Need help in reading JPEG file using libjpeg

Question

I followed the example code in the libjpeg example file, however I was not able to read the image data.

I have the following struct, and I created an instance of this struct.

 struct ImageData {
        unsigned char *pixels;
        long  width;
        long height;
    };

    ImageData *imageData;

Below is my read_JPEG_file function:

int read_JPEG_file (char * filename)
{
    struct jpeg_decompress_struct cinfo;
    struct my_error_mgr jerr;

    /* More stuff */
    FILE * infile;      /* source file */
    JSAMPARRAY buffer;      /* Output row buffer */
    int row_stride;     /* physical row width in output buffer */

    if ((infile = fopen(filename, "rb")) == NULL) {
        fprintf(stderr, "can't open %s\n", filename);
        return 0;
    }

    /* Step 1: allocate and initialize JPEG decompression object */

    /* We set up the normal JPEG error routines, then override error_exit. */
    cinfo.err = jpeg_std_error(&jerr.pub);
    jerr.pub.error_exit = my_error_exit;
    /* Establish the setjmp return context for my_error_exit to use. */
    if (setjmp(jerr.setjmp_buffer)) {

        jpeg_destroy_decompress(&cinfo);
        fclose(infile);
        return 0;
    }
    /* Now we can initialize the JPEG decompression object. */
    jpeg_create_decompress(&cinfo);

    /* Step 2: specify data source (eg, a file) */

    jpeg_stdio_src(&cinfo, infile);

    /* Step 3: read file parameters with jpeg_read_header() */

    (void) jpeg_read_header(&cinfo, TRUE);
    /* Step 4: set parameters for decompression */

    /* In this example, we don't need to change any of the defaults set by
     * jpeg_read_header(), so we do nothing here.
     */

    /* Step 5: Start decompressor */

    (void) jpeg_start_decompress(&cinfo);


    row_stride = cinfo.output_width * cinfo.output_components;
    /* Make a one-row-high sample array that will go away when done with image */
    buffer = (*cinfo.mem->alloc_sarray) ((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);

    imageData = new ImageData;
    imageData->width = cinfo.output_width;
    imageData->height = cinfo.output_height;

    imageData->pixels = new unsigned char [cinfo.output_width * cinfo.output_height * cinfo.output_components];
    long counter = 0;

   //step 6, read the image line by line
    while (cinfo.output_scanline < cinfo.output_height) {
        //IT ALWAYS crash ON THIS JPEG_READ_SCANLINES FUNCTION CALL BELOW
        (void) jpeg_read_scanlines(&cinfo, (JSAMPARRAY)(imageData->pixels), 1);
        counter +=row_stride;

    }
       /* Step 7: Finish decompression */

    (void) jpeg_finish_decompress(&cinfo);
    /* Step 8: Release JPEG decompression object */

    /* This is an important step since it will release a good deal of memory. */
    jpeg_destroy_decompress(&cinfo);

    fclose(infile);
    /* And we're done! */
    return 1;
}

It always fails on this JPEG_READ_SCANLINES function, in the step 6 above. I got an "EXC_BAD_ACCESS" signal on that line.

Does anyone have any idea, or have some working examples on reading .jpg file with libjpeg that you can share here? I have checked the size of my imageData->pixels, and compared it with the size of the jpeg file itself, and it has the same size. The memory for this variable has also dynamically allocated, so I know that it was not a memory problem.

Any ideas?

Answer 1

Like dacap said, it is expecting a JSAMPARRAY. That being said, you can write straight to your imageData->pixels array if you would like. You just need to do something like this:

// Allocate imageData->pixels to be the correct size, start decompress and all
// that jazz, like you did in your code. Skip allocating buffer though.
// ...

JSAMPROW output_data;
unsigned int scanline_len = cinfo.output_width * cinfo.output_components;

unsigned int scanline_count = 0;
while (cinfo.output_scanline < cinfo.output_height)
{
    output_data = (imageData->pixels + (scanline_count * scanline_len));
    jpeg_read_scanlines(&cinfo, &output_data, 1);
    scanline_count++;
}

You can then skip allocating buffer altogether. Using memcpy works fine, but why do the extra copy if you don't have to?

Answer 2

Here's a sample for reading a jpeg image:

/***************************************************
    To read a jpg image file and download
    it as a texture map for openGL
    Derived from Tom Lane's example.c
    -- Obtain & install jpeg stuff from web 
    (jpeglib.h, jerror.h jmore.h, jconfig.h,jpeg.lib)
****************************************************/
#include <jpeglib.h>    
#include <jerror.h>
//================================
GLuint LoadJPEG(char* FileName)
//================================
{
  unsigned long x, y;
  unsigned int texture_id;
  unsigned long data_size;     // length of the file
  int channels;               //  3 =>RGB   4 =>RGBA 
  unsigned int type;  
  unsigned char * rowptr[1];    // pointer to an array
  unsigned char * jdata;        // data for the image
  struct jpeg_decompress_struct info; //for our jpeg info
  struct jpeg_error_mgr err;          //the error handler

  FILE* file = fopen(FileName, "rb");  //open the file

  info.err = jpeg_std_error(& err);     
  jpeg_create_decompress(& info);   //fills info structure

  //if the jpeg file doesn't load
  if(!file) {
     fprintf(stderr, "Error reading JPEG file %s!", FileName);
     return 0;
  }

  jpeg_stdio_src(&info, file);    
  jpeg_read_header(&info, TRUE);   // read jpeg file header

  jpeg_start_decompress(&info);    // decompress the file

  //set width and height
  x = info.output_width;
  y = info.output_height;
  channels = info.num_components;
  type = GL_RGB;
  if(channels == 4) type = GL_RGBA;

  data_size = x * y * 3;

  //--------------------------------------------
  // read scanlines one at a time & put bytes 
  //    in jdata[] array. Assumes an RGB image
  //--------------------------------------------
  jdata = (unsigned char *)malloc(data_size);
  while (info.output_scanline < info.output_height) // loop
  {
    // Enable jpeg_read_scanlines() to fill our jdata array
    rowptr[0] = (unsigned char *)jdata +  // secret to method
            3* info.output_width * info.output_scanline; 

    jpeg_read_scanlines(&info, rowptr, 1);
  }
  //---------------------------------------------------

  jpeg_finish_decompress(&info);   //finish decompressing

  //----- create OpenGL tex map (omit if not needed) --------
  glGenTextures(1,&texture_id);
  glBindTexture(GL_TEXTURE_2D, texture_id);
  gluBuild2DMipmaps(GL_TEXTURE_2D,3,x,y,GL_RGB,GL_UNSIGNED_BYTE,jdata);

  jpeg_destroy_decompress(&info);
  fclose(file);                    //close the file
  free(jdata);

  return texture_id;    // for OpenGL tex maps
}

Answer 3

jpeg_read_scanlines function receives an array of pointers (not the direct pointer of pixels as imageData->pixels). So we should create a JSAMPARRAY first:

int buffer_height = 1;
JSAMPARRAY buffer = (JSAMPARRAY)malloc(sizeof(JSAMPROW) * buffer_height);
buffer[0] = (JSAMPROW)malloc(sizeof(JSAMPLE) * row_stride);

In your code you've created a "buffer" with "cinfo.mem->alloc_sarray" but you never use it. The final step is to pass the "buffer" as argument of jpeg_read_scanlines:

while (cinfo.output_scanline < cinfo.output_height) {
  jpeg_read_scanlines(&cinfo, buffer, 1);
  memcpy(imageData->pixels+counter, buffer[0], row_stride);
  counter += row_stride;
}

See that we're using "imageData->pixels+counter", not just "imageData->pixels" as in your code. In this way we write each row after another in the whole "imageData->pixels" memory chunk.