kc3-lang/libpng/example.c

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• Hash : 27e548af
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  Date : 2024-01-23T21:25:03
  

  chore: Clean up the return statements and update example.c accordingly
  

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• example.c

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  #if 0 /* in case someone actually tries to compile this */
  
  /* example.c - an example of using libpng
   *
   * Maintained 2018-2024 Cosmin Truta
   * Maintained 1998-2016 Glenn Randers-Pehrson
   * Maintained 1996-1997 Andreas Dilger
   * Written 1995-1996 Guy Eric Schalnat, Group 42, Inc.
   *
   * To the extent possible under law, the authors have waived
   * all copyright and related or neighboring rights to this file.
   * This work is published from: United States, Canada.
   */
  
  /* This is an example of how to use libpng to read and write PNG files.
   * The file libpng-manual.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.
   */
  
  /* The simple, but restricted approach to reading a PNG file or data stream
   * requires just two function calls, as in the following complete program.
   * Writing a file needs just one function call, so long as the data has an
   * appropriate layout.
   *
   * The following code reads PNG image data from a file and writes it, in a
   * potentially new format, to a new file.  While this code will compile, there
   * is minimal (insufficient) error checking.  For a more realistic version,
   * see contrib/examples/pngtopng.c
   */
  
  #include <stddef.h>
  #include <stdlib.h>
  #include <string.h>
  #include <stdio.h>
  #include <png.h>
  #include <zlib.h>
  
  int main(int argc, const char **argv)
  {
     if (argc == 3)
     {
        png_image image; /* The control structure used by libpng */
  
        /* Initialize the 'png_image' structure. */
        memset(&image, 0, (sizeof image));
        image.version = PNG_IMAGE_VERSION;
  
        /* The first argument is the file to read: */
        if (png_image_begin_read_from_file(&image, argv[1]) != 0)
        {
           png_bytep buffer;
  
           /* Set the format in which to read the PNG file; this code chooses a
            * simple sRGB format with a non-associated alpha channel, adequate to
            * store most images.
            */
           image.format = PNG_FORMAT_RGBA;
  
           /* Now allocate enough memory to hold the image in this format; the
            * PNG_IMAGE_SIZE macro uses the information about the image (width,
            * height and format) stored in 'image'.
            */
           buffer = malloc(PNG_IMAGE_SIZE(image));
  
           /* If enough memory was available, read the image in the desired
            * format, then write the result out to the new file.  'background' is
            * not necessary when reading the image, because the alpha channel is
            * preserved; if it were to be removed, for example if we requested
            * PNG_FORMAT_RGB, then either a solid background color would have to
            * be supplied, or the output buffer would have to be initialized to
            * the actual background of the image.
            *
            * The fourth argument to png_image_finish_read is the 'row_stride' -
            * this is the number of components allocated for the image in each
            * row.  It has to be at least as big as the value returned by
            * PNG_IMAGE_ROW_STRIDE, but if you just allocate space for the
            * default, minimum size, using PNG_IMAGE_SIZE as above, you can pass
            * zero.
            *
            * The final argument is a pointer to a buffer for the colormap;
            * colormaps have exactly the same format as a row of image pixels
            * (so you choose what format to make the colormap by setting
            * image.format).  A colormap is only returned if
            * PNG_FORMAT_FLAG_COLORMAP is also set in image.format, so in this
            * case NULL is passed as the final argument.  If you do want to force
            * all images into an index/color-mapped format, then you can use:
            *
            *    PNG_IMAGE_COLORMAP_SIZE(image)
            *
            * to find the maximum size of the colormap in bytes.
            */
           if (buffer != NULL &&
              png_image_finish_read(&image, NULL/*background*/, buffer,
                  0/*row_stride*/, NULL/*colormap*/) != 0)
           {
              /* Now write the image out to the second argument.  In the write
               * call 'convert_to_8bit' allows 16-bit data to be squashed down to
               * 8 bits; this isn't necessary here because the original read was
               * to the 8-bit format.
               */
              if (png_image_write_to_file(&image, argv[2], 0/*convert_to_8bit*/,
                  buffer, 0/*row_stride*/, NULL/*colormap*/) != 0)
              {
                 /* The image has been written successfully. */
                 exit(0);
              }
           }
           else
           {
              /* Calling png_image_free is optional unless the simplified API was
               * not run to completion.  In this case, if there wasn't enough
               * memory for 'buffer', we didn't complete the read, so we must
               * free the image:
               */
              if (buffer == NULL)
                 png_image_free(&image);
              else
                 free(buffer);
           }
        }
  
        /* Something went wrong reading or writing the image.  libpng stores a
         * textual message in the 'png_image' structure:
         */
        fprintf(stderr, "pngtopng: error: %s\n", image.message);
        exit(1);
     }
  
     fprintf(stderr, "pngtopng: usage: pngtopng input-file output-file\n");
     exit(2);
  }
  
  /* That's it ;-)  Of course you probably want to do more with PNG files than
   * just converting them all to 32-bit RGBA PNG files; you can do that between
   * the call to png_image_finish_read and png_image_write_to_file.  You can also
   * ask for the image data to be presented in a number of different formats.
   * You do this by simply changing the 'format' parameter set before allocating
   * the buffer.
   *
   * The format parameter consists of five flags that define various aspects of
   * the image.  You can simply add these together to get the format, or you can
   * use one of the predefined macros from png.h (as above):
   *
   * PNG_FORMAT_FLAG_COLOR: if set, the image will have three color components
   *    per pixel (red, green and blue); if not set, the image will just have one
   *    luminance (grayscale) component.
   *
   * PNG_FORMAT_FLAG_ALPHA: if set, each pixel in the image will have an
   *    additional alpha value; a linear value that describes the degree the
   *    image pixel covers (overwrites) the contents of the existing pixel on the
   *    display.
   *
   * PNG_FORMAT_FLAG_LINEAR: if set, the components of each pixel will be
   *    returned as a series of 16-bit linear values; if not set, the components
   *    will be returned as a series of 8-bit values encoded according to the
   *    sRGB standard.  The 8-bit format is the normal format for images intended
   *    for direct display, because almost all display devices do the inverse of
   *    the sRGB transformation to the data they receive.  The 16-bit format is
   *    more common for scientific data and image data that must be further
   *    processed; because it is linear, simple math can be done on the component
   *    values.  Regardless of the setting of this flag, the alpha channel is
   *    always linear, although it will be 8 bits or 16 bits wide as specified by
   *    the flag.
   *
   * PNG_FORMAT_FLAG_BGR: if set, the components of a color pixel will be
   *    returned in the order blue, then green, then red.  If not set, the pixel
   *    components are in the order red, then green, then blue.
   *
   * PNG_FORMAT_FLAG_AFIRST: if set, the alpha channel (if present) precedes the
   *    color or grayscale components.  If not set, the alpha channel follows the
   *    components.
   *
   * You do not have to read directly from a file.  You can read from memory or,
   * on systems that support it, from a <stdio.h> FILE*.  This is controlled by
   * the particular png_image_read_from_ function you call at the start.
   * Likewise, on write, you can write to a FILE* if your system supports it.
   * Check the macro PNG_STDIO_SUPPORTED to see if stdio support has been
   * included in your libpng build.
   *
   * If you read 16-bit (PNG_FORMAT_FLAG_LINEAR) data, you may need to write it
   * in the 8-bit format for display.  You do this by setting the convert_to_8bit
   * flag to 'true'.
   *
   * Don't repeatedly convert between the 8-bit and 16-bit forms.  There is
   * significant data loss when 16-bit data is converted to the 8-bit encoding,
   * and the current libpng implementation of conversion to 16-bit is also
   * significantly lossy.  The latter will be fixed in the future, but the former
   * is unavoidable - the 8-bit format just doesn't have enough resolution.
   */
  
  /* If your program needs more information from the PNG data it reads, or if you
   * need to do more complex transformations, or minimize transformations, on the
   * data you read, then you must use one of the several lower level libpng
   * interfaces.
   *
   * All these interfaces require that you do your own error handling - your
   * program must be able to arrange for control to return to your own code, any
   * time libpng encounters a problem.  There are several ways to do this, but
   * the standard way is to use the <setjmp.h> interface to establish a return
   * point within your own code.  You must do this if you do not use the
   * simplified interface (above).
   *
   * The first step is to include the header files you need, including the libpng
   * header file.  Include any standard headers and feature test macros your
   * program requires before including png.h:
   */
  #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)->png_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 otherwise.
   *
   * The function check_if_png() shown here, but not used, returns nonzero (true)
   * if the file can be opened and is a PNG, and 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 true if they match.
      */
     return png_sig_cmp(buf, 0, PNG_BYTES_TO_CHECK) == 0;
  }
  
  /* 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;
     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, 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, NULL, 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, 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(), 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
      * quantizing, 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, 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, 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 bits/color files down to 8 bits/color.
      * Use accurate scaling if it's available, otherwise just chop off the
      * low byte.
      */
  #ifdef PNG_READ_SCALE_16_TO_8_SUPPORTED
     png_set_scale_16(png_ptr);
  #else
     png_set_strip_16(png_ptr);
  #endif
  
     /* 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 or 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_to_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_expand_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) != 0)
        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) != 0)
        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 = PNG_DEFAULT_sRGB; /* A good guess for a PC monitor
                                            in a dimly lit room */
        screen_gamma = PNG_GAMMA_MAC_18 or 1.0; /* Good guesses 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.  Gamma correction support in your application
      * is strongly recommended.
      */
  
     int intent;
  
     if (png_get_sRGB(png_ptr, info_ptr, &intent) != 0)
        png_set_gamma(png_ptr, screen_gamma, PNG_DEFAULT_sRGB);
     else
     {
        double image_gamma;
        if (png_get_gAMA(png_ptr, info_ptr, &image_gamma) != 0)
           png_set_gamma(png_ptr, screen_gamma, image_gamma);
        else
           png_set_gamma(png_ptr, screen_gamma, 0.45455);
     }
  
  #ifdef PNG_READ_QUANTIZE_SUPPORTED
     /* Quantize 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) != 0)
     {
        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 quantized. */
           png_color std_color_cube[MAX_SCREEN_COLORS];
           png_set_quantize(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) != 0)
        {
           png_uint_16p histogram = NULL;
           png_get_hIST(png_ptr, info_ptr, &histogram);
           png_set_quantize(png_ptr, palette, num_palette,
               max_screen_colors, histogram, 0);
        }
     }
  #endif /* READ_QUANTIZE */
  
     /* 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) != 0)
     {
        png_color_8p sig_bit_p;
        png_get_sBIT(png_ptr, info_ptr, &sig_bit_p);
        png_set_shift(png_ptr, sig_bit_p);
     }
  
     /* Flip the RGB pixels to BGR (or RGBA to BGRA). */
     if ((color_type & PNG_COLOR_MASK_COLOR) != 0)
        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, 0xffff, PNG_FILLER_AFTER);
  
  #ifdef PNG_READ_INTERLACING_SUPPORTED
     /* 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);
  #else /* !READ_INTERLACING */
     number_passes = 1;
  #endif /* READ_INTERLACING */
  
     /* 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 (i.e. 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. */
     png_bytep row_pointers[height];
     for (row = 0; row < height; row++)
        row_pointers[row] = NULL; /* Clear the pointer array */
     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], 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], NULL,
               number_of_rows);
  #else no_sparkle /* Read the image using the "rectangle" effect */
           png_read_rows(png_ptr, 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, 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.
      */
     *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)
     {
        *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, NULL);
        return ERROR;
     }
     if (setjmp(png_jmpbuf((*png_ptr))))
     {
        png_destroy_read_struct(png_ptr, info_ptr, NULL);
        return ERROR;
     }
  
     /* You will need to provide all three function callbacks,
      * even if you aren't using all of them.
      * If you aren't using all functions, you can specify NULL
      * parameters.  Even when all three functions are NULL,
      * you need to call png_set_progressive_read_fn().
      * 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_jmpbuf((*png_ptr))))
     {
        /* Free the png_ptr and info_ptr memory on error. */
        png_destroy_read_struct(png_ptr, info_ptr, NULL);
        return ERROR;
     }
  
     /* Give 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 interlaced, and you turned on the interlace handler,
      * this function will be called for every row in every pass.
      *
      * In this function you will receive a pointer to new row data from
      * libpng called new_row that is to replace a corresponding row (of
      * the same data format) in a buffer allocated by your application.
      *
      * The new row data pointer "new_row" may be NULL, indicating there is
      * no new data to be replaced (in cases of interlace loading).
      *
      * If new_row is not NULL, then you need to call
      * png_progressive_combine_row(), to replace the corresponding row as
      * shown below:
      */
  
     /* Get pointer to corresponding row in our PNG read buffer. */
     png_bytep old_row = ((png_bytep *)our_data)[row_num];
  
  #ifdef PNG_READ_INTERLACING_SUPPORTED
     /* If both rows are allocated, then copy the new row
      * data to the corresponding row data.
      */
     if (old_row != NULL && new_row != NULL)
        png_progressive_combine_row(png_ptr, old_row, new_row);
  
     /* 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 new row and the
      * old row, as demonstrated above.  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 as new_row, and the function will combine
      * the old row and the new row.
      */
  #endif /* READ_INTERLACING */
  }
  
  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;
     png_colorp palette;
  
     /* Open the file */
     fp = fopen(file_name, "wb");
     if (fp == NULL)
        return ERROR;
  
     /* 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,
         png_voidp user_error_ptr, user_error_fn, user_warning_fn);
     if (png_ptr == NULL)
     {
        fclose(fp);
        return ERROR;
     }
  
     /* 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,  NULL);
        return ERROR;
     }
  
     /* Set up error handling.  REQUIRED if you aren't supplying your own
      * error handling functions in the png_create_write_struct() call.
      */
     if (setjmp(png_jmpbuf(png_ptr)))
     {
        /* If we get here, we had a problem writing the file. */
        fclose(fp);
        png_destroy_write_struct(&png_ptr, &info_ptr);
        return ERROR;
     }
  
     /* One of the following I/O initialization functions is REQUIRED. */
  
  #ifdef streams /* I/O initialization method 1 */
     /* Set up the output control if you are using standard C streams. */
     png_init_io(png_ptr, fp);
  
  #else no_streams /* I/O initialization method 2 */
     /* If you are using replacement write functions, instead of calling
      * png_init_io(), 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. */
  #endif no_streams /* Only use one initialization method */
  
  #ifdef hilevel
     /* This is the easy way.  Use it if you already have all the
      * image info living in the structure.  You could "|" many
      * PNG_TRANSFORM flags into the png_transforms integer here.
      */
     png_write_png(png_ptr, info_ptr, png_transforms, NULL);
  
  #else
     /* This is the hard way. */
  
     /* 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_colorp)png_malloc(png_ptr,
         PNG_MAX_PALETTE_LENGTH * (sizeof (png_color)));
     /* ... Set palette colors ... */
     png_set_PLTE(png_ptr, info_ptr, palette, PNG_MAX_PALETTE_LENGTH);
     /* You must not free palette here, because png_set_PLTE only makes a link
      * to the palette that you allocated.  Wait until you are about to destroy
      * the png structure.
      */
  
     /* Optional significant bit (sBIT) chunk. */
     png_color_8 sig_bit;
  
     /* 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. */
     {
        png_text text_ptr[3];
  
        char key0[] = "Title";
        char text0[] = "Mona Lisa";
        text_ptr[0].key = key0;
        text_ptr[0].text = text0;
        text_ptr[0].compression = PNG_TEXT_COMPRESSION_NONE;
        text_ptr[0].itxt_length = 0;
        text_ptr[0].lang = NULL;
        text_ptr[0].lang_key = NULL;
  
        char key1[] = "Author";
        char text1[] = "Leonardo DaVinci";
        text_ptr[1].key = key1;
        text_ptr[1].text = text1;
        text_ptr[1].compression = PNG_TEXT_COMPRESSION_NONE;
        text_ptr[1].itxt_length = 0;
        text_ptr[1].lang = NULL;
        text_ptr[1].lang_key = NULL;
  
        char key2[] = "Description";
        char text2[] = "<long text>";
        text_ptr[2].key = key2;
        text_ptr[2].text = text2;
        text_ptr[2].compression = PNG_TEXT_COMPRESSION_zTXt;
        text_ptr[2].itxt_length = 0;
        text_ptr[2].lang = NULL;
        text_ptr[2].lang_key = NULL;
  
        png_set_text(write_ptr, write_info_ptr, text_ptr, 3);
     }
  
     /* Other optional chunks like cHRM, bKGD, tRNS, tIME, oFFs, pHYs. */
  
     /* Note that if sRGB is present, the gAMA and cHRM chunks must be ignored
      * on read and, if your application chooses to write them, they must
      * be written in accordance with the sRGB profile.
      */
  
     /* Write the file header information.  REQUIRED. */
     png_write_info(png_ptr, info_ptr);
  
     /* If you want, you can write the info in two steps, in case you need to
      * write your private chunk ahead of PLTE:
      *
      *   png_write_info_before_PLTE(write_ptr, write_info_ptr);
      *   write_my_chunk();
      *   png_write_info(png_ptr, info_ptr);
      *
      * However, given the level of known- and unknown-chunk support in 1.2.0
      * and up, this should no longer be necessary.
      */
  
     /* Once we write out the header, the compression type on the text
      * chunk 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 monochrome pixels. */
     png_set_invert_mono(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-bit, 2-bit, 4-bit packed pixel formats. */
     png_set_packswap(png_ptr);
  
     /* Turn on interlace handling if you are not using png_write_image(). */
     if (interlacing != 0)
        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_uint_32 k, height, width;
  
     /* In this example, "image" is a one-dimensional array of bytes. */
  
     /* Guard against integer overflow. */
     if (height > PNG_SIZE_MAX / (width * bytes_per_pixel))
        png_error(png_ptr, "Image data buffer would be too large");
  
     png_byte image[height * width * bytes_per_pixel];
     png_bytep row_pointers[height];
  
     if (height > PNG_UINT_32_MAX / (sizeof (png_bytep)))
        png_error(png_ptr, "Image is too tall to process in memory");
  
     /* Set up pointers into your "image" byte array. */
     for (k = 0; k < height; k++)
        row_pointers[k] = image + k * width * bytes_per_pixel;
  
     /* One of the following output methods is REQUIRED. */
  
  #ifdef entire /* 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. */
  
  #else no_entire /* 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[first_row], number_of_rows);
  
        /* If you are only writing one row at a time, this works. */
        for (y = 0; y < height; y++)
           png_write_rows(png_ptr, &row_pointers[y], 1);
     }
  #endif no_entire /* Use only one output method */
  
     /* You can write optional chunks like tEXt, zTXt, and tIME at the end
      * as well.  Shouldn't be necessary in 1.2.0 and up, as all the public
      * chunks are supported, and you can use png_set_unknown_chunks() to
      * register unknown chunks into the info structure to be written out.
      */
  
     /* It is REQUIRED to call this to finish writing the rest of the file. */
     png_write_end(png_ptr, info_ptr);
  #endif hilevel
  
     /* If you png_malloced a palette, free it here.
      * (Don't free info_ptr->palette, as shown in versions 1.0.5m and earlier of
      * this example; if libpng mallocs info_ptr->palette, libpng will free it).
      * If you allocated it with malloc() instead of png_malloc(), use free()
      * instead of png_free().
      */
     png_free(png_ptr, palette);
     palette = NULL;
  
     /* Similarly, if you png_malloced any data that you passed in with
      * png_set_something(), such as a hist or trans array, free it here,
      * when you can be sure that libpng is through with it.
      */
     png_free(png_ptr, trans);
     trans = NULL;
  
     /* Whenever you use png_free(), it is a good idea to set the pointer to
      * NULL in case your application inadvertently tries to png_free() it
      * again.  When png_free() sees a NULL it returns without action, avoiding
      * the double-free problem.
      */
  
     /* Clean up after the write, and free any allocated memory. */
     png_destroy_write_struct(&png_ptr, &info_ptr);
  
     /* Close the file. */
     fclose(fp);
  
     /* That's it! */
     return OK;
  }
  
  #endif /* if 0 */
  

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