example.c

Linux下的基于X11的图形开发环境。

#if 0 /* in case someone actually tries to compile this */

/* 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, is hereby placed
 * in the public domain, 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;
 * see also the programs in the contrib directory.
 */

#include "png.h"

 /* The png_jmpbuf() macro, used in error handling, became available in
  * libpng version 1.0.6.  If you want to be able to run your code with older
  * versions of libpng, you must define the macro yourself (but only if it
  * is not already defined by libpng!).
  */

#ifndef png_jmpbuf
#  define png_jmpbuf(png_ptr) ((png_ptr)->jmpbuf)
#endif

/* Check to see if a file is a PNG file using png_sig_cmp().  png_sig_cmp()
 * returns zero if the image is a PNG and nonzero if it isn't a PNG.
 *
 * The function check_if_png() shown here, but not used, returns nonzero (true)
 * if the file can be opened and is a PNG, 0 (false) otherwise.
 *
 * 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_sig_cmp() 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 some of 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 nonzero (true) if they match */

   return(!png_sig_cmp(buf, (png_size_t)0, 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).
 */
#ifdef open_file /* 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 (ERROR);
#else no_open_file /* 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;
#endif no_open_file /* 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,
      png_voidp user_error_ptr, user_error_fn, user_warning_fn);

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

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

   /* 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_jmpbuf(png_ptr)))
   {
      /* 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 (ERROR);
   }

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

#else no_streams /* 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 */
#endif no_streams /* Use only one I/O method! */

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

#ifdef hilevel
   /*
    * If you have enough memory to read in the entire image at once,
    * and you need to specify only transforms that can be controlled
    * with one of the PNG_TRANSFORM_* bits (this presently excludes
    * dithering, filling, setting background, and doing gamma
    * adjustment), then you can read the entire image (including
    * pixels) into the info structure with this call:
    */
   png_read_png(png_ptr, info_ptr, png_transforms, png_voidp_NULL);
#else
   /* OK, you're doing it the hard way, with the lower-level functions */

   /* 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, int_p_NULL, int_p_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 the
    * 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_palette_rgb(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_gray_1_2_4_to_8(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_tRNS_to_alpha(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 */

   /* Note that screen gamma is the display_exponent, which includes
    * the CRT_exponent and any correction for viewing conditions */
   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("SCREEN_GAMMA")) != NULL)
   {
      screen_gamma = atof(gamma_str);
   }
   /* If we don't have another value */
   else
   {
      screen_gamma = 2.2;  /* A good guess for a PC monitors in a dimly
                              lit room */
      screen_gamma = 1.7 or 1.0;  /* A good guess for Mac systems */
   }

   /* Tell libpng to handle the gamma conversion for you.  The final 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.
    */

   int intent;

   if (png_get_sRGB(png_ptr, info_ptr, &intent))
      png_set_gamma(png_ptr, screen_gamma, 0.45455);
   else
   {
      double image_gamma;
      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, 0.45455);
   }

   /* 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)
   {
      int 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, png_uint_16p_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_uint_16p histogram = NULL;

         png_get_hIST(png_ptr, info_ptr, &histogram);

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

   /* invert monochrome files to have 0 as white and 1 as black */
   png_set_invert_mono(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_color_8p 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) */
   if (color_type & PNG_COLOR_MASK_COLOR)
      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] = png_malloc(png_ptr, png_get_rowbytes(png_ptr,
         info_ptr));
   }

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

#else no_entire /* 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++)
   {
#ifdef single /* Read the image a single row at a time */
      for (y = 0; y < height; y++)
      {
         png_read_rows(png_ptr, &row_pointers[y], png_bytepp_NULL, 1);
      }

#else no_single /* Read the image several rows at a time */
      for (y = 0; y < height; y += number_of_rows)
      {
#ifdef sparkle /* Read the image using the "sparkle" effect. */
         png_read_rows(png_ptr, &row_pointers[y], png_bytepp_NULL,
            number_of_rows);
#else no_sparkle /* Read the image using the "rectangle" effect */
         png_read_rows(png_ptr, png_bytepp_NULL, &row_pointers[y],
            number_of_rows);
#endif no_sparkle /* use only one of these two methods */
      }

      /* if you want to display the image after every pass, do
         so here */
#endif no_single /* use only one of these two methods */
   }
#endif no_entire /* 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);
#endif hilevel

   /* At this point you have read the entire image */

   /* 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 (OK);
}

/* 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.