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.  For an example of a minimal
   working PNG reader/writer, see pngtest.c, included in this distribution.
*/

#include <png.h>

/* Check to see if a file is a PNG file using png_check_sig().  Returns
   non-zero if the image is a PNG, and 0 if it isn't a PNG.

   If this call is successful, and you are going to keep the file open,
   you should call png_set_sig_bytes(png_ptr, PNG_BYTES_TO_CHECK); once
   you have created the png_ptr, so that libpng knows your application
   has read that many bytes from the start of the file.  Make sure you
   don't call png_set_sig_bytes() with more than 8 bytes read or give it
   an incorrect number of bytes read, or you will either have read too
   many bytes (your fault), or you are telling libpng to read the wrong
   number of magic bytes (also your fault).

   Many applications already read the first 2 or 4 bytes from the start
   of the image to determine the file type, so it would be easiest just
   to pass the bytes to png_check_sig() or even skip that if you know
   you have a PNG file, and call png_set_sig_bytes().
*/
#define PNG_BYTES_TO_CHECK 4
int check_if_png(char *file_name, FILE **fp)
{
   char buf[PNG_BYTES_TO_CHECK];

   /* Open the prospective PNG file. */
   if ((*fp = fopen(file_name, "rb")) != NULL);
      return 0;

   /* Read in the signature bytes */
   if (fread(buf, 1, PNG_BYTES_TO_CHECK, *fp) != PNG_BYTES_TO_CHECK)
      return 0;

   /* Compare the first PNG_BYTES_TO_CHECK bytes of the signature. */
   return(png_check_sig(buf, PNG_BYTES_TO_CHECK));
}

/* Read a PNG file.  You may want to return an error code if the read
   fails (depending upon the failure).  There are two "prototypes" given
   here - one where we are given the filename, and we need to open the
   file, and the other where we are given an open file (possibly with
   some or all of the magic bytes read - see comments above). */
**** prototype 1 ****
void read_png(char *file_name)  /* We need to open the file */
{
   png_structp png_ptr;
   png_infop info_ptr;
   unsigned int sig_read = 0;
   png_uint_32 width, height;
   int bit_depth, color_type, interlace_type;
   FILE *fp;

   if ((fp = fopen(file_name, "rb")) == NULL)
      return;
**** prototype 2 ****
void read_png(FILE *fp, unsigned int sig_read)  /* file is already open */
{
   png_structp png_ptr;
   png_infop info_ptr;
   png_uint_32 width, height;
   int bit_depth, color_type, interlace_type;
**** only use one prototype! ****

   /* Create and initialize the png_struct with the desired error handler
    * functions.  If you want to use the default stderr and longjump method,
    * you can supply NULL for the last three parameters.  We also supply the
    * the compiler header file version, so that we know if the application
    * was compiled with a compatible version of the library.  REQUIRED
    */
   png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
      (void *)user_error_ptr, user_error_fn, user_warning_fn);

   if (png_ptr == NULL)
   {
      fclose(fp);
      return;
   }

   /* Allocate/initialize the memory for image information.  REQUIRED. */
   info_ptr = png_create_info_struct();
   if (info_ptr == NULL)
   {
      fclose(fp);
      png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL);
      return;
   }

   /* Set error handling if you are using the setjmp/longjmp method (this is
    * the normal method of doing things with libpng).  REQUIRED unless you
    * set up your own error handlers in the png_create_read_struct() earlier.
    */
   if (setjmp(png_ptr->jmpbuf))
   {
      /* Free all of the memory associated with the png_ptr and info_ptr */
      png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
      fclose(fp);
      /* If we get here, we had a problem reading the file */
      return;
   }

   /* One of the following I/O initialization methods is REQUIRED */
**** PNG file I/O method 1 ****
   /* Set up the input control if you are using standard C streams */
   png_init_io(png_ptr, fp);

**** PNG file I/O method 2 ****
   /* If you are using replacement read functions, instead of calling
    * png_init_io() here you would call */
   png_set_read_fn(png_ptr, (void *)user_io_ptr, user_read_fn);
   /* where user_io_ptr is a structure you want available to the callbacks */
**** Use only one I/O method! ****

   /* If we have already read some of the signature */
   png_set_sig_bytes_read(png_ptr, sig_read);

   /* The call to png_read_info() gives us all of the information from the
    * PNG file before the first IDAT (image data chunk).  REQUIRED
    */
   png_read_info(png_ptr, info_ptr);

   png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
       &interlace_type, NULL, NULL);

/**** Set up the data transformations you want.  Note that these are all
 **** optional.  Only call them if you want/need them.  Many of the
 **** transformations only work on specific types of images, and many
 **** are mutually exclusive.
 ****/

   /* tell libpng to strip 16 bit/color files down to 8 bits/color */
   png_set_strip_16(png_ptr);

   /* strip alpha bytes from the input data without combining with th
    * background (not recommended) */
   png_set_strip_alpha(png_ptr);

   /* extract multiple pixels with bit depths of 1, 2, and 4 from a single
    * byte into separate bytes (useful for paletted and grayscale images).
    */
   png_set_packing(png_ptr);

   /* change the order of packed pixels to least significant bit first
    * (not useful if you are using png_set_packing). */
   png_set_packswap(png_ptr);

   /* expand paletted colors into true RGB triplets */
   if (color_type == PNG_COLOR_TYPE_PALETTE)
      png_set_expand(png_ptr);

   /* expand grayscale images to the full 8 bits from 1, 2, or 4 bits/pixel */
   if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
      png_set_expand(png_ptr);

   /* expand paletted or RGB images with transparency to full alpha channels
    * so the data will be available as RGBA quartets */
   if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
      png_set_expand(png_ptr);

   /* Set the background color to draw transparent and alpha images over.
    * It is possible to set the red, green, and blue components directly
    * for paletted images instead of supplying a palette index.  Note that
    * even if the PNG file supplies a background, you are not required to
    * use it - you should use the (solid) application background if it has one.
    */

   png_color_16 my_background, *image_background);

   if (png_get_bKGD(png_ptr, info_ptr, &image_background);
      png_set_background(png_ptr, image_background),
                         PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
   else
      png_set_background(png_ptr, &my_background,
                         PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);

   /* Some suggestions as to how to get a screen gamma value */
   if (/* We have a user-defined screen gamma value */)
   {
      screen_gamma = user-defined screen_gamma;
   }
   /* This is one way that applications share the same screen gamma value */
   else if ((gamma_str = getenv("DISPLAY_GAMMA")) != NULL)
   {
      screen_gamma = atof(gamma_str);
   }
   /* If we don't have another value */
   else
   {
      screen_gamma = 2.2;  /* A good guess for PC monitors */
      screen_gamma = 1.7 or 1.0;  /* A good guess for Mac systems */
   }

   /* Tell libpng to handle the gamma conversion for you.  The second call
    * is a good guess for PC generated images, but it should be configurable
    * by the user at run time by the user.  It is strongly suggested that
    * your application support gamma correction.
    */
   if (png_get_gAMA(png_ptr, info_ptr, &image_gamma);
      png_set_gamma(png_ptr, screen_gamma, image_gamma);
   else
      png_set_gamma(png_ptr, screen_gamma, 0.45);

   /* Dither RGB files down to 8 bit palette or reduce palettes
      to the number of colors available on your screen */
   if (color_type & PNG_COLOR_MASK_COLOR)
   {
      png_uint_32 num_palette;
      png_colorp palette;

      /* This reduces the image to the application supplied palette */
      if (we have our own palette)
      {
         /* An array of colors to which the image should be dithered */
         png_color std_color_cube[MAX_SCREEN_COLORS];

         png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
            MAX_SCREEN_COLORS, NULL, 0);
      }
      /* This reduces the image to the palette supplied in the file */
      else if (png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette)))
      {
         png_color16p histogram;

         png_get_hIST(png_ptr, info_ptr, &histogram);

         png_set_dither(png_ptr, palette, num_palette,
                        max_screen_colors, histogram, 0);
      }
   }

   /* invert monocrome files to have 0 as white and 1 as black */
   png_set_invert(png_ptr);

   /* If you want to shift the pixel values from the range [0,255] or
    * [0,65535] to the original [0,7] or [0,31], or whatever range the
    * colors were originally in:
    */
   if (png_get_valid(png_ptr, info_ptr, PNG_INFO_sBIT))
   {
      png_color8p sig_bit;

      png_get_sBIT(png_ptr, info_ptr, &sig_bit);
      png_set_shift(png_ptr, sig_bit);
   }

   /* flip the RGB pixels to BGR (or RGBA to BGRA) */
   png_set_bgr(png_ptr);

   /* swap the RGBA or GA data to ARGB or AG (or BGRA to ABGR) */
   png_set_swap_alpha(png_ptr);

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

   /* Add filler (or alpha) byte (before/after each RGB triplet) */
   png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);

   /* Turn on interlace handling.  REQUIRED if you are not using
    * png_read_image().  To see how to handle interlacing passes,
    * see the png_read_row() method below.
    */
   number_passes = png_set_interlace_handling(png_ptr);

   /* optional call to gamma correct and add the background to the palette
    * and update info structure.  REQUIRED if you are expecting libpng to
    * update the palette for you (ie you selected such a transform above).
    */
   png_read_update_info(png_ptr, info_ptr);

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

   /* the easiest way to read the image */
   png_bytep row_pointers[height];

   for (row = 0; row < height; row++)
   {
      row_pointers[row] = malloc(png_get_rowbytes(png_ptr, info_ptr));
   }

   /* Now it's time to read the image.  One of these methods is REQUIRED */
**** Read the entire image in one go ****
   png_read_image(png_ptr, row_pointers);

**** Read the image one or more scanlines at a time ****
   /* the other way to read images - deal with interlacing */

   for (pass = 0; pass < number_passes; pass++)
   {
[[[[[[[ Read the image a single row at a time ]]]]]]]
      for (y = 0; y < height; y++)
      {
         png_bytep row_pointers = row[y];
         png_read_rows(png_ptr, &row_pointers, NULL, 1);
      }

[[[[[[[ Read the image several rows at a time ]]]]]]]
      for (y = 0; y < height; y += number_of_rows)
      {
<<<<<<<<<< Read the image using the "sparkle" effect. >>>>>>>>>>
         png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);
        
<<<<<<<<<< Read the image using the "rectangle" effect >>>>>>>>>>
         png_read_rows(png_ptr, NULL, row_pointers, number_of_rows);
<<<<<<<<<< use only one of these two methods >>>>>>>>>>
      }
     
      /* if you want to display the image after every pass, do
         so here */
[[[[[[[ use only one of these two methods ]]]]]]]
   }
**** use only one of these two methods ****

   /* read rest of file, and get additional chunks in info_ptr - REQUIRED */
   png_read_end(png_ptr, info_ptr);

   /* clean up after the read, and free any memory allocated - REQUIRED */
   png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);

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

   /* that's it */
   return;
}

/* progressively read a file */

int
initialize_png_reader(png_structp *png_ptr, png_infop *info_ptr)
{
   /* Create and initialize the png_struct with the desired error handler
    * functions.  If you want to use the default stderr and longjump method,
    * you can supply NULL for the last three parameters.  We also check that
    * the library version is compatible in case we are using dynamically
    * linked libraries.
    */
   *png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
       (void *)user_error_ptr, user_error_fn, user_warning_fn);

   if (*png_ptr == NULL)
   {
      *info_ptr = NULL;
      return ERROR;
   }

   *info_ptr = png_create_info_struct(png_ptr);

   if (*info_ptr == NULL)
   {
      png_destroy_read_struct(png_ptr, info_ptr, (png_infopp)NULL);
      return ERROR;
   }

   if (setjmp((*png_ptr)->jmpbuf))
   {
      png_destroy_read_struct(png_ptr, info_ptr, (png_infopp)NULL);
      return ERROR;
   }

   /* this one's new.  You will need to provide all three
    * function callbacks, even if you aren't using them all.
    * These functions shouldn't be dependent on global or
    * static variables if you are decoding several images
    * simultaneously.  You should store stream specific data
    * in a separate struct, given as the second parameter,
    * and retrieve the pointer from inside the callbacks using
    * the function png_get_progressive_ptr(png_ptr).
    */
   png_set_progressive_read_fn(*png_ptr, (void *)stream_data,
      info_callback, row_callback, end_callback);

   return OK;
}

int
process_data(png_structp *png_ptr, png_infop *info_ptr,
   png_bytep buffer, png_uint_32 length)
{
   if (setjmp((*png_ptr)->jmpbuf))
   {
      /* Free the png_ptr and info_ptr memory on error */
      png_destroy_read_struct(png_ptr, info_ptr, (png_infopp)NULL);
      return ERROR;
   }

   /* This one's new also.  Simply give it chunks of data as
    * they arrive from the data stream (in order, of course).
    * On Segmented machines, don't give it any more than 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 with less
    * than 256 bytes yet).  When this function returns, you may
    * want to display any rows that were generated in the row
    * callback, if you aren't already displaying them there.
    */
   png_process_data(*png_ptr, *info_ptr, buffer, length);
   return OK;
}

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 == NULL)
      return;

   /* Create and initialize the png_struct with the desired error handler
    * functions.  If you want to use the default stderr and longjump method,
    * you can supply NULL for the last three parameters.  We also check that
    * the library version is compatible with the one used at compile time,
    * in case we are using dynamically linked libraries.  REQUIRED.
    */
   png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
      (void *)user_error_ptr, user_error_fn, user_warning_fn);

   if (png_ptr == NULL)
   {
      fclose(fp);
      return;
   }

   /* Allocate/initialize the image information data.  REQUIRED */
   info_ptr = png_create_info_struct(png_ptr);
   if (info_ptr == NULL)
   {
      fclose(fp);
      png_destroy_write_struct(&png_ptr,  (png_infopp)NULL);
      return;
   }

   /* Set error handling.  REQUIRED if you aren't supplying your own
    * error hadnling functions in the png_create_write_struct() call.
    */
   if (setjmp(png_ptr->jmpbuf))
   {
      /* If we get here, we had a problem reading the file */
      fclose(fp);
      png_destroy_write_struct(&png_ptr,  (png_infopp)NULL);
      return;
   }

   /* One of the following I/O initialization functions is REQUIRED */
**** I/O initialization method 1 ****
   /* set up the output control if you are using standard C streams */
   png_init_io(png_ptr, fp);
**** I/O initialization method 2 ****
   /* If you are using replacement read functions, instead of calling
    * png_init_io() here you would call */
   png_set_write_fn(png_ptr, (void *)user_io_ptr, user_write_fn,
      user_IO_flush_function);
   /* where user_io_ptr is a structure you want available to the callbacks */
**** only use 1 initialization method ****

   /* Set the image information here.  Width and height are up to 2^31,
    * bit_depth is one of 1, 2, 4, 8, or 16, but valid values also depend on
    * the color_type selected. color_type is one of PNG_COLOR_TYPE_GRAY,
    * PNG_COLOR_TYPE_GRAY_ALPHA, PNG_COLOR_TYPE_PALETTE, PNG_COLOR_TYPE_RGB,
    * or PNG_COLOR_TYPE_RGB_ALPHA.  interlace is either PNG_INTERLACE_NONE or
    * PNG_INTERLACE_ADAM7, and the compression_type and filter_type MUST
    * currently be PNG_COMPRESSION_TYPE_BASE and PNG_FILTER_TYPE_BASE. REQUIRED
    */
   png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth, PNG_COLOR_TYPE_???,
      PNG_INTERLACE_????, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);

   /* set the palette if there is one.  REQUIRED for indexed-color images */
   palette = png_malloc(png_ptr, 256 * sizeof (png_color));
   ... set palette colors ...
   png_set_PLTE(png_ptr, info_ptr, palette, 256);

   /* optional significant bit chunk */
   /* if we are dealing with a grayscale image then */
   sig_bit.gray = true_bit_depth;
   /* otherwise, if we are dealing with a color image then */
   sig_bit.red = true_red_bit_depth;
   sig_bit.green = true_green_bit_depth;
   sig_bit.blue = true_blue_bit_depth;
   /* if the image has an alpha channel then */
   sig_bit.alpha = true_alpha_bit_depth;
   png_set_sBIT(png_ptr, info_ptr, sig_bit);

  
   /* Optional gamma chunk is strongly suggested if you have any guess
    * as to the correct gamma of the image. */
   png_set_gAMA(png_ptr, info_ptr, gamma);

   /* Optionally write comments into the image */
   text_ptr[0].key = "Title";
   text_ptr[0].text = "Mona Lisa";
   text_ptr[0].compression = PNG_TEXT_COMPRESSION_NONE;
   text_ptr[1].key = "Author";
   text_ptr[1].text = "Leonardo DaVinci";
   text_ptr[1].compression = PNG_TEXT_COMPRESSION_NONE;
   text_ptr[2].key = "Description";
   text_ptr[2].text = "<long text>";
   text_ptr[2].compression = PNG_TEXT_COMPRESSION_zTXt;
   png_set_text(png_ptr, info_ptr, text_ptr, 2);

   /* other optional chunks like cHRM, bKGD, tRNS, tIME, oFFs, pHYs, */

   /* Write the file header information.  REQUIRED */
   png_write_info(png_ptr, info_ptr);

   /* Once we write out the header, the compression type on the text
    * chunks gets changed to PNG_TEXT_COMPRESSION_NONE_WR or
    * PNG_TEXT_COMPRESSION_zTXt_WR, so it doesn't get written out again
    * at the end.
    */

   /* 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, &sig_bit);

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

   /* swap location of alpha bytes from ARGB to RGBA */
   png_set_swap_alpha(png_ptr);

   /* Get rid of filler (OR ALPHA) bytes, pack XRGB/RGBX/ARGB/RGBA into
    * RGB (4 channels -> 3 channels). The second parameter is not used. */
   png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

   /* flip BGR pixels to RGB */
   png_set_bgr(png_ptr);

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

   /* swap bits of 1, 2, 4 bit packed pixel formats */
   png_set_packswap(png_ptr);

   /* 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 (you may have a different memory
    * layout, however, so choose what fits your needs best).  You need to
    * use the first method if you aren't handling interlacing yourself.
    */
   png_byte row_pointers[height][width];

   /* One of the following output methods is REQUIRED */
**** write out the entire image data in one call ***
   png_write_image(png_ptr, row_pointers);

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

**** write out the image data by one or more scanlines ****
   /* The number of passes is either 1 for non-interlaced images,
    * or 7 for interlaced images.
    */
   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);
      }
   }
**** use only one output method ****

   /* You can write optional chunks like tEXt, zTXt, and tIME at the end
    * as well.
    */

   /* It is REQUIRED to call this to finish writing the rest of the file */
   png_write_end(png_ptr, info_ptr);

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

   /* if you allocated any text comments, free them here */

   /* clean up after the write, and free any memory allocated */
   png_destroy_write_struct(&png_ptr, (png_infopp)NULL);

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

   /* that's it */
   return;
}