workbench/libs/png/contrib/examples/pngpixel.c

   1 /*- pngpixel
   2  *
   3  * COPYRIGHT: Written by John Cunningham Bowler, 2011.
   4  * To the extent possible under law, the author has waived all copyright and
   5  * related or neighboring rights to this work.  This work is published from:
   6  * United States.
   7  *
   8  * Read a single pixel value from a PNG file.
   9  *
  10  * This code illustrates basic 'by-row' reading of a PNG file using libpng.
  11  * Rows are read until a particular pixel is found, the value of this pixel is
  12  * then printed on stdout.
  13  *
  14  * The code illustrates how to do this on interlaced as well as non-interlaced
  15  * images.  Normally you would call png_set_interlace_handling() to have libpng
  16  * deal with the interlace for you, but that obliges you to buffer half of the
  17  * image to assemble the interlaced rows.  In this code
  18  * png_set_interlace_handling() is not called and, instead, the code handles the
  19  * interlace passes directly looking for the required pixel.
  20  */
  21 #include <stdlib.h>
  22 #include <stdio.h>
  23 #include <setjmp.h> /* required for error handling */
  24
  25 /* Normally use <png.h> here to get the installed libpng, but this is done to
  26  * ensure the code picks up the local libpng implementation:
  27  */
  28 #include "../../png.h"
  29
  30 /* Return component 'c' of pixel 'x' from the given row. */
  31 static unsigned int
  32 component(png_const_bytep row, png_uint_32 x, unsigned int c,
  33    unsigned int bit_depth, unsigned int channels)
  34 {
  35    /* PNG images can be up to 2^31 pixels wide, but this means they can be up to
  36     * 2^37 bits wide (for a 64-bit pixel - the largest possible) and hence 2^34
  37     * bytes wide.  Since the row fitted into memory, however, the following must
  38     * work:
  39     */
  40    png_uint_32 bit_offset_hi = bit_depth * ((x >> 6) * channels);
  41    png_uint_32 bit_offset_lo = bit_depth * ((x & 0x3f) * channels + c);
  42
  43    row = (png_const_bytep)(((PNG_CONST png_byte (*)[8])row) + bit_offset_hi);
  44    row += bit_offset_lo >> 3;
  45    bit_offset_lo &= 0x07;
  46
  47    /* PNG pixels are packed into bytes to put the first pixel in the highest
  48     * bits of the byte and into two bytes for 16-bit values with the high 8 bits
  49     * first, so:
  50     */
  51    switch (bit_depth)
  52    {
  53       case 1: return (row[0] >> (7-bit_offset_lo)) & 0x01;
  54       case 2: return (row[0] >> (6-bit_offset_lo)) & 0x03;
  55       case 4: return (row[0] >> (4-bit_offset_lo)) & 0x0f;
  56       case 8: return row[0];
  57       case 16: return (row[0] << 8) + row[1];
  58       default:
  59          /* This should never happen, it indicates a bug in this program or in
  60           * libpng itself:
  61           */
  62          fprintf(stderr, "pngpixel: invalid bit depth %u\n", bit_depth);
  63          exit(1);
  64    }
  65 }
  66
  67 /* Print a pixel from a row returned by libpng; determine the row format, find
  68  * the pixel, and print the relevant information to stdout.
  69  */
  70 static void
  71 print_pixel(png_structp png_ptr, png_infop info_ptr, png_const_bytep row,
  72    png_uint_32 x)
  73 {
  74    PNG_CONST unsigned int bit_depth = png_get_bit_depth(png_ptr, info_ptr);
  75
  76    switch (png_get_color_type(png_ptr, info_ptr))
  77    {
  78       case PNG_COLOR_TYPE_GRAY:
  79          printf("GRAY %u\n", component(row, x, 0, bit_depth, 1));
  80          return;
  81
  82       /* The palette case is slightly more difficult - the palette and, if
  83        * present, the tRNS ('transparency', though the values are really
  84        * opacity) data must be read to give the full picture:
  85        */
  86       case PNG_COLOR_TYPE_PALETTE:
  87          {
  88             PNG_CONST unsigned int index = component(row, x, 0, bit_depth, 1);
  89             png_colorp palette = NULL;
  90             int num_palette = 0;
  91
  92             if ((png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette) &
  93                PNG_INFO_PLTE) && num_palette > 0 && palette != NULL)
  94             {
  95                png_bytep trans_alpha = NULL;
  96                int num_trans = 0;
  97                if ((png_get_tRNS(png_ptr, info_ptr, &trans_alpha, &num_trans,
  98                   NULL) & PNG_INFO_tRNS) && num_trans > 0 &&
  99                   trans_alpha != NULL)
 100                   printf("INDEXED %u = %d %d %d %d\n", index,
 101                      palette[index].red, palette[index].green,
 102                      palette[index].blue,
 103                      index < num_trans ? trans_alpha[index] : 255);
 104
 105                else /* no transparency */
 106                   printf("INDEXED %u = %d %d %d\n", index,
 107                      palette[index].red, palette[index].green,
 108                      palette[index].blue);
 109             }
 110
 111             else
 112                printf("INDEXED %u = invalid index\n", index);
 113          }
 114          return;
 115
 116       case PNG_COLOR_TYPE_RGB:
 117          printf("RGB %u %u %u\n", component(row, x, 0, bit_depth, 3),
 118             component(row, x, 1, bit_depth, 3),
 119             component(row, x, 2, bit_depth, 3));
 120          return;
 121
 122       case PNG_COLOR_TYPE_GRAY_ALPHA:
 123          printf("GRAY+ALPHA %u %u\n", component(row, x, 0, bit_depth, 2),
 124             component(row, x, 1, bit_depth, 2));
 125          return;
 126
 127       case PNG_COLOR_TYPE_RGB_ALPHA:
 128          printf("RGBA %u %u %u %u\n", component(row, x, 0, bit_depth, 4),
 129             component(row, x, 1, bit_depth, 4),
 130             component(row, x, 2, bit_depth, 4),
 131             component(row, x, 3, bit_depth, 4));
 132          return;
 133
 134       default:
 135          png_error(png_ptr, "invalid color type");
 136    }
 137 }
 138
 139 int main(int argc, const char **argv)
 140 {
 141    /* This program uses the default, <setjmp.h> based, libpng error handling
 142     * mechanism, therefore any local variable that exists before the call to
 143     * setjmp and is changed after the call to setjmp returns successfully must
 144     * be declared with 'volatile' to ensure that their values don't get
 145     * destroyed by longjmp:
 146     */
 147    volatile int result = 1/*fail*/;
 148
 149    if (argc == 4)
 150    {
 151       long x = atol(argv[1]);
 152       long y = atol(argv[2]);
 153       FILE *f = fopen(argv[3], "rb");
 154       volatile png_bytep row = NULL;
 155
 156       if (f != NULL)
 157       {
 158          /* libpng requires a callback function for handling errors; this
 159           * callback must not return.  The default callback function uses a
 160           * stored <setjmp.h> style jmp_buf which is held in a png_struct and
 161           * writes error messages to stderr.  Creating the png_struct is a
 162           * little tricky; just copy the following code.
 163           */
 164          png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
 165             NULL, NULL, NULL);
 166
 167          if (png_ptr != NULL)
 168          {
 169             png_infop info_ptr = png_create_info_struct(png_ptr);
 170
 171             if (info_ptr != NULL)
 172             {
 173                /* Declare stack variables to hold pointers to locally allocated
 174                 * data.
 175                 */
 176
 177                /* Initialize the error control buffer: */
 178                if (setjmp(png_jmpbuf(png_ptr)) == 0)
 179                {
 180                   png_uint_32 width, height;
 181                   int bit_depth, color_type, interlace_method,
 182                      compression_method, filter_method;
 183                   png_bytep row_tmp;
 184
 185                   /* Now associate the recently opened (FILE*) with the default
 186                    * libpng initialization functions.  Sometimes libpng is
 187                    * compiled without stdio support (it can be difficult to do
 188                    * in some environments); in that case you will have to write
 189                    * your own read callback to read data from the (FILE*).
 190                    */
 191                   png_init_io(png_ptr, f);
 192
 193                   /* And read the first part of the PNG file - the header and
 194                    * all the information up to the first pixel.
 195                    */
 196                   png_read_info(png_ptr, info_ptr);
 197
 198                   /* This fills in enough information to tell us the width of
 199                    * each row in bytes, allocate the appropriate amount of
 200                    * space.  In this case png_malloc is used - it will not
 201                    * return if memory isn't available.
 202                    */
 203                   row = png_malloc(png_ptr, png_get_rowbytes(png_ptr,
 204                      info_ptr));
 205
 206                   /* To avoid the overhead of using a volatile auto copy row_tmp
 207                    * to a local here - just use row for the png_free below.
 208                    */
 209                   row_tmp = row;
 210
 211                   /* All the information we need is in the header is returned by
 212                    * png_get_IHDR, if this fails we can now use 'png_error' to
 213                    * signal the error and return control to the setjmp above.
 214                    */
 215                   if (png_get_IHDR(png_ptr, info_ptr, &width, &height,
 216                      &bit_depth, &color_type, &interlace_method,
 217                      &compression_method, &filter_method))
 218                   {
 219                      int passes, pass;
 220
 221                      /* png_set_interlace_handling returns the number of
 222                       * passes required as well as turning on libpng's
 223                       * handling, but since we do it ourselves this is
 224                       * necessary:
 225                       */
 226                      switch (interlace_method)
 227                      {
 228                         case PNG_INTERLACE_NONE:
 229                            passes = 1;
 230                            break;
 231
 232                         case PNG_INTERLACE_ADAM7:
 233                            passes = PNG_INTERLACE_ADAM7_PASSES;
 234                            break;
 235
 236                         default:
 237                            png_error(png_ptr, "pngpixel: unknown interlace");
 238                      }
 239
 240                      /* Now read the pixels, pass-by-pass, row-by-row: */
 241                      png_start_read_image(png_ptr);
 242
 243                      for (pass=0; pass<passes; ++pass)
 244                      {
 245                         png_uint_32 ystart, xstart, ystep, xstep;
 246                         png_uint_32 py;
 247
 248                         if (interlace_method == PNG_INTERLACE_ADAM7)
 249                         {
 250                            /* Sometimes the whole pass is empty because the
 251                             * image is too narrow or too short.  libpng
 252                             * expects to be called for each row that is
 253                             * present in the pass, so it may be necessary to
 254                             * skip the loop below (over py) if the image is
 255                             * too narrow.
 256                             */
 257                            if (PNG_PASS_COLS(width, pass) == 0)
 258                               continue;
 259
 260                            /* We need the starting pixel and the offset
 261                             * between each pixel in this pass; use the macros
 262                             * in png.h:
 263                             */
 264                            xstart = PNG_PASS_START_COL(pass);
 265                            ystart = PNG_PASS_START_ROW(pass);
 266                            xstep = PNG_PASS_COL_OFFSET(pass);
 267                            ystep = PNG_PASS_ROW_OFFSET(pass);
 268                         }
 269
 270                         else
 271                         {
 272                            ystart = xstart = 0;
 273                            ystep = xstep = 1;
 274                         }
 275
 276                         /* To find the pixel, loop over 'py' for each pass
 277                          * reading a row and then checking to see if it
 278                          * contains the pixel.
 279                          */
 280                         for (py = ystart; py < height; py += ystep)
 281                         {
 282                            png_uint_32 px, ppx;
 283
 284                            /* png_read_row takes two pointers.  When libpng
 285                             * handles the interlace the first is filled in
 286                             * pixel-by-pixel, and the second receives the same
 287                             * pixels but they are replicated across the
 288                             * unwritten pixels so far for each pass.  When we
 289                             * do the interlace, however, they just contain
 290                             * the pixels from the interlace pass - giving
 291                             * both is wasteful and pointless, so we pass a
 292                             * NULL pointer.
 293                             */
 294                            png_read_row(png_ptr, row_tmp, NULL);
 295
 296                            /* Now find the pixel if it is in this row; there
 297                             * are, of course, much better ways of doing this
 298                             * than using a for loop:
 299                             */
 300                            if (y == py) for (px = xstart, ppx = 0;
 301                               px < width; px += xstep, ++ppx) if (x == px)
 302                            {
 303                               /* 'ppx' is the index of the pixel in the row
 304                                * buffer.
 305                                */
 306                               print_pixel(png_ptr, info_ptr, row_tmp, ppx);
 307
 308                               /* Now terminate the loops early - we have
 309                                * found and handled the required data.
 310                                */
 311                               goto pass_loop_end;
 312                            } /* x loop */
 313                         } /* y loop */
 314                      } /* pass loop */
 315
 316                      /* Finally free the temporary buffer: */
 317                   pass_loop_end:
 318                      row = NULL;
 319                      png_free(png_ptr, row_tmp);
 320                   }
 321
 322                   else
 323                      png_error(png_ptr, "pngpixel: png_get_IHDR failed");
 324
 325                }
 326
 327                else
 328                {
 329                   /* Else libpng has raised an error.  An error message has
 330                    * already been output, so it is only necessary to clean up
 331                    * locally allocated data:
 332                    */
 333                   if (row != NULL)
 334                   {
 335                      /* The default implementation of png_free never errors out
 336                       * (it just crashes if something goes wrong), but the safe
 337                       * way of using it is still to clear 'row' before calling
 338                       * png_free:
 339                       */
 340                      png_bytep row_tmp = row;
 341                      row = NULL;
 342                      png_free(png_ptr, row_tmp);
 343                   }
 344                }
 345
 346                png_destroy_info_struct(png_ptr, &info_ptr);
 347             }
 348
 349             else
 350                fprintf(stderr, "pngpixel: out of memory allocating png_info\n");
 351
 352             png_destroy_read_struct(&png_ptr, NULL, NULL);
 353          }
 354
 355          else
 356             fprintf(stderr, "pngpixel: out of memory allocating png_struct\n");
 357       }
 358
 359       else
 360          fprintf(stderr, "pngpixel: %s: could not open file\n", argv[3]);
 361    }
 362
 363    else
 364       /* Wrong number of arguments */
 365       fprintf(stderr, "pngpixel: usage: pngpixel x y png-file\n");
 366
 367    return result;
 368 }