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371 lines
15 KiB
C
371 lines
15 KiB
C
/*- pngpixel
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*
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* COPYRIGHT: Written by John Cunningham Bowler, 2011.
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* To the extent possible under law, the author has waived all copyright and
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* related or neighboring rights to this work. This work is published from:
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* United States.
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*
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* Read a single pixel value from a PNG file.
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*
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* This code illustrates basic 'by-row' reading of a PNG file using libpng.
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* Rows are read until a particular pixel is found; the value of this pixel is
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* then printed on stdout.
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*
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* The code illustrates how to do this on interlaced as well as non-interlaced
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* images. Normally you would call png_set_interlace_handling() to have libpng
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* deal with the interlace for you, but that obliges you to buffer half of the
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* image to assemble the interlaced rows. In this code
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* png_set_interlace_handling() is not called and, instead, the code handles the
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* interlace passes directly looking for the required pixel.
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*/
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#include <stdlib.h>
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#include <stdio.h>
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#include <setjmp.h> /* required for error handling */
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/* Normally use <png.h> here to get the installed libpng, but this is done to
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* ensure the code picks up the local libpng implementation:
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*/
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#include "../../png.h"
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#if defined(PNG_READ_SUPPORTED) && defined(PNG_SEQUENTIAL_READ_SUPPORTED)
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/* Return component 'c' of pixel 'x' from the given row. */
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static unsigned int
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component(png_const_bytep row, png_uint_32 x, unsigned int c,
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unsigned int bit_depth, unsigned int channels)
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{
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/* PNG images can be up to 2^31 pixels wide, but this means they can be up to
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* 2^37 bits wide (for a 64-bit pixel - the largest possible) and hence 2^34
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* bytes wide. Since the row fitted into memory, however, the following must
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* work:
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*/
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png_uint_32 bit_offset_hi = bit_depth * ((x >> 6) * channels);
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png_uint_32 bit_offset_lo = bit_depth * ((x & 0x3f) * channels + c);
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row = (png_const_bytep)(((const png_byte (*)[8])row) + bit_offset_hi);
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row += bit_offset_lo >> 3;
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bit_offset_lo &= 0x07;
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/* PNG pixels are packed into bytes to put the first pixel in the highest
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* bits of the byte and into two bytes for 16-bit values with the high 8 bits
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* first, so:
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*/
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switch (bit_depth)
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{
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case 1: return (row[0] >> (7-bit_offset_lo)) & 0x01;
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case 2: return (row[0] >> (6-bit_offset_lo)) & 0x03;
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case 4: return (row[0] >> (4-bit_offset_lo)) & 0x0f;
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case 8: return row[0];
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case 16: return (row[0] << 8) + row[1];
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default:
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/* This should never happen; it indicates a bug in this program or in
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* libpng itself:
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*/
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fprintf(stderr, "pngpixel: invalid bit depth %u\n", bit_depth);
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exit(1);
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}
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}
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/* Print a pixel from a row returned by libpng; determine the row format, find
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* the pixel, and print the relevant information to stdout.
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*/
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static void
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print_pixel(png_structp png_ptr, png_infop info_ptr, png_const_bytep row,
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png_uint_32 x)
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{
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unsigned int bit_depth = png_get_bit_depth(png_ptr, info_ptr);
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switch (png_get_color_type(png_ptr, info_ptr))
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{
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case PNG_COLOR_TYPE_GRAY:
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printf("GRAY %u\n", component(row, x, 0, bit_depth, 1));
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return;
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/* The palette case is slightly more difficult - the palette and, if
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* present, the tRNS ('transparency', though the values are really
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* opacity) data must be read to give the full picture:
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*/
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case PNG_COLOR_TYPE_PALETTE:
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{
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int index = component(row, x, 0, bit_depth, 1);
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png_colorp palette = NULL;
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int num_palette = 0;
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if ((png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette) &
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PNG_INFO_PLTE) && num_palette > 0 && palette != NULL)
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{
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png_bytep trans_alpha = NULL;
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int num_trans = 0;
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if ((png_get_tRNS(png_ptr, info_ptr, &trans_alpha, &num_trans,
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NULL) & PNG_INFO_tRNS) && num_trans > 0 &&
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trans_alpha != NULL)
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printf("INDEXED %u = %d %d %d %d\n", index,
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palette[index].red, palette[index].green,
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palette[index].blue,
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index < num_trans ? trans_alpha[index] : 255);
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else /* no transparency */
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printf("INDEXED %u = %d %d %d\n", index,
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palette[index].red, palette[index].green,
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palette[index].blue);
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}
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else
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printf("INDEXED %u = invalid index\n", index);
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}
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return;
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case PNG_COLOR_TYPE_RGB:
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printf("RGB %u %u %u\n", component(row, x, 0, bit_depth, 3),
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component(row, x, 1, bit_depth, 3),
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component(row, x, 2, bit_depth, 3));
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return;
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case PNG_COLOR_TYPE_GRAY_ALPHA:
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printf("GRAY+ALPHA %u %u\n", component(row, x, 0, bit_depth, 2),
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component(row, x, 1, bit_depth, 2));
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return;
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case PNG_COLOR_TYPE_RGB_ALPHA:
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printf("RGBA %u %u %u %u\n", component(row, x, 0, bit_depth, 4),
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component(row, x, 1, bit_depth, 4),
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component(row, x, 2, bit_depth, 4),
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component(row, x, 3, bit_depth, 4));
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return;
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default:
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png_error(png_ptr, "pngpixel: invalid color type");
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}
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}
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int main(int argc, const char **argv)
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{
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/* This program uses the default, <setjmp.h> based, libpng error handling
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* mechanism, therefore any local variable that exists before the call to
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* setjmp and is changed after the call to setjmp returns successfully must
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* be declared with 'volatile' to ensure that their values don't get
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* destroyed by longjmp:
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*/
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volatile int result = 1/*fail*/;
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if (argc == 4)
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{
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long x = atol(argv[1]);
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long y = atol(argv[2]);
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FILE *f = fopen(argv[3], "rb");
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volatile png_bytep row = NULL;
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if (f != NULL)
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{
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/* libpng requires a callback function for handling errors; this
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* callback must not return. The default callback function uses a
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* stored <setjmp.h> style jmp_buf which is held in a png_struct and
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* writes error messages to stderr. Creating the png_struct is a
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* little tricky; just copy the following code.
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*/
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png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
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NULL, NULL, NULL);
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if (png_ptr != NULL)
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{
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png_infop info_ptr = png_create_info_struct(png_ptr);
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if (info_ptr != NULL)
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{
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/* Declare stack variables to hold pointers to locally allocated
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* data.
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*/
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/* Initialize the error control buffer: */
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if (setjmp(png_jmpbuf(png_ptr)) == 0)
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{
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png_uint_32 width, height;
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int bit_depth, color_type, interlace_method,
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compression_method, filter_method;
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png_bytep row_tmp;
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/* Now associate the recently opened (FILE*) with the default
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* libpng initialization functions. Sometimes libpng is
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* compiled without stdio support (it can be difficult to do
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* in some environments); in that case you will have to write
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* your own read callback to read data from the (FILE*).
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*/
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png_init_io(png_ptr, f);
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/* And read the first part of the PNG file - the header and
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* all the information up to the first pixel.
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*/
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png_read_info(png_ptr, info_ptr);
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/* This fills in enough information to tell us the width of
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* each row in bytes, allocate the appropriate amount of
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* space. In this case png_malloc is used - it will not
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* return if memory isn't available.
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*/
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row = png_malloc(png_ptr, png_get_rowbytes(png_ptr,
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info_ptr));
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/* To avoid the overhead of using a volatile auto copy row_tmp
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* to a local here - just use row for the png_free below.
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*/
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row_tmp = row;
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/* All the information we need is in the header is returned by
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* png_get_IHDR, if this fails we can now use 'png_error' to
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* signal the error and return control to the setjmp above.
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*/
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if (png_get_IHDR(png_ptr, info_ptr, &width, &height,
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&bit_depth, &color_type, &interlace_method,
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&compression_method, &filter_method))
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{
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int passes, pass;
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/* png_set_interlace_handling returns the number of
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* passes required as well as turning on libpng's
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* handling, but since we do it ourselves this is
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* necessary:
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*/
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switch (interlace_method)
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{
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case PNG_INTERLACE_NONE:
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passes = 1;
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break;
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case PNG_INTERLACE_ADAM7:
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passes = PNG_INTERLACE_ADAM7_PASSES;
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break;
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default:
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png_error(png_ptr, "pngpixel: unknown interlace");
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}
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/* Now read the pixels, pass-by-pass, row-by-row: */
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png_start_read_image(png_ptr);
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for (pass=0; pass<passes; ++pass)
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{
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png_uint_32 ystart, xstart, ystep, xstep;
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png_uint_32 py;
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if (interlace_method == PNG_INTERLACE_ADAM7)
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{
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/* Sometimes the whole pass is empty because the
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* image is too narrow or too short. libpng
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* expects to be called for each row that is
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* present in the pass, so it may be necessary to
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* skip the loop below (over py) if the image is
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* too narrow.
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*/
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if (PNG_PASS_COLS(width, pass) == 0)
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continue;
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/* We need the starting pixel and the offset
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* between each pixel in this pass; use the macros
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* in png.h:
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*/
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xstart = PNG_PASS_START_COL(pass);
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ystart = PNG_PASS_START_ROW(pass);
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xstep = PNG_PASS_COL_OFFSET(pass);
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ystep = PNG_PASS_ROW_OFFSET(pass);
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}
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else
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{
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ystart = xstart = 0;
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ystep = xstep = 1;
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}
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/* To find the pixel, loop over 'py' for each pass
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* reading a row and then checking to see if it
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* contains the pixel.
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*/
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for (py = ystart; py < height; py += ystep)
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{
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png_uint_32 px, ppx;
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/* png_read_row takes two pointers. When libpng
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* handles the interlace the first is filled in
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* pixel-by-pixel, and the second receives the same
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* pixels but they are replicated across the
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* unwritten pixels so far for each pass. When we
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* do the interlace, however, they just contain
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* the pixels from the interlace pass - giving
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* both is wasteful and pointless, so we pass a
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* NULL pointer.
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*/
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png_read_row(png_ptr, row_tmp, NULL);
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/* Now find the pixel if it is in this row; there
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* are, of course, much better ways of doing this
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* than using a for loop:
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*/
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if (y == py) for (px = xstart, ppx = 0;
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px < width; px += xstep, ++ppx) if (x == px)
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{
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/* 'ppx' is the index of the pixel in the row
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* buffer.
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*/
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print_pixel(png_ptr, info_ptr, row_tmp, ppx);
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/* Now terminate the loops early - we have
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* found and handled the required data.
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*/
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goto pass_loop_end;
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} /* x loop */
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} /* y loop */
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} /* pass loop */
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/* Finally free the temporary buffer: */
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pass_loop_end:
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row = NULL;
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png_free(png_ptr, row_tmp);
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}
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else
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png_error(png_ptr, "pngpixel: png_get_IHDR failed");
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}
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else
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{
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/* Else libpng has raised an error. An error message has
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* already been output, so it is only necessary to clean up
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* locally allocated data:
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*/
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if (row != NULL)
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{
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/* The default implementation of png_free never errors out
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* (it just crashes if something goes wrong), but the safe
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* way of using it is still to clear 'row' before calling
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* png_free:
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*/
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png_bytep row_tmp = row;
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row = NULL;
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png_free(png_ptr, row_tmp);
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}
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}
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png_destroy_info_struct(png_ptr, &info_ptr);
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}
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else
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fprintf(stderr, "pngpixel: out of memory allocating png_info\n");
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png_destroy_read_struct(&png_ptr, NULL, NULL);
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}
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else
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fprintf(stderr, "pngpixel: out of memory allocating png_struct\n");
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}
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else
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fprintf(stderr, "pngpixel: %s: could not open file\n", argv[3]);
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}
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else
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/* Wrong number of arguments */
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fprintf(stderr, "pngpixel: usage: pngpixel x y png-file\n");
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return result;
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}
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#endif /* READ && SEQUENTIAL_READ */
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