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libpng/example.c
2009-04-06 16:04:12 -05:00

688 lines
24 KiB
C

/* example.c - an example of using libpng */
/* This is an example of how to use libpng to read and write PNG files.
* The file libpng.txt is much more verbose then this. If you have not
* read it, do so first. This was designed to be a starting point of an
* implementation. This is not officially part of libpng, 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.
*/
#include <png.h>
/* Check to see if a file is a PNG file using png_check_sig(). Returns
* non-zero if the image is a PNG, and 0 if it isn't a PNG.
*
* 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_check_sig() 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 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(png_check_sig(buf, PNG_BYTES_TO_CHECK));
}
/* Read a PNG file. You may want to return an error code if the read
* fails (depending upon the failure). There are two "prototypes" given
* here - one where we are given the filename, and we need to open the
* file, and the other where we are given an open file (possibly with
* some or all of the magic bytes read - see comments above).
*/
**** prototype 1 ****
void read_png(char *file_name) /* We need to open the file */
{
png_structp png_ptr;
png_infop info_ptr;
unsigned int sig_read = 0;
png_uint_32 width, height;
int bit_depth, color_type, interlace_type;
FILE *fp;
if ((fp = fopen(file_name, "rb")) == NULL)
return;
**** prototype 2 ****
void read_png(FILE *fp, unsigned int sig_read) /* file is already open */
{
png_structp png_ptr;
png_infop info_ptr;
png_uint_32 width, height;
int bit_depth, color_type, interlace_type;
**** 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,
(void *)user_error_ptr, user_error_fn, user_warning_fn);
if (png_ptr == NULL)
{
fclose(fp);
return;
}
/* Allocate/initialize the memory for image information. REQUIRED. */
info_ptr = png_create_info_struct();
if (info_ptr == NULL)
{
fclose(fp);
png_destroy_read_struct(&png_ptr, (png_infopp)NULL, (png_infopp)NULL);
return;
}
/* 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_ptr->jmpbuf))
{
/* Free all of the memory associated with the png_ptr and info_ptr */
png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
fclose(fp);
/* If we get here, we had a problem reading the file */
return;
}
/* One of the following I/O initialization methods is REQUIRED */
**** PNG file I/O method 1 ****
/* Set up the input control if you are using standard C streams */
png_init_io(png_ptr, fp);
**** PNG file I/O method 2 ****
/* If you are using replacement read functions, instead of calling
* png_init_io() here you would call:
*/
png_set_read_fn(png_ptr, (void *)user_io_ptr, user_read_fn);
/* where user_io_ptr is a structure you want available to the callbacks */
**** Use only one I/O method! ****
/* If we have already read some of the signature */
png_set_sig_bytes_read(png_ptr, sig_read);
/* 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 bit/color files down to 8 bits/color */
png_set_strip_16(png_ptr);
/* Strip alpha bytes from the input data without combining with th
* background (not recommended).
*/
png_set_strip_alpha(png_ptr);
/* Extract multiple pixels with bit depths of 1, 2, and 4 from a single
* byte into separate bytes (useful for paletted and grayscale images).
*/
png_set_packing(png_ptr);
/* Change the order of packed pixels to least significant bit first
* (not useful if you are using png_set_packing). */
png_set_packswap(png_ptr);
/* Expand paletted colors into true RGB triplets */
if (color_type == PNG_COLOR_TYPE_PALETTE)
png_set_expand(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(png_ptr);
/* Expand paletted or RGB images with transparency to full alpha channels
* so the data will be available as RGBA quartets.
*/
if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
png_set_expand(png_ptr);
/* Set the background color to draw transparent and alpha images over.
* It is possible to set the red, green, and blue components directly
* for paletted images instead of supplying a palette index. Note that
* even if the PNG file supplies a background, you are not required to
* use it - you should use the (solid) application background if it has one.
*/
png_color_16 my_background, *image_background);
if (png_get_bKGD(png_ptr, info_ptr, &image_background);
png_set_background(png_ptr, image_background),
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
else
png_set_background(png_ptr, &my_background,
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
/* Some suggestions as to how to get a screen gamma value */
/* Note that screen gamma is (display_gamma/viewing_gamma)
if (/* We have a user-defined screen gamma value */)
{
screen_gamma = user-defined screen_gamma;
}
/* This is one way that applications share the same screen gamma value */
else if ((gamma_str = getenv("SCREEN_GAMMA")) != NULL)
{
screen_gamma = atof(gamma_str);
}
/* If we don't have another value */
else
{
screen_gamma = 2.2; /* A good guess for a PC monitors in a dimly
lit room */
screen_gamma = 1.7 or 1.0; /* A good guess for Mac systems */
}
/* Tell libpng to handle the gamma conversion for you. The second call
* is a good guess for PC generated images, but it should be configurable
* by the user at run time by the user. It is strongly suggested that
* your application support gamma correction.
*/
int intent;
if (png_get_sRGB(png_ptr, info_ptr, &intent)
png_set_sRGB(png_ptr, intent, 0);
else
if (png_get_gAMA(png_ptr, info_ptr, &image_gamma)
png_set_gamma(png_ptr, screen_gamma, image_gamma);
else
png_set_gamma(png_ptr, screen_gamma, 0.50);
/* Dither RGB files down to 8 bit palette or reduce palettes
* to the number of colors available on your screen.
*/
if (color_type & PNG_COLOR_MASK_COLOR)
{
png_uint_32 num_palette;
png_colorp palette;
/* This reduces the image to the application supplied palette */
if (we have our own palette)
{
/* An array of colors to which the image should be dithered */
png_color std_color_cube[MAX_SCREEN_COLORS];
png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
MAX_SCREEN_COLORS, NULL, 0);
}
/* This reduces the image to the palette supplied in the file */
else if (png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette)))
{
png_color16p histogram;
png_get_hIST(png_ptr, info_ptr, &histogram);
png_set_dither(png_ptr, palette, num_palette,
max_screen_colors, histogram, 0);
}
}
/* invert monocrome files to have 0 as white and 1 as black */
png_set_invert_mono(png_ptr);
/* If you want to shift the pixel values from the range [0,255] or
* [0,65535] to the original [0,7] or [0,31], or whatever range the
* colors were originally in:
*/
if (png_get_valid(png_ptr, info_ptr, PNG_INFO_sBIT))
{
png_color8p sig_bit;
png_get_sBIT(png_ptr, info_ptr, &sig_bit);
png_set_shift(png_ptr, sig_bit);
}
/* flip the RGB pixels to BGR (or RGBA to BGRA) */
png_set_bgr(png_ptr);
/* swap the RGBA or GA data to ARGB or AG (or BGRA to ABGR) */
png_set_swap_alpha(png_ptr);
/* swap bytes of 16 bit files to least significant byte first */
png_set_swap(png_ptr);
/* Add filler (or alpha) byte (before/after each RGB triplet) */
png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);
/* Turn on interlace handling. REQUIRED if you are not using
* png_read_image(). To see how to handle interlacing passes,
* see the png_read_row() method below:
*/
number_passes = png_set_interlace_handling(png_ptr);
/* Optional call to gamma correct and add the background to the palette
* and update info structure. REQUIRED if you are expecting libpng to
* update the palette for you (ie you selected such a transform above).
*/
png_read_update_info(png_ptr, info_ptr);
/* Allocate the memory to hold the image using the fields of info_ptr. */
/* The easiest way to read the image: */
png_bytep row_pointers[height];
for (row = 0; row < height; row++)
{
row_pointers[row] = malloc(png_get_rowbytes(png_ptr, info_ptr));
}
/* Now it's time to read the image. One of these methods is REQUIRED */
**** Read the entire image in one go ****
png_read_image(png_ptr, row_pointers);
**** 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++)
{
[[[[[[[ Read the image a single row at a time ]]]]]]]
for (y = 0; y < height; y++)
{
png_bytep row_pointers = row[y];
png_read_rows(png_ptr, &row_pointers, NULL, 1);
}
[[[[[[[ Read the image several rows at a time ]]]]]]]
for (y = 0; y < height; y += number_of_rows)
{
<<<<<<<<<< Read the image using the "sparkle" effect. >>>>>>>>>>
png_read_rows(png_ptr, row_pointers, NULL, number_of_rows);
<<<<<<<<<< Read the image using the "rectangle" effect >>>>>>>>>>
png_read_rows(png_ptr, NULL, row_pointers, number_of_rows);
<<<<<<<<<< use only one of these two methods >>>>>>>>>>
}
/* if you want to display the image after every pass, do
so here */
[[[[[[[ use only one of these two methods ]]]]]]]
}
**** 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);
/* clean up after the read, and free any memory allocated - REQUIRED */
png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp)NULL);
/* close the file */
fclose(fp);
/* that's it */
return;
}
/* 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,
(void *)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, (png_infopp)NULL);
return ERROR;
}
if (setjmp((*png_ptr)->jmpbuf))
{
png_destroy_read_struct(png_ptr, info_ptr, (png_infopp)NULL);
return ERROR;
}
/* this one's new. You will need to provide all three
* function callbacks, even if you aren't using them all.
* 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_ptr)->jmpbuf))
{
/* Free the png_ptr and info_ptr memory on error */
png_destroy_read_struct(png_ptr, info_ptr, (png_infopp)NULL);
return ERROR;
}
/* This one's new also. Simply give it 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 interlacing, and you turned on the interlace handler,
* this function will be called for every row in every pass.
* Some of these rows will not be changed from the previous pass.
* When the row is not changed, the new_row variable will be NULL.
* 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 row and the
* old row. 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, and the function will combine the
* old row and the new row.
*/
}
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;
/* open the file */
fp = fopen(file_name, "wb");
if (fp == NULL)
return;
/* 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,
(void *)user_error_ptr, user_error_fn, user_warning_fn);
if (png_ptr == NULL)
{
fclose(fp);
return;
}
/* 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, (png_infopp)NULL);
return;
}
/* Set error handling. REQUIRED if you aren't supplying your own
* error hadnling functions in the png_create_write_struct() call.
*/
if (setjmp(png_ptr->jmpbuf))
{
/* If we get here, we had a problem reading the file */
fclose(fp);
png_destroy_write_struct(&png_ptr, (png_infopp)NULL);
return;
}
/* One of the following I/O initialization functions is REQUIRED */
**** I/O initialization method 1 ****
/* set up the output control if you are using standard C streams */
png_init_io(png_ptr, fp);
**** I/O initialization method 2 ****
/* If you are using replacement read functions, instead of calling
* png_init_io() here 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 */
**** only use 1 initialization method ****
/* 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, 256 * sizeof (png_color));
... set palette colors ...
png_set_PLTE(png_ptr, info_ptr, palette, 256);
/* optional significant bit chunk */
/* 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 */
text_ptr[0].key = "Title";
text_ptr[0].text = "Mona Lisa";
text_ptr[0].compression = PNG_TEXT_COMPRESSION_NONE;
text_ptr[1].key = "Author";
text_ptr[1].text = "Leonardo DaVinci";
text_ptr[1].compression = PNG_TEXT_COMPRESSION_NONE;
text_ptr[2].key = "Description";
text_ptr[2].text = "<long text>";
text_ptr[2].compression = PNG_TEXT_COMPRESSION_zTXt;
png_set_text(png_ptr, info_ptr, text_ptr, 2);
/* other optional chunks like cHRM, bKGD, tRNS, tIME, oFFs, pHYs, */
/* note that if sRGB is present the cHRM chunk must be ignored
* on read and must be written in accordance with the sRGB profile */
/* Write the file header information. REQUIRED */
png_write_info(png_ptr, info_ptr);
/* Once we write out the header, the compression type on the text
* chunks 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 monocrome 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, 2, 4 bit packed pixel formats */
png_set_packswap(png_ptr);
/* turn on interlace handling if you are not using png_write_image() */
if (interlacing)
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_byte row_pointers[height][width];
/* One of the following output methods is REQUIRED */
**** 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 */
**** 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, number_of_rows);
/* If you are only writing one row at a time, this works */
for (y = 0; y < height; y++)
{
png_bytep row_pointers = row[y];
png_write_rows(png_ptr, &row_pointers, 1);
}
}
**** use only one output method ****
/* You can write optional chunks like tEXt, zTXt, and tIME at the end
* as well.
*/
/* It is REQUIRED to call this to finish writing the rest of the file */
png_write_end(png_ptr, info_ptr);
/* if you malloced the palette, free it here */
free(info_ptr->palette);
/* if you allocated any text comments, free them here */
/* clean up after the write, and free any memory allocated */
png_destroy_write_struct(&png_ptr, (png_infopp)NULL);
/* close the file */
fclose(fp);
/* that's it */
return;
}