' ) {
    while (  getc( fp )  != '\n' )
    ;
  }
  ungetc ( c, fp );
}

class ppm_error
{
  public:
    ppm_error() {
      printf("\nIncorrect PPM format!\n");
      exit ( 1 );
    }
};


//change from (R, G, B) to (B, G, R)
void  ppm2sdl ( CImage *ibuf, int width, int height )
{
  unsigned char temp;

  for ( int i = 0; i < height; ++i ) {
    int row_offset = i * width;
    for ( int j = 0; j < width; ++j ){
      int offset =  row_offset + j;
      temp = ibuf[offset].red;
      ibuf[offset].red = ibuf[offset].blue;
      ibuf[offset].blue = temp;
    }
  }
}

int main( int argc, char* args[] )
{
  int ppmh[20], c;          //PPM header
  int width, height;            //image width and height
  SDL_Surface *screen;
  char filename[] = "C:/data/beach.ppm";

  FILE *input = fopen (filename, "rb"); //PPM file for testing read
  if ( !input ) {
    printf("\nError opening input file %s!\n", filename);
    return 1;
  }

  //read PPM input file
  ppm_read_comments ( input );      //read comments
  char temp[100];
  fscanf ( input, "%2s", temp );
  temp[3] = 0;
  if ( strncmp ( temp, "P6", 2 ) )
    throw ppm_error();
  ppm_read_comments ( input );
  fscanf ( input, "%d", &width );
  ppm_read_comments ( input );
  fscanf ( input, "%d", &height );
  ppm_read_comments ( input );
  int colorlevels;
  fscanf ( input, "%d", &colorlevels );
  printf("\n%s PPM file: ", temp );
  printf(" \n\twidth=%d\theight=%d\tcolorlevles=%d\n", width,height,colorlevels+1 );
  ppm_read_comments ( input );
  while ( ( c = getc ( input )) == '\n' );     //get rid of extra line returns
  ungetc ( c ,input );

  // May use CImage ibuf[width][height] if we do not use SDL_QUIT;
  CImage *ibuf = (CImage *) malloc ( width * height * 3 );
  fread ( ibuf,  3, width * height, input );   //read image data from file
  fclose ( input );

  //initialize video system
  if ( SDL_Init( SDL_INIT_VIDEO ) < 0 ) {
    fprintf(stderr, "Unable to init SDL: %s\n", SDL_GetError());
    exit(1);
  }
  //ensure SDL_Quit is called when the program exits
  atexit(SDL_Quit); //if not use this, we need to
                        //  do house cleaning manually when program ends

  //set video mode of width x height with 24-bit pixels
  screen = SDL_SetVideoMode( width, height, 24, SDL_SWSURFACE);
  if ( screen == NULL ) {
        fprintf(stderr, "Unable to set %dx%d video: %s\n", width, height,          SDL_GetError());
        exit(1);
  }

  //convert PPM format (R, G, B)  to SDL format (B, G, R)
      ppm2sdl ( ibuf,  width, height );

  screen->pixels = ibuf;  //point framebuffer to data buffer

    //  ibuf needs to be dynamically allocated if SDL_QUIT is used
    printf("update rect\n"   );
    SDL_UpdateRect ( screen, 0, 0, 0, 0 );  //blit data to screen

   // SDL_Flip(screen);
   // SDL_CreateRGBSurface(screen, 0, 0, 0, 0,0,0,0);

  SDL_Delay ( 4000 );     //delay 4 seconds before exit
  printf("Displaying PPM image %s successful!\n", filename );

  //do NOT free( ibuf ) if use SDL_QUIT which does the house cleaning
  return 0;
}

Answer 1

I haven't worked with SDL for some time, but I think you're not supposed to manually change the pixels data of your screen surface, i.e. you shouldn't change pixels of the SDL_Surface you get from SDL_SetVideoMode().

You can use SDL_CreateRGBSurfaceFrom() to create a surface from the pixel data you read from your ppm file, then blit it to the screen with SDL_BlitSurface().