/*- pngpixel

 *

 * COPYRIGHT: Written by John Cunningham Bowler, 2011.

 * To the extent possible under law, the author has waived all copyright and

 * related or neighboring rights to this work.  This work is published from:

 * United States.

 *

 * Read a single pixel value from a PNG file.

 *

 * This code illustrates basic 'by-row' reading of a PNG file using libpng.

 * Rows are read until a particular pixel is found; the value of this pixel is

 * then printed on stdout.

 *

 * The code illustrates how to do this on interlaced as well as non-interlaced

 * images.  Normally you would call png_set_interlace_handling() to have libpng

 * deal with the interlace for you, but that obliges you to buffer half of the

 * image to assemble the interlaced rows.  In this code

 * png_set_interlace_handling() is not called and, instead, the code handles the

 * interlace passes directly looking for the required pixel.

 */

#include <stdlib.h>

#include <stdio.h>

#include <setjmp.h> /* required for error handling */

/* Normally use <png.h> here to get the installed libpng, but this is done to

 * ensure the code picks up the local libpng implementation:

 */

#include "../../png.h"

#if defined(PNG_READ_SUPPORTED) && defined(PNG_SEQUENTIAL_READ_SUPPORTED)

/* Return component 'c' of pixel 'x' from the given row. */

static unsigned int

component(png_const_bytep row, png_uint_32 x, unsigned int c,

   unsigned int bit_depth, unsigned int channels)

{

   /* PNG images can be up to 2^31 pixels wide, but this means they can be up to

    * 2^37 bits wide (for a 64-bit pixel - the largest possible) and hence 2^34

    * bytes wide.  Since the row fitted into memory, however, the following must

    * work:

    */

   png_uint_32 bit_offset_hi = bit_depth * ((x >> 6) * channels);

   png_uint_32 bit_offset_lo = bit_depth * ((x & 0x3f) * channels + c);

   row = (png_const_bytep)(((PNG_CONST png_byte (*)[8])row) + bit_offset_hi);

   row += bit_offset_lo >> 3;

   bit_offset_lo &= 0x07;

   /* PNG pixels are packed into bytes to put the first pixel in the highest

    * bits of the byte and into two bytes for 16-bit values with the high 8 bits

    * first, so:

    */

   switch (bit_depth)

   {

      case 1: return (row[0] >> (7-bit_offset_lo)) & 0x01;

      case 2: return (row[0] >> (6-bit_offset_lo)) & 0x03;

      case 4: return (row[0] >> (4-bit_offset_lo)) & 0x0f;

      case 8: return row[0];

      case 16: return (row[0] << 8) + row[1];

      default:

         /* This should never happen; it indicates a bug in this program or in

          * libpng itself:

          */

         fprintf(stderr, "pngpixel: invalid bit depth %u\n", bit_depth);

         exit(1);

   }

}

/* Print a pixel from a row returned by libpng; determine the row format, find

 * the pixel, and print the relevant information to stdout.

 */

static void

print_pixel(png_structp png_ptr, png_infop info_ptr, png_const_bytep row,

   png_uint_32 x)

{

   PNG_CONST unsigned int bit_depth = png_get_bit_depth(png_ptr, info_ptr);

   switch (png_get_color_type(png_ptr, info_ptr))

   {

      case PNG_COLOR_TYPE_GRAY:

         printf("GRAY %u\n", component(row, x, 0, bit_depth, 1));

         return;

      /* The palette case is slightly more difficult - the palette and, if

       * present, the tRNS ('transparency', though the values are really

       * opacity) data must be read to give the full picture:

       */

      case PNG_COLOR_TYPE_PALETTE:

         {

            PNG_CONST int index = component(row, x, 0, bit_depth, 1);

            png_colorp palette = NULL;

            int num_palette = 0;

            if ((png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette) &

               PNG_INFO_PLTE) && num_palette > 0 && palette != NULL)

            {

               png_bytep trans_alpha = NULL;

               int num_trans = 0;

               if ((png_get_tRNS(png_ptr, info_ptr, &trans_alpha, &num_trans,

                  NULL) & PNG_INFO_tRNS) && num_trans > 0 &&

                  trans_alpha != NULL)

                  printf("INDEXED %u = %d %d %d %d\n", index,

                     palette[index].red, palette[index].green,

                     palette[index].blue,

                     index < num_trans ? trans_alpha[index] : 255);

               else /* no transparency */

                  printf("INDEXED %u = %d %d %d\n", index,

                     palette[index].red, palette[index].green,

                     palette[index].blue);

            }

            else

               printf("INDEXED %u = invalid index\n", index);

         }

         return;

      case PNG_COLOR_TYPE_RGB:

         printf("RGB %u %u %u\n", component(row, x, 0, bit_depth, 3),

            component(row, x, 1, bit_depth, 3),

            component(row, x, 2, bit_depth, 3));

         return;

      case PNG_COLOR_TYPE_GRAY_ALPHA:

         printf("GRAY+ALPHA %u %u\n", component(row, x, 0, bit_depth, 2),

            component(row, x, 1, bit_depth, 2));

         return;

      case PNG_COLOR_TYPE_RGB_ALPHA:

         printf("RGBA %u %u %u %u\n", component(row, x, 0, bit_depth, 4),

            component(row, x, 1, bit_depth, 4),

            component(row, x, 2, bit_depth, 4),

            component(row, x, 3, bit_depth, 4));

         return;

      default:

         png_error(png_ptr, "pngpixel: invalid color type");

   }

}

int main(int argc, const char **argv)

{

   /* This program uses the default, <setjmp.h> based, libpng error handling

    * mechanism, therefore any local variable that exists before the call to

    * setjmp and is changed after the call to setjmp returns successfully must

    * be declared with 'volatile' to ensure that their values don't get

    * destroyed by longjmp:

    */

   volatile int result = 1/*fail*/;

   if (argc == 4)

   {

      long x = atol(argv[1]);

      long y = atol(argv[2]);

      FILE *f = fopen(argv[3], "rb");

      volatile png_bytep row = NULL;

      if (f != NULL)

      {

         /* libpng requires a callback function for handling errors; this

          * callback must not return.  The default callback function uses a

          * stored <setjmp.h> style jmp_buf which is held in a png_struct and

          * writes error messages to stderr.  Creating the png_struct is a

          * little tricky; just copy the following code.

          */

         png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,

            NULL, NULL, NULL);

         if (png_ptr != NULL)

         {

            png_infop info_ptr = png_create_info_struct(png_ptr);

            if (info_ptr != NULL)

            {

               /* Declare stack variables to hold pointers to locally allocated

                * data.

                */

               /* Initialize the error control buffer: */

               if (setjmp(png_jmpbuf(png_ptr)) == 0)

               {

                  png_uint_32 width, height;

                  int bit_depth, color_type, interlace_method,

                     compression_method, filter_method;

                  png_bytep row_tmp;

                  /* Now associate the recently opened (FILE*) with the default

                   * libpng initialization functions.  Sometimes libpng is

                   * compiled without stdio support (it can be difficult to do

                   * in some environments); in that case you will have to write

                   * your own read callback to read data from the (FILE*).

                   */

                  png_init_io(png_ptr, f);

                  /* And read the first part of the PNG file - the header and

                   * all the information up to the first pixel.

                   */

                  png_read_info(png_ptr, info_ptr);

                  /* This fills in enough information to tell us the width of

                   * each row in bytes, allocate the appropriate amount of

                   * space.  In this case png_malloc is used - it will not

                   * return if memory isn't available.

                   */

                  row = png_malloc(png_ptr, png_get_rowbytes(png_ptr,

                     info_ptr));

                  /* To avoid the overhead of using a volatile auto copy row_tmp

                   * to a local here - just use row for the png_free below.

                   */

                  row_tmp = row;

                  /* All the information we need is in the header is returned by

                   * png_get_IHDR, if this fails we can now use 'png_error' to

                   * signal the error and return control to the setjmp above.

                   */

                  if (png_get_IHDR(png_ptr, info_ptr, &width, &height,

                     &bit_depth, &color_type, &interlace_method,

                     &compression_method, &filter_method))

                  {

                     int passes, pass;

                     /* png_set_interlace_handling returns the number of

                      * passes required as well as turning on libpng's

                      * handling, but since we do it ourselves this is

                      * necessary:

                      */

                     switch (interlace_method)

                     {

                        case PNG_INTERLACE_NONE:

                           passes = 1;

                           break;

                        case PNG_INTERLACE_ADAM7:

                           passes = PNG_INTERLACE_ADAM7_PASSES;

                           break;

                        default:

                           png_error(png_ptr, "pngpixel: unknown interlace");

                     }

                     /* Now read the pixels, pass-by-pass, row-by-row: */

                     png_start_read_image(png_ptr);

                     for (pass=0; pass

                     {

                        png_uint_32 ystart, xstart, ystep, xstep;

                        png_uint_32 py;

                        if (interlace_method == PNG_INTERLACE_ADAM7)

                        {

                           /* Sometimes the whole pass is empty because the

                            * image is too narrow or too short.  libpng

                            * expects to be called for each row that is

                            * present in the pass, so it may be necessary to

                            * skip the loop below (over py) if the image is

                            * too narrow.

                            */

                           if (PNG_PASS_COLS(width, pass) == 0)

                              continue;

                           /* We need the starting pixel and the offset

                            * between each pixel in this pass; use the macros

                            * in png.h:

                            */

                           xstart = PNG_PASS_START_COL(pass);

                           ystart = PNG_PASS_START_ROW(pass);

                           xstep = PNG_PASS_COL_OFFSET(pass);

                           ystep = PNG_PASS_ROW_OFFSET(pass);

                        }

                        else

                        {

                           ystart = xstart = 0;

                           ystep = xstep = 1;

                        }

                        /* To find the pixel, loop over 'py' for each pass

                         * reading a row and then checking to see if it

                         * contains the pixel.

                         */

                        for (py = ystart; py < height; py += ystep)

                        {

                           png_uint_32 px, ppx;

                           /* png_read_row takes two pointers.  When libpng

                            * handles the interlace the first is filled in

                            * pixel-by-pixel, and the second receives the same

                            * pixels but they are replicated across the

                            * unwritten pixels so far for each pass.  When we

                            * do the interlace, however, they just contain

                            * the pixels from the interlace pass - giving

                            * both is wasteful and pointless, so we pass a

                            * NULL pointer.

                            */

                           png_read_row(png_ptr, row_tmp, NULL);

                           /* Now find the pixel if it is in this row; there

                            * are, of course, much better ways of doing this

                            * than using a for loop:

                            */

                           if (y == py) for (px = xstart, ppx = 0;

                              px < width; px += xstep, ++ppx) if (x == px)

                           {

                              /* 'ppx' is the index of the pixel in the row

                               * buffer.

                               */

                              print_pixel(png_ptr, info_ptr, row_tmp, ppx);

                              /* Now terminate the loops early - we have

                               * found and handled the required data.

                               */

                              goto pass_loop_end;

                           } /* x loop */

                        } /* y loop */

                     } /* pass loop */

                     /* Finally free the temporary buffer: */

                  pass_loop_end:

                     row = NULL;

                     png_free(png_ptr, row_tmp);

                  }

                  else

                     png_error(png_ptr, "pngpixel: png_get_IHDR failed");

               }

               else

               {

                  /* Else libpng has raised an error.  An error message has

                   * already been output, so it is only necessary to clean up

                   * locally allocated data:

                   */

                  if (row != NULL)

                  {

                     /* The default implementation of png_free never errors out

                      * (it just crashes if something goes wrong), but the safe

                      * way of using it is still to clear 'row' before calling

                      * png_free:

                      */

                     png_bytep row_tmp = row;

                     row = NULL;

                     png_free(png_ptr, row_tmp);

                  }

               }

               png_destroy_info_struct(png_ptr, &info_ptr);

            }

            else

               fprintf(stderr, "pngpixel: out of memory allocating png_info\n");

            png_destroy_read_struct(&png_ptr, NULL, NULL);

         }

         else

            fprintf(stderr, "pngpixel: out of memory allocating png_struct\n");

      }

      else

         fprintf(stderr, "pngpixel: %s: could not open file\n", argv[3]);

   }

   else

      /* Wrong number of arguments */

      fprintf(stderr, "pngpixel: usage: pngpixel x y png-file\n");

   return result;

}

#endif /* READ && SEQUENTIAL_READ */