example.c

/* example.c - an example of using libpng */

/* this is an example of how to use libpng to read and write
   png files.  The file libpng.txt is much more verbose then
   this.  If you have not read it, do so first.  This was
   designed to be a starting point of an implementation.
   This is not officially part of libpng, and therefore
   does not require a copyright notice.

   This file does not currently compile, because it is missing
   certain parts, like allocating memory to hold an image.
   You will have to supply these parts to get it to compile.
   */

#include <png.h>

/* check to see if a file is a png file using png_check_sig() */
int check_png(char * file_name)
{
   FILE *fp;
   char buf[8];
   int ret;

   fp = fopen(file_name, "rb");
   if (!fp)
      return 0;
   ret = fread(buf, 1, 8, fp);
   fclose(fp);

   if (ret != 8)
      return 0;

   ret = png_check_sig(buf, 8);

   return (ret);
}

/* read a png file.  You may want to return an error code if the read
   fails (depending upon the failure). */
void read_png(char *file_name)
{
   FILE *fp;
   png_structp png_ptr;
   png_infop info_ptr;

   /* open the file */
   fp = fopen(file_name, "rb");
   if (!fp)
      return;

   /* allocate the necessary structures */
   png_ptr = malloc(sizeof (png_struct));
   if (!png_ptr)
   {
      fclose(fp);
      return;
   }

   info_ptr = malloc(sizeof (png_info));
   if (!info_ptr)
   {
      fclose(fp);
      free(png_ptr);
      return;
   }

   /* set error handling */
   if (setjmp(png_ptr->jmpbuf))
   {
      png_read_destroy(png_ptr, info_ptr, (png_info *)0);
      fclose(fp);
      free(png_ptr);
      free(info_ptr);
      /* If we get here, we had a problem reading the file */
      return;
   }

   /* initialize the structures, info first for error handling */
   png_info_init(info_ptr);
   png_read_init(png_ptr);

   /* set up the input control if you are using standard C streams */
   png_init_io(png_ptr, fp);

   /* if you are using replacement read functions, here you would call */
   png_set_read_fn(png_ptr, (void *)io_ptr, user_read_fn);
   /* where io_ptr is a structure you want available to the callbacks */

   /* if you are using replacement message functions, here you would call */
   png_set_message_fn(png_ptr, (void *)msg_ptr, user_error_fn, user_warning_fn);
   /* where msg_ptr is a structure you want available to the callbacks */

   /* read the file information */
   png_read_info(png_ptr, info_ptr);

   /* set up the transformations you want.  Note that these are
      all optional.  Only call them if you want them */

   /* expand paletted colors into true rgb */
   if (info_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
      png_set_expand(png_ptr);

   /* expand grayscale images to the full 8 bits */
   if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY &&
      info_ptr->bit_depth < 8)
      png_set_expand(png_ptr);

   /* expand images with transparency to full alpha channels */
   if (info_ptr->valid & PNG_INFO_tRNS)
      png_set_expand(png_ptr);

   /* Set the background color to draw transparent and alpha
      images over */
   png_color_16 my_background;

   if (info_ptr->valid & PNG_INFO_bKGD)
      png_set_background(png_ptr, &(info_ptr->background),
         PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
   else
      png_set_background(png_ptr, &my_background,
         PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);

   /* tell libpng to handle the gamma conversion for you */
   if (info_ptr->valid & PNG_INFO_gAMA)
      png_set_gamma(png_ptr, screen_gamma, info_ptr->gamma);
   else
      png_set_gamma(png_ptr, screen_gamma, 0.45);

   /* tell libpng to strip 16 bit depth files down to 8 bits */
   if (info_ptr->bit_depth == 16)
      png_set_strip_16(png_ptr);

   /* dither rgb files down to 8 bit palettes & reduce palettes
      to the number of colors available on your screen */
   if (info_ptr->color_type & PNG_COLOR_MASK_COLOR)
   {
      if (info_ptr->valid & PNG_INFO_PLTE)
         png_set_dither(png_ptr, info_ptr->palette,
            info_ptr->num_palette, max_screen_colors,
               info_ptr->histogram);
      else
      {
         png_color std_color_cube[MAX_SCREEN_COLORS] =
            {/* ... colors ... */};

         png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
            MAX_SCREEN_COLORS, NULL);
      }
   }

   /* invert monocrome files */
   if (info_ptr->bit_depth == 1 &&
      info_ptr->color_type == PNG_COLOR_GRAY)
      png_set_invert(png_ptr);

   /* shift the pixels down to their true bit depth */
   if (info_ptr->valid & PNG_INFO_sBIT &&
      info_ptr->bit_depth > info_ptr->sig_bit)
      png_set_shift(png_ptr, &(info_ptr->sig_bit));

   /* pack pixels into bytes */
   if (info_ptr->bit_depth < 8)
      png_set_packing(png_ptr);

   /* flip the rgb pixels to bgr */
   if (info_ptr->color_type == PNG_COLOR_TYPE_RGB ||
      info_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
      png_set_bgr(png_ptr);

   /* swap bytes of 16 bit files to least significant bit first */
   if (info_ptr->bit_depth == 16)
      png_set_swap(png_ptr);

   /* add a filler byte to rgb files */
   if (info_ptr->bit_depth == 8 &&
      info_ptr->color_type == PNG_COLOR_TYPE_RGB)
      png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);

   /* turn on interlace handling if you are not using png_read_image() */
   if (info_ptr->interlace_type)
      number_passes = png_set_interlace_handling(png_ptr);
   else
      number_passes = 1;

   /* optional call to update palette with transformations */
   png_start_read_image(png_ptr);

   /* optional call to update the info structure */
   png_read_update_info(png_ptr, info_ptr);

   /* allocate the memory to hold the image using the fields
      of png_info. */

   /* the easiest way to read the image */
   png_bytep row_pointers[height];
   png_read_image(png_ptr, row_pointers);

   /* the other way to read images - deal with interlacing */

   for (pass = 0; pass < number_passes; pass++)
   {
      /* Read the image using the "sparkle" effect. */
      png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);

      /* If you are only reading on row at a time, this works */
      for (y = 0; y < height; y++)
      {
         png_bytep row_pointers = row[y];
         png_read_rows(png_ptr, &row_pointers, NULL, 1);
      }

      /* to get the rectangle effect, use the third parameter */
      png_read_rows(png_ptr, NULL, row_pointers, number_of_rows);

      /* if you want to display the image after every pass, do
         so here */
   }

   /* read the rest of the file, getting any additional chunks
      in info_ptr */
   png_read_end(png_ptr, info_ptr);

   /* clean up after the read, and free any memory allocated */
   png_read_destroy(png_ptr, info_ptr, (png_infop)0);

   /* free the structures */
   free(png_ptr);
   free(info_ptr);

   /* close the file */
   fclose(fp);

   /* that's it */
   return;
}

/* progressively read a file */

/* these will normally not be global unless you are only
   reading in one image at a time */
png_structp png_ptr;
png_infop info_ptr;

int
initialize_png_reader()
{
   png_ptr = malloc(sizeof (png_struct));
   if (!png_ptr)
      return -1;
   info_ptr = malloc(sizeof (png_info));
   if (!info_ptr)
   {
      free(png_ptr);
      return -1;
   }

   if (setjmp(png_ptr->jmpbuf))
   {
      png_read_destroy(png_ptr, info_ptr, (png_info *)0);
      /* free pointers before returning, if necessary */
      free(png_ptr);
      free(info_ptr);
      return -1;
   }

   png_info_init(info_ptr);
   png_read_init(png_ptr);

   /* this one's new.  You will need to provide all three
      function callbacks, even if you aren't using them all.
      You can put a void pointer in place of the NULL, and
      retrieve the pointer from inside the callbacks using
      the function png_get_progressive_ptr(png_ptr); */
   png_set_progressive_read_fn(png_ptr, NULL,
      info_callback, row_callback, end_callback);

   return 0;
}

int
process_data(png_bytep buffer, png_uint_32 length)
{
   if (setjmp(png_ptr->jmpbuf))
   {
      png_read_destroy(png_ptr, info_ptr, (png_info *)0);
      free(png_ptr);
      free(info_ptr);
      return -1;
   }

   /* this one's new also.  Simply give it a chunk of data
      from the file stream (in order, of course).  On Segmented
      machines, don't give it any more then 64K.  The library
      seems to run fine with sizes of 4K, although you can give
      it much less if necessary (I assume you can give it chunks
      of 1 byte, but I haven't tried less then 256 bytes yet).
      When this function returns, you may want to display any
      rows that were generated in the row callback. */
   png_process_data(png_ptr, info_ptr, buffer, length);
   return 0;
}

info_callback(png_structp png_ptr, png_infop info)
{
/* do any setup here, including setting any of the transformations
   mentioned in the Reading PNG files section.  For now, you _must_
   call either png_start_read_image() or png_read_update_info()
   after all the transformations are set (even if you don't set
   any).  You may start getting rows before png_process_data()
   returns, so this is your last chance to prepare for that. */
}

row_callback(png_structp png_ptr, png_bytep new_row,
   png_uint_32 row_num, int pass)
{
/* this function is called for every row in the image.  If the
   image is interlacing, and you turned on the interlace handler,
   this function will be called for every row in every pass.
   Some of these rows will not be changed from the previous pass.
   When the row is not changed, the new_row variable will be NULL.
   The rows and passes are called in order, so you don't really
   need the row_num and pass, but I'm supplying them because it
   may make your life easier.

   For the non-NULL rows of interlaced images, you must call
   png_progressive_combine_row() passing in the row and the
   old row.  You can call this function for NULL rows (it will
   just return) and for non-interlaced images (it just does the
   memcpy for you) if it will make the code easier.  Thus, you
   can just do this for all cases: */

   png_progressive_combine_row(png_ptr, old_row, new_row);

/* where old_row is what was displayed for previous rows.  Note
   that the first pass (pass == 0 really) will completely cover
   the old row, so the rows do not have to be initialized.  After
   the first pass (and only for interlaced images), you will have
   to pass the current row, and the function will combine the
   old row and the new row. */
}

end_callback(png_structp png_ptr, png_infop info)
{
/* this function is called when the whole image has been read,
   including any chunks after the image (up to and including
   the IEND).  You will usually have the same info chunk as you
   had in the header, although some data may have been added
   to the comments and time fields.

   Most people won't do much here, perhaps setting a flag that
   marks the image as finished. */
}

/* write a png file */
void write_png(char *file_name, ... other image information ...)
{
   FILE *fp;
   png_structp png_ptr;
   png_infop info_ptr;

   /* open the file */
   fp = fopen(file_name, "wb");
   if (!fp)
      return;

   /* allocate the necessary structures */
   png_ptr = malloc(sizeof (png_struct));
   if (!png_ptr)
   {
      fclose(fp);
      return;
   }

   info_ptr = malloc(sizeof (png_info));
   if (!info_ptr)
   {
      fclose(fp);
      free(png_ptr);
      return;
   }

   /* set error handling */
   if (setjmp(png_ptr->jmpbuf))
   {
      png_write_destroy(png_ptr);
      fclose(fp);
      free(png_ptr);
      free(info_ptr);
      /* If we get here, we had a problem reading the file */
      return;
   }

   /* initialize the structures */
   png_info_init(info_ptr);
   png_write_init(png_ptr);

   /* set up the output control if you are using standard C streams */
   png_init_io(png_ptr, fp);

   /* if you are using replacement write functions, here you would call */
   png_set_write_fn(png_ptr, (void *)io_ptr, user_write_fn, user_flush_fn);
   /* where io_ptr is a structure you want available to the callbacks */

   /* if you are using replacement message functions, here you would call */
   png_set_message_fn(png_ptr, (void *)msg_ptr, user_error_fn, user_warning_fn);
   /* where msg_ptr is a structure you want available to the callbacks */

   /* set the file information here */
   info_ptr->width = ;
   info_ptr->height = ;
   etc.

   /* set the palette if there is one */
   info_ptr->valid |= PNG_INFO_PLTE;
   info_ptr->palette = malloc(256 * sizeof (png_color));
   info_ptr->num_palette = 256;
   ... set palette colors ...

   /* optional significant bit chunk */
   info_ptr->valid |= PNG_INFO_sBIT;
   /* if we are dealing with a grayscale image then */
   info_ptr->sig_bit.gray = true_bit_depth;
   /* otherwise, if we are dealing with a color image then */
   info_ptr->sig_bit.red = true_red_bit_depth;
   info_ptr->sig_bit.green = true_green_bit_depth;
   info_ptr->sig_bit.blue = true_blue_bit_depth;
   /* if the image has an alpha channel then */
   info_ptr->sig_bit.alpha = true_alpha_bit_depth;
  
   /* optional gamma chunk is strongly suggested if you have any guess
      as to the correct gamma of the image */
   info_ptr->valid |= PNG_INFO_gAMA;
   info_ptr->gamma = gamma;

   /* other optional chunks */

   /* write the file information */
   png_write_info(png_ptr, info_ptr);

   /* set up the transformations you want.  Note that these are
      all optional.  Only call them if you want them */

   /* invert monocrome pixels */
   png_set_invert(png_ptr);

   /* shift the pixels up to a legal bit depth and fill in
      as appropriate to correctly scale the image */
   png_set_shift(png_ptr, &(info_ptr->sig_bit));

   /* pack pixels into bytes */
   png_set_packing(png_ptr);

   /* flip bgr pixels to rgb */
   png_set_bgr(png_ptr);

   /* swap bytes of 16 bit files to most significant bit first */
   png_set_swap(png_ptr);

   /* get rid of filler bytes, pack rgb into 3 bytes.  The
      filler number is not used. */
   png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

   /* turn on interlace handling if you are not using png_write_image() */
   if (interlacing)
      number_passes = png_set_interlace_handling(png_ptr);
   else
      number_passes = 1;

   /* the easiest way to write the image */
   png_bytep row_pointers[height];
   png_write_image(png_ptr, row_pointers);

   /* the other way to write the image - deal with interlacing */

   for (pass = 0; pass < number_passes; pass++)
   {
      /* Write a few rows at a time. */
      png_write_rows(png_ptr, row_pointers, number_of_rows);

      /* If you are only writing one row at a time, this works */
      for (y = 0; y < height; y++)
      {
         png_bytep row_pointers = row[y];
         png_write_rows(png_ptr, &row_pointers, 1);
      }
   }

   /* write the rest of the file */
   png_write_end(png_ptr, info_ptr);

   /* clean up after the write, and free any memory allocated */
   png_write_destroy(png_ptr);

   /* if you malloced the palette, free it here */
   if (info_ptr->palette)
      free(info_ptr->palette);

   /* free the structures */
   free(png_ptr);
   free(info_ptr);

   /* close the file */
   fclose(fp);

   /* that's it */
   return;
}