Extract RGB from JPEG image, using libjpeg C++ Library

Question

I am using the libjpeg library to read and copy a jpeg into an editing program that I am writing in C++

I have a display buffer which is a vector of a datatype called ColorData

All ColorData consists of is 3 floats (RGB)

Here is my code that opens the jpeg files

PixelBuffer * IOManager::load_jpg_to_pixel_buffer(const char *file_name){
  struct jpeg_decompress_struct cinfo;

  FILE * infile;

  JSAMPARRAY buffer;

  if ((infile = fopen(file_name, "rb")) == NULL) {
    std::cout << "Could not open the jpg file: " << file_name << std::endl;
    return nullptr;
  }

  struct jpeg_error_mgr jerr; 

  cinfo.err = jpeg_std_error(&jerr);

  jpeg_create_decompress(&cinfo);

  jpeg_stdio_src(&cinfo, infile);
  jpeg_read_header(&cinfo, TRUE);
  jpeg_start_decompress(&cinfo);

  int width = static_cast<int>(cinfo.output_width);

  int height = static_cast<int>(cinfo.output_height);

  std::cout << typeid(cinfo.colormap).name() << std::endl;

  std::cout << "Width: " << width << "Height: " << height << std::endl;

  PixelBuffer * image_buffer = new PixelBuffer(width, height, ColorData());

  std::cout << cinfo.output_components << std::endl;

   buffer = (*cinfo.mem->alloc_sarray)
    ((j_common_ptr) &cinfo, JPOOL_IMAGE, cinfo.output_width * cinfo.output_components, 1);

  /* Step 6: while (scan lines remain to be read) */
  /*           jpeg_read_scanlines(...); */

  /* Here we use the library's state variable cinfo.output_scanline as the
   * loop counter, so that we don't have to keep track ourselves.
   */
  while (cinfo.output_scanline < cinfo.output_height) {
    /* jpeg_read_scanlines expects an array of pointers to scanlines.
     * Here the array is only one element long, but you could ask for
     * more than one scanline at a time if that's more convenient.
     */
    (void) jpeg_read_scanlines(&cinfo, buffer, 1);
    /* Assume put_scanline_someplace wants a pointer and sample count. */

  }

  return nullptr;


}

How can I get the RGB value from the jpeg using the libjpeg?

JPEG files are YCbCr encoded, possibly with chroma subsampling. You'll have to either write the necessary color conversion code, or use a library that can do it for you. — Cornstalks
@Cornstalks, libjpeg has ready-made conversions built-in, you just need to specify proper 'out_color_space' — Dmitry

samgak samgak · Accepted Answer · 2016-11-05T06:07:56

The RGB values are in buffer. It's actually an array of arrays, so you have to index buffer[0].

Something like this:

while (cinfo.output_scanline < cinfo.output_height)
{
    (void) jpeg_read_scanlines(&cinfo, buffer, 1);

    // get the pointer to the row:
    unsigned char* pixel_row = (unsigned char*)(buffer[0]);
    // iterate over the pixels:
    for(int i = 0; i < cinfo.output_width; i++)
    {
        // convert the RGB values to a float in the range 0 - 1
        float red = (float)(*pixel_row++) / 255.0f;
        float green = (float)(*pixel_row++) / 255.0f;
        float blue = (float)(*pixel_row++) / 255.0f;
    }
}

This is assuming cinfo.output_components is 3.

Extract RGB from JPEG image, using libjpeg C++ Library

1 Answers