gd_png.c

/* $Id$ */
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <stdio.h>
#include <math.h>
#include <string.h>
#include <stdlib.h>
#include "gd.h"
#include "gd_errors.h"

/* JCE: Arrange HAVE_LIBPNG so that it can be set in gd.h */
#ifdef HAVE_LIBPNG

#include "gdhelpers.h"
#include "png.h"		/* includes zlib.h and setjmp.h */

#define TRUE 1
#define FALSE 0

/*---------------------------------------------------------------------------

  gd_png.c                 Copyright 1999 Greg Roelofs and Thomas Boutell

  The routines in this file, gdImagePng*() and gdImageCreateFromPng*(),
  are drop-in replacements for gdImageGif*() and gdImageCreateFromGif*(),
  except that these functions are noisier in the case of errors (comment
  out all fprintf() statements to disable that).

  GD 2.0 supports RGBA truecolor and will read and write truecolor PNGs.
  GD 2.0 supports 8 bits of color resolution per channel and
  7 bits of alpha channel resolution. Images with more than 8 bits
  per channel are reduced to 8 bits. Images with an alpha channel are
  only able to resolve down to '1/128th opaque' instead of '1/256th',
  and this conversion is also automatic. I very much doubt you can see it.
  Both tRNS and true alpha are supported.

  Gamma is ignored, and there is no support for text annotations.

  Last updated:  9 February 2001

  ---------------------------------------------------------------------------*/

#ifdef PNG_SETJMP_SUPPORTED
typedef struct _jmpbuf_wrapper {
	jmp_buf jmpbuf;
}
jmpbuf_wrapper;

static void
gdPngErrorHandler (png_structp png_ptr, png_const_charp msg)
{
	jmpbuf_wrapper *jmpbuf_ptr;

	/* This function, aside from the extra step of retrieving the "error
	 * pointer" (below) and the fact that it exists within the application
	 * rather than within libpng, is essentially identical to libpng's
	 * default error handler.  The second point is critical:  since both
	 * setjmp() and longjmp() are called from the same code, they are
	 * guaranteed to have compatible notions of how big a jmp_buf is,
	 * regardless of whether _BSD_SOURCE or anything else has (or has not)
	 * been defined. */

	gd_error_ex(GD_ERROR, "gd-png: fatal libpng error: %s\n", msg);

	jmpbuf_ptr = png_get_error_ptr (png_ptr);
	if (jmpbuf_ptr == NULL) {				/* we are completely hosed now */
		gd_error_ex(GD_ERROR, "gd-png: EXTREMELY fatal error: jmpbuf unrecoverable; terminating.\n");
		exit (99);
	}

	longjmp (jmpbuf_ptr->jmpbuf, 1);
}
#endif

static void
gdPngReadData (png_structp png_ptr, png_bytep data, png_size_t length)
{
	int check;
	check = gdGetBuf (data, length, (gdIOCtx *) png_get_io_ptr (png_ptr));
	if (check != (int)length) {
		png_error(png_ptr, "Read Error: truncated data");
	}
}

static void
gdPngWriteData (png_structp png_ptr, png_bytep data, png_size_t length)
{
	gdPutBuf (data, length, (gdIOCtx *) png_get_io_ptr (png_ptr));
}

static void
gdPngFlushData (png_structp png_ptr)
{
	(void)png_ptr;
}

/*
  Function: gdImageCreateFromPng

    <gdImageCreateFromPng> is called to load images from PNG format
    files. Invoke <gdImageCreateFromPng> with an already opened
    pointer to a FILE containing the desired
    image. <gdImageCreateFromPng> returns a <gdImagePtr> to the new
    image, or NULL if unable to load the image (most often because the
    file is corrupt or does not contain a PNG
    image). <gdImageCreateFromPng> does not close the file. You can
    inspect the sx and sy members of the image to determine its
    size. The image must eventually be destroyed using
    gdImageDestroy().

    If the PNG image being loaded is a truecolor image, the resulting
    gdImagePtr will refer to a truecolor image. If the PNG image being
    loaded is a palette or grayscale image, the resulting gdImagePtr
    will refer to a palette image. gd retains only 8 bits of
    resolution for each of the red, green and blue channels, and only
    7 bits of resolution for the alpha channel. The former restriction
    affects only a handful of very rare 48-bit color and 16-bit
    grayscale PNG images. The second restriction affects all
    semitransparent PNG images, but the difference is essentially
    invisible to the eye. 7 bits of alpha channel resolution is, in
    practice, quite a lot.

  Variants:

    <gdImageCreateFromPngPtr> creates an image from PNG data (i.e. the
    contents of a PNG file) already in memory.

    <gdImageCreateFromPngCtx> reads in an image using the functions in
    a <gdIOCtx> struct.

    <gdImageCreateFromPngSource> is similar to
    <gdImageCreateFromPngCtx> but uses the old <gdSource> interface.
    It is *obsolete*.

  Parameters:

    infile - The input FILE pointer.

  Returns:

    A pointer to the new image or NULL if an error occurred.

  Example:

    > gdImagePtr im;
    > ... inside a function ...
    > FILE *in;
    > in = fopen("mypng.png", "rb");
    > im = gdImageCreateFromPng(in);
    > fclose(in);
    > // ... Use the image ...
    > gdImageDestroy(im);

 */
BGD_DECLARE(gdImagePtr) gdImageCreateFromPng (FILE * inFile)
{
	gdImagePtr im;
	gdIOCtx *in = gdNewFileCtx (inFile);
	if (in == NULL) return NULL;
	im = gdImageCreateFromPngCtx (in);
	in->gd_free (in);
	return im;
}

/*
  Function: gdImageCreateFromPngPtr

  See <gdImageCreateFromPng>.
*/
BGD_DECLARE(gdImagePtr) gdImageCreateFromPngPtr (int size, void *data)
{
	gdImagePtr im;
	gdIOCtx *in = gdNewDynamicCtxEx (size, data, 0);
	if(!in)
		return 0;
	im = gdImageCreateFromPngCtx (in);
	in->gd_free (in);
	return im;
}



/* This routine is based in part on the Chapter 13 demo code in
 * "PNG: The Definitive Guide" (http://www.libpng.org/pub/png/book/).
 */

/*
  Function: gdImageCreateFromPngCtx

  See <gdImageCreateFromPng>.
*/
BGD_DECLARE(gdImagePtr) gdImageCreateFromPngCtx (gdIOCtx * infile)
{
	png_byte sig[8];
#ifdef PNG_SETJMP_SUPPORTED
	jmpbuf_wrapper jbw;
#endif
	png_structp png_ptr;
	png_infop info_ptr;
	png_uint_32 width, height, rowbytes, w, h, res_x, res_y;
	int bit_depth, color_type, interlace_type, unit_type;
	int num_palette = 0, num_trans;
	png_colorp palette;
	png_color_16p trans_gray_rgb;
	png_color_16p trans_color_rgb;
	png_bytep trans;
	png_bytep image_data = NULL;
	png_bytepp row_pointers = NULL;
	gdImagePtr im = NULL;
	int i, j, *open = NULL;
	volatile int transparent = -1;
	volatile int palette_allocated = FALSE;

	/* Make sure the signature can't match by dumb luck -- TBB */
	/* GRR: isn't sizeof(infile) equal to the size of the pointer? */
	memset (sig, 0, sizeof (sig));

	/* first do a quick check that the file really is a PNG image; could
	 * have used slightly more general png_sig_cmp() function instead */
	if (gdGetBuf (sig, 8, infile) < 8) {
		return NULL;
	}

	if (png_sig_cmp(sig, 0, 8) != 0) { /* bad signature */
		return NULL;		/* bad signature */
	}

#ifdef PNG_SETJMP_SUPPORTED
	png_ptr = png_create_read_struct (PNG_LIBPNG_VER_STRING, &jbw, gdPngErrorHandler, NULL);
#else
	png_ptr = png_create_read_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
#endif
	if (png_ptr == NULL) {
		gd_error("gd-png error: cannot allocate libpng main struct\n");
		return NULL;
	}

	info_ptr = png_create_info_struct (png_ptr);
	if (info_ptr == NULL) {
		gd_error("gd-png error: cannot allocate libpng info struct\n");
		png_destroy_read_struct (&png_ptr, NULL, NULL);

		return NULL;
	}

	/* we could create a second info struct here (end_info), but it's only
	 * useful if we want to keep pre- and post-IDAT chunk info separated
	 * (mainly for PNG-aware image editors and converters)
	 */

	/* setjmp() must be called in every non-callback function that calls a
	 * PNG-reading libpng function */
#ifdef PNG_SETJMP_SUPPORTED
	if (setjmp(jbw.jmpbuf)) {
		gd_error("gd-png error: setjmp returns error condition 1\n");
		png_destroy_read_struct (&png_ptr, &info_ptr, NULL);

		return NULL;
	}
#endif

	png_set_sig_bytes (png_ptr, 8);	/* we already read the 8 signature bytes */

	png_set_read_fn (png_ptr, (void *) infile, gdPngReadData);
	png_read_info (png_ptr, info_ptr);	/* read all PNG info up to image data */

	png_get_IHDR (png_ptr, info_ptr, &width, &height, &bit_depth, &color_type, &interlace_type, NULL, NULL);
	if ((color_type == PNG_COLOR_TYPE_RGB) || (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
	        || color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {
		im = gdImageCreateTrueColor ((int) width, (int) height);
	} else {
		im = gdImageCreate ((int) width, (int) height);
	}
	if (im == NULL) {
		gd_error("gd-png error: cannot allocate gdImage struct\n");
		png_destroy_read_struct (&png_ptr, &info_ptr, NULL);

		return NULL;
	}

	if (bit_depth == 16) {
		png_set_strip_16 (png_ptr);
	} else if (bit_depth < 8) {
		png_set_packing (png_ptr);	/* expand to 1 byte per pixel */
	}

	/* setjmp() must be called in every non-callback function that calls a
	 * PNG-reading libpng function
	 */
#ifdef PNG_SETJMP_SUPPORTED
	if (setjmp(jbw.jmpbuf)) {
		gd_error("gd-png error: setjmp returns error condition 2\n");
		png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
		gdFree(image_data);
		gdFree(row_pointers);
		if (im) {
			gdImageDestroy(im);
		}
		return NULL;
	}
#endif

#ifdef PNG_pHYs_SUPPORTED
	/* check if the resolution is specified */
	if (png_get_valid(png_ptr, info_ptr, PNG_INFO_pHYs)) {
		if (png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, &unit_type)) {
			switch (unit_type) {
			case PNG_RESOLUTION_METER:
				im->res_x = DPM2DPI(res_x);
				im->res_y = DPM2DPI(res_y);
				break;
			}
		}
	}
#endif

	switch (color_type) {
	case PNG_COLOR_TYPE_PALETTE:
		png_get_PLTE (png_ptr, info_ptr, &palette, &num_palette);
#ifdef DEBUG
		gd_error("gd-png color_type is palette, colors: %d\n", num_palette);
#endif /* DEBUG */
		if (png_get_valid (png_ptr, info_ptr, PNG_INFO_tRNS)) {
			/* gd 2.0: we support this rather thoroughly now. Grab the
			 * first fully transparent entry, if any, as the value of
			 * the simple-transparency index, mostly for backwards
			 * binary compatibility. The alpha channel is where it's
			 * really at these days.
			 */
			int firstZero = 1;
			png_get_tRNS (png_ptr, info_ptr, &trans, &num_trans, NULL);
			for (i = 0; i < num_trans; ++i) {
				im->alpha[i] = gdAlphaMax - (trans[i] >> 1);
				if ((trans[i] == 0) && (firstZero)) {
					/* 2.0.5: long-forgotten patch from Wez Furlong */
					transparent = i;
					firstZero = 0;
				}
			}
		}
		break;

	case PNG_COLOR_TYPE_GRAY:
		/* create a fake palette and check for single-shade transparency */
		if ((palette = (png_colorp) gdMalloc (256 * sizeof (png_color))) == NULL) {
			gd_error("gd-png error: cannot allocate gray palette\n");
			png_destroy_read_struct (&png_ptr, &info_ptr, NULL);
			gdImageDestroy(im);
			return NULL;
		}
		palette_allocated = TRUE;
		if (bit_depth < 8) {
			num_palette = 1 << bit_depth;
			for (i = 0; i < 256; ++i) {
				j = (255 * i) / (num_palette - 1);
				palette[i].red = palette[i].green = palette[i].blue = j;
			}
		} else {
			num_palette = 256;
			for (i = 0; i < 256; ++i) {
				palette[i].red = palette[i].green = palette[i].blue = i;
			}
		}
		if (png_get_valid (png_ptr, info_ptr, PNG_INFO_tRNS)) {
			png_get_tRNS (png_ptr, info_ptr, NULL, NULL, &trans_gray_rgb);
			if (bit_depth == 16) {	/* png_set_strip_16() not yet in effect */
				transparent = trans_gray_rgb->gray >> 8;
			} else {
				transparent = trans_gray_rgb->gray;
			}
			/* Note slight error in 16-bit case:  up to 256 16-bit shades
			 * may get mapped to a single 8-bit shade, and only one of them
			 * is supposed to be transparent.  IOW, both opaque pixels and
			 * transparent pixels will be mapped into the transparent entry.
			 * There is no particularly good way around this in the case
			 * that all 256 8-bit shades are used, but one could write some
			 * custom 16-bit code to handle the case where there are gdFree
			 * palette entries.  This error will be extremely rare in
			 * general, though.  (Quite possibly there is only one such
			 * image in existence.) */
		}
		break;

	case PNG_COLOR_TYPE_GRAY_ALPHA:
		png_set_gray_to_rgb(png_ptr);

	case PNG_COLOR_TYPE_RGB:
	case PNG_COLOR_TYPE_RGB_ALPHA:
		/* gd 2.0: we now support truecolor. See the comment above
		   for a rare situation in which the transparent pixel may not
		   work properly with 16-bit channels. */
		if (png_get_valid (png_ptr, info_ptr, PNG_INFO_tRNS)) {
			png_get_tRNS (png_ptr, info_ptr, NULL, NULL, &trans_color_rgb);
			if (bit_depth == 16) {	/* png_set_strip_16() not yet in effect */
				transparent = gdTrueColor (trans_color_rgb->red >> 8,
				                           trans_color_rgb->green >> 8,
				                           trans_color_rgb->blue >> 8);
			} else {
				transparent = gdTrueColor (trans_color_rgb->red,
				                           trans_color_rgb->green,
				                           trans_color_rgb->blue);
			}
		}
		break;
	default:
		gd_error("gd-png color_type is unknown: %d\n", color_type);
		png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
		gdFree(image_data);
		gdFree(row_pointers);
		if (im) {
			gdImageDestroy(im);
		}
		return NULL;
		break;
	}

	png_read_update_info (png_ptr, info_ptr);

	/* allocate space for the PNG image data */
	rowbytes = png_get_rowbytes (png_ptr, info_ptr);
	if (overflow2(rowbytes, height)) {
		png_destroy_read_struct (&png_ptr, &info_ptr, NULL);
		if (im) {
			gdImageDestroy(im);
		}
		if (palette_allocated) {
			gdFree (palette);
		}
		return NULL;
	}
	image_data = (png_bytep) gdMalloc (rowbytes * height);
	if (!image_data) {
		gd_error("gd-png error: cannot allocate image data\n");
		png_destroy_read_struct (&png_ptr, &info_ptr, NULL);
		if (im) {
			gdImageDestroy(im);
		}
		if (palette_allocated) {
			gdFree (palette);
		}
		return NULL;
	}
	if (overflow2(height, sizeof (png_bytep))) {
		png_destroy_read_struct (&png_ptr, &info_ptr, NULL);
		gdFree (image_data);
		if (im) {
			gdImageDestroy(im);
		}
		if (palette_allocated) {
			gdFree (palette);
		}
		return NULL;
	}

	row_pointers = (png_bytepp) gdMalloc (height * sizeof (png_bytep));
	if (!row_pointers) {
		gd_error("gd-png error: cannot allocate row pointers\n");
		png_destroy_read_struct (&png_ptr, &info_ptr, NULL);
		if (im) {
			gdImageDestroy(im);
		}
		gdFree (image_data);
		if (palette_allocated) {
			gdFree (palette);
		}
		return NULL;
	}

	/* set the individual row_pointers to point at the correct offsets */
	for (h = 0; h < height; ++h) {
		row_pointers[h] = image_data + h * rowbytes;
	}

	png_read_image (png_ptr, row_pointers);	/* read whole image... */
	png_read_end (png_ptr, NULL);	/* ...done! */

	if (!im->trueColor) {
		im->colorsTotal = num_palette;
		/* load the palette and mark all entries "open" (unused) for now */
		open = im->open;
		for (i = 0; i < num_palette; ++i) {
			im->red[i] = palette[i].red;
			im->green[i] = palette[i].green;
			im->blue[i] = palette[i].blue;
			open[i] = 1;
		}
		for (i = num_palette; i < gdMaxColors; ++i) {
			open[i] = 1;
		}
	}
	/* 2.0.12: Slaven Rezic: palette images are not the only images
	   with a simple transparent color setting */
	im->transparent = transparent;
	im->interlace = (interlace_type == PNG_INTERLACE_ADAM7);

	/* can't nuke structs until done with palette */
	png_destroy_read_struct (&png_ptr, &info_ptr, NULL);
	switch (color_type) {
	case PNG_COLOR_TYPE_RGB:
		for (h = 0; h < height; h++) {
			int boffset = 0;
			for (w = 0; w < width; w++) {
				register png_byte r = row_pointers[h][boffset++];
				register png_byte g = row_pointers[h][boffset++];
				register png_byte b = row_pointers[h][boffset++];
				im->tpixels[h][w] = gdTrueColor (r, g, b);
			}
		}
		break;

	case PNG_COLOR_TYPE_GRAY_ALPHA:
	case PNG_COLOR_TYPE_RGB_ALPHA:
		for (h = 0; h < height; h++) {
			int boffset = 0;
			for (w = 0; w < width; w++) {
				register png_byte r = row_pointers[h][boffset++];
				register png_byte g = row_pointers[h][boffset++];
				register png_byte b = row_pointers[h][boffset++];

				/* gd has only 7 bits of alpha channel resolution, and
				 * 127 is transparent, 0 opaque. A moment of convenience,
				 *  a lifetime of compatibility.
				 */

				register png_byte a = gdAlphaMax - (row_pointers[h][boffset++] >> 1);
				im->tpixels[h][w] = gdTrueColorAlpha(r, g, b, a);
			}
		}
		break;
	default:
		/* Palette image, or something coerced to be one */
		for (h = 0; h < height; ++h) {
			for (w = 0; w < width; ++w) {
				register png_byte idx = row_pointers[h][w];
				im->pixels[h][w] = idx;
				open[idx] = 0;
			}
		}
	}
#ifdef DEBUG
	if (!im->trueColor) {
		for (i = num_palette; i < gdMaxColors; ++i) {
			if (!open[i]) {
				fprintf (stderr,
				         "gd-png warning: image data references out-of-range"
				         " color index (%d)\n", i);
			}
		}
	}
#endif

	if (palette_allocated) {
		gdFree (palette);
	}
	gdFree (image_data);
	gdFree (row_pointers);

	return im;
}


/*
  Function: gdImagePngEx

    <gdImagePngEx> outputs the specified image to the specified file in
    PNG format. The file must be open for writing. Under MSDOS and all
    versions of Windows, it is important to use "wb" as opposed to
    simply "w" as the mode when opening the file, and under Unix there
    is no penalty for doing so. <gdImagePngEx> does not close the file;
    your code must do so.

    In addition, <gdImagePngEx> allows the level of compression to be
    specified. A compression level of 0 means "no compression." A
    compression level of 1 means "compressed, but as quickly as
    possible." A compression level of 9 means "compressed as much as
    possible to produce the smallest possible file." A compression level
    of -1 will use the default compression level at the time zlib was
    compiled on your system.

  Variants:

    <gdImagePng> is equivalent to calling <gdImagePngEx> with
    compression of -1.

    <gdImagePngCtx> and <gdImagePngCtxEx> write via a <gdIOCtx>
    instead of a file handle.

    <gdImagePngPtr> and <gdImagePngPtrEx> store the image file to
    memory.

  Parameters:

    im      - the image to write
    outFile - the output FILE* object.
    level   - compression level: 0 -> none, 1-9 -> level, -1 -> default

  Returns:

    Nothing.

  Example:

    > gdImagePtr im;
    > int black, white;
    > FILE *out;
    > 
    > im = gdImageCreate(100, 100);              // Create the image 
    > white = gdImageColorAllocate(im, 255, 255, 255); // Alloc background 
    > black = gdImageColorAllocate(im, 0, 0, 0); // Allocate drawing color 
    > gdImageRectangle(im, 0, 0, 99, 99, black); // Draw rectangle 
    > out = fopen("rect.png", "wb");             // Open output file (binary) 
    > gdImagePngEx(im, out, 9);                  // Write PNG, max compression
    > fclose(out);                               // Close file 
    > gdImageDestroy(im);                        // Destroy image 

*/
BGD_DECLARE(void) gdImagePngEx (gdImagePtr im, FILE * outFile, int level)
{
	gdIOCtx *out = gdNewFileCtx (outFile);
	if (out == NULL) return;
	gdImagePngCtxEx (im, out, level);
	out->gd_free (out);
}

/*
  Function: gdImagePng

    Equivalent to calling <gdImagePngEx> with compression of -1.

  Parameters:

    im      - the image to save.
    outFile - the output FILE*.

  Returns:

    Nothing.
*/
BGD_DECLARE(void) gdImagePng (gdImagePtr im, FILE * outFile)
{
	gdIOCtx *out = gdNewFileCtx (outFile);
	if (out == NULL) return;
	gdImagePngCtxEx (im, out, -1);
	out->gd_free (out);
}


/*
  Function: gdImagePngPtr

    Equivalent to calling <gdImagePngPtrEx> with compression of -1.

    See <gdImagePngEx> for more information.

  Parameters:

    im      - the image to save.
    size    - Output: size in bytes of the result.

  Returns:

    A pointer to memory containing the image data or NULL on error.

*/
BGD_DECLARE(void *) gdImagePngPtr (gdImagePtr im, int *size)
{
	void *rv;
	gdIOCtx *out = gdNewDynamicCtx (2048, NULL);
	if (out == NULL) return NULL;
	gdImagePngCtxEx (im, out, -1);
	rv = gdDPExtractData (out, size);
	out->gd_free (out);
	return rv;
}

/*
  Function: gdImagePngPtrEx

    Identical to <gdImagePngEx> except that it returns a pointer to a
    memory area with the PNG data. This memory must be freed by the
    caller when it is no longer needed. **The caller must invoke
    gdFree(), not free()**

    The 'size' parameter receives the total size of the block of
    memory.

    See <gdImagePngEx> for more information.

  Parameters:

    im      - the image to save.
    size    - Output: size in bytes of the result.
    level   - compression level: 0 -> none, 1-9 -> level, -1 -> default

  Returns:

    A pointer to memory containing the image data or NULL on error.

*/
BGD_DECLARE(void *) gdImagePngPtrEx (gdImagePtr im, int *size, int level)
{
	void *rv;
	gdIOCtx *out = gdNewDynamicCtx (2048, NULL);
	if (out == NULL) return NULL;
	gdImagePngCtxEx (im, out, level);
	rv = gdDPExtractData (out, size);
	out->gd_free (out);
	return rv;
}



/*
  Function: gdImagePngCtx

    Equivalent to calling <gdImagePngCtxEx> with compression of -1.
    See <gdImagePngEx> for more information.

  Parameters:

    im      - the image to save.
    outfile - the <gdIOCtx> to write to.

  Returns:

    Nothing.

*/
BGD_DECLARE(void) gdImagePngCtx (gdImagePtr im, gdIOCtx * outfile)
{
	/* 2.0.13: 'return' here was an error, thanks to Kevin Smith */
	gdImagePngCtxEx (im, outfile, -1);
}




/*
  Function: gdImagePngCtxEx

    Outputs the given image as PNG data, but using a <gdIOCtx> instead
    of a file.  See <gdIamgePnEx>.

  Parameters:

    im      - the image to save.
    outfile - the <gdIOCtx> to write to.
    level   - compression level: 0 -> none, 1-9 -> level, -1 -> default

  Returns:

    Nothing.

*/

/* This routine is based in part on code from Dale Lutz (Safe Software Inc.)
 *  and in part on demo code from Chapter 15 of "PNG: The Definitive Guide"
 *  (http://www.libpng.org/pub/png/book/).
 */
BGD_DECLARE(void) gdImagePngCtxEx (gdImagePtr im, gdIOCtx * outfile, int level)
{
	int i, j, bit_depth = 0, interlace_type;
	int width = im->sx;
	int height = im->sy;
	int colors = im->colorsTotal;
	int *open = im->open;
	int mapping[gdMaxColors];	/* mapping[gd_index] == png_index */
	png_byte trans_values[256];
	png_color_16 trans_rgb_value;
	png_color palette[gdMaxColors];
	png_structp png_ptr;
	png_infop info_ptr;
	volatile int transparent = im->transparent;
	volatile int remap = FALSE;
#ifdef PNG_SETJMP_SUPPORTED
	jmpbuf_wrapper jbw;
#endif

	/* width or height of value 0 is invalid in IHDR;
	   see http://www.w3.org/TR/PNG-Chunks.html */
	if (width == 0 || height ==0) return;

#ifdef PNG_SETJMP_SUPPORTED
	png_ptr = png_create_write_struct (PNG_LIBPNG_VER_STRING,
	                                   &jbw, gdPngErrorHandler,
	                                   NULL);
#else
	png_ptr = png_create_write_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
#endif
	if (png_ptr == NULL) {
		gd_error("gd-png error: cannot allocate libpng main struct\n");
		return;
	}

	info_ptr = png_create_info_struct (png_ptr);
	if (info_ptr == NULL) {
		gd_error("gd-png error: cannot allocate libpng info struct\n");
		png_destroy_write_struct (&png_ptr, (png_infopp) NULL);
		return;
	}

#ifdef PNG_SETJMP_SUPPORTED
	if (setjmp(jbw.jmpbuf)) {
		gd_error("gd-png error: setjmp returns error condition\n");
		png_destroy_write_struct (&png_ptr, &info_ptr);
		return;
	}
#endif

	png_set_write_fn (png_ptr, (void *) outfile, gdPngWriteData,
	                  gdPngFlushData);

	/* This is best for palette images, and libpng defaults to it for
	   palette images anyway, so we don't need to do it explicitly.
	   What to ideally do for truecolor images depends, alas, on the image.
	   gd is intentionally imperfect and doesn't spend a lot of time
	   fussing with such things. */

	/* Faster if this is uncommented, but may produce larger truecolor files.
	   Wait for gdImagePngCtxEx. */
#if 0
	png_set_filter (png_ptr, 0, PNG_FILTER_NONE);
#endif

	/* 2.0.12: this is finally a parameter */
	png_set_compression_level (png_ptr, level);

#ifdef PNG_pHYs_SUPPORTED
	/* 2.1.0: specify the resolution */
	png_set_pHYs(png_ptr, info_ptr, DPI2DPM(im->res_x), DPI2DPM(im->res_y),
	             PNG_RESOLUTION_METER);
#endif

	/* can set this to a smaller value without compromising compression if all
	 * image data is 16K or less; will save some decoder memory [min == 8] */
	/*  png_set_compression_window_bits(png_ptr, 15);  */

	if (!im->trueColor) {
		if (transparent >= im->colorsTotal ||
		        (transparent >= 0 && open[transparent]))
			transparent = -1;
	}
	if (!im->trueColor) {
		for (i = 0; i < gdMaxColors; ++i)
			mapping[i] = -1;
	}
	if (!im->trueColor) {
		/* count actual number of colors used (colorsTotal == high-water mark) */
		colors = 0;
		for (i = 0; i < im->colorsTotal; ++i) {
			if (!open[i]) {
				mapping[i] = colors;
				++colors;
			}
		}
		if (colors == 0) {
			gd_error("gd-png error: no colors in palette\n");
			goto bail;
		}
		if (colors < im->colorsTotal) {
			remap = TRUE;
		}
		if (colors <= 2)
			bit_depth = 1;
		else if (colors <= 4)
			bit_depth = 2;
		else if (colors <= 16)
			bit_depth = 4;
		else
			bit_depth = 8;
	}
	interlace_type = im->interlace ? PNG_INTERLACE_ADAM7 : PNG_INTERLACE_NONE;

	if (im->trueColor) {
		if (im->saveAlphaFlag) {
			png_set_IHDR (png_ptr, info_ptr, width, height, 8,
			              PNG_COLOR_TYPE_RGB_ALPHA, interlace_type,
			              PNG_COMPRESSION_TYPE_DEFAULT,
			              PNG_FILTER_TYPE_DEFAULT);
		} else {
			png_set_IHDR (png_ptr, info_ptr, width, height, 8,
			              PNG_COLOR_TYPE_RGB, interlace_type,
			              PNG_COMPRESSION_TYPE_DEFAULT,
			              PNG_FILTER_TYPE_DEFAULT);
		}
	} else {
		png_set_IHDR (png_ptr, info_ptr, width, height, bit_depth,
		              PNG_COLOR_TYPE_PALETTE, interlace_type,
		              PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
	}
	if (im->trueColor && (!im->saveAlphaFlag) && (transparent >= 0)) {
		/* 2.0.9: fixed by Thomas Winzig */
		trans_rgb_value.red = gdTrueColorGetRed (im->transparent);
		trans_rgb_value.green = gdTrueColorGetGreen (im->transparent);
		trans_rgb_value.blue = gdTrueColorGetBlue (im->transparent);
		png_set_tRNS (png_ptr, info_ptr, 0, 0, &trans_rgb_value);
	}
	if (!im->trueColor) {
		/* Oy veh. Remap the PNG palette to put the
		   entries with interesting alpha channel
		   values first. This minimizes the size
		   of the tRNS chunk and thus the size
		   of the PNG file as a whole. */
		int tc = 0;
		int i;
		int j;
		int k;
		for (i = 0; (i < im->colorsTotal); i++) {
			if ((!im->open[i]) && (im->alpha[i] != gdAlphaOpaque)) {
				tc++;
			}
		}
		if (tc) {
#if 0
			for (i = 0; (i < im->colorsTotal); i++) {
				trans_values[i] = 255 -
				                  ((im->alpha[i] << 1) + (im->alpha[i] >> 6));
			}
			png_set_tRNS (png_ptr, info_ptr, trans_values, 256, NULL);
#endif
			if (!remap) {
				remap = TRUE;
			}
			/* (Semi-)transparent indexes come up from the bottom
			   of the list of real colors; opaque
			   indexes come down from the top */
			j = 0;
			k = colors - 1;
			for (i = 0; (i < im->colorsTotal); i++) {
				if (!im->open[i]) {
					if (im->alpha[i] != gdAlphaOpaque) {
						/* Andrew Hull: >> 6, not >> 7! (gd 2.0.5) */
						trans_values[j] = 255 -
						                  ((im->alpha[i] << 1) + (im->alpha[i] >> 6));
						mapping[i] = j++;
					} else {
						mapping[i] = k--;
					}
				}
			}
			png_set_tRNS (png_ptr, info_ptr, trans_values, tc, NULL);
		}
	}

	/* convert palette to libpng layout */
	if (!im->trueColor) {
		if (remap)
			for (i = 0; i < im->colorsTotal; ++i) {
				if (mapping[i] < 0)
					continue;
				palette[mapping[i]].red = im->red[i];
				palette[mapping[i]].green = im->green[i];
				palette[mapping[i]].blue = im->blue[i];
			}
		else
			for (i = 0; i < colors; ++i) {
				palette[i].red = im->red[i];
				palette[i].green = im->green[i];
				palette[i].blue = im->blue[i];
			}
		png_set_PLTE (png_ptr, info_ptr, palette, colors);
	}

	/* write out the PNG header info (everything up to first IDAT) */
	png_write_info (png_ptr, info_ptr);

	/* make sure < 8-bit images are packed into pixels as tightly as possible */
	png_set_packing (png_ptr);

	/* This code allocates a set of row buffers and copies the gd image data
	 * into them only in the case that remapping is necessary; in gd 1.3 and
	 * later, the im->pixels array is laid out identically to libpng's row
	 * pointers and can be passed to png_write_image() function directly.
	 * The remapping case could be accomplished with less memory for non-
	 * interlaced images, but interlacing causes some serious complications. */
	if (im->trueColor) {
		/* performance optimizations by Phong Tran */
		int channels = im->saveAlphaFlag ? 4 : 3;
		/* Our little 7-bit alpha channel trick costs us a bit here. */
		png_bytep *row_pointers;
		unsigned char *pOutputRow;
		int **ptpixels = im->tpixels;
		int *pThisRow;
		unsigned char a;
		int thisPixel;
		png_bytep *prow_pointers;
		int saveAlphaFlag = im->saveAlphaFlag;
		if (overflow2(sizeof (png_bytep), height)) {
			goto bail;
		}
		row_pointers = gdMalloc (sizeof (png_bytep) * height);
		if (row_pointers == NULL) {
			gd_error("gd-png error: unable to allocate row_pointers\n");
			goto bail;
		}
		prow_pointers = row_pointers;
		for (j = 0; j < height; ++j) {
			if (overflow2(width, channels) || ((*prow_pointers =
			                                        (png_bytep) gdMalloc (width * channels)) == NULL)) {
				gd_error("gd-png error: unable to allocate rows\n");
				for (i = 0; i < j; ++i)
					gdFree (row_pointers[i]);
				/* 2.0.29: memory leak TBB */
				gdFree(row_pointers);
				goto bail;
			}
			pOutputRow = *prow_pointers++;
			pThisRow = *ptpixels++;
			for (i = 0; i < width; ++i) {
				thisPixel = *pThisRow++;
				*pOutputRow++ = gdTrueColorGetRed (thisPixel);
				*pOutputRow++ = gdTrueColorGetGreen (thisPixel);
				*pOutputRow++ = gdTrueColorGetBlue (thisPixel);

				if (saveAlphaFlag) {
					/* convert the 7-bit alpha channel to an 8-bit alpha channel.
					   We do a little bit-flipping magic, repeating the MSB
					   as the LSB, to ensure that 0 maps to 0 and
					   127 maps to 255. We also have to invert to match
					   PNG's convention in which 255 is opaque. */
					a = gdTrueColorGetAlpha (thisPixel);
					/* Andrew Hull: >> 6, not >> 7! (gd 2.0.5) */
					*pOutputRow++ = 255 - ((a << 1) + (a >> 6));
				}
			}
		}

		png_write_image (png_ptr, row_pointers);
		png_write_end (png_ptr, info_ptr);

		for (j = 0; j < height; ++j)
			gdFree (row_pointers[j]);
		gdFree (row_pointers);
	} else {
		if (remap) {
			png_bytep *row_pointers;
			if (overflow2(sizeof (png_bytep), height)) {
				goto bail;
			}
			row_pointers = gdMalloc (sizeof (png_bytep) * height);
			if (row_pointers == NULL) {
				gd_error("gd-png error: unable to allocate row_pointers\n");
				goto bail;
			}
			for (j = 0; j < height; ++j) {
				if ((row_pointers[j] = (png_bytep) gdMalloc (width)) == NULL) {
					gd_error("gd-png error: unable to allocate rows\n");
					for (i = 0; i < j; ++i)
						gdFree (row_pointers[i]);
					/* TBB: memory leak */
					gdFree (row_pointers);
					goto bail;
				}
				for (i = 0; i < width; ++i)
					row_pointers[j][i] = mapping[im->pixels[j][i]];
			}

			png_write_image (png_ptr, row_pointers);
			png_write_end (png_ptr, info_ptr);

			for (j = 0; j < height; ++j)
				gdFree (row_pointers[j]);
			gdFree (row_pointers);
		} else {
			png_write_image (png_ptr, im->pixels);
			png_write_end (png_ptr, info_ptr);
		}
	}
	/* 1.6.3: maybe we should give that memory BACK! TBB */
bail:
	png_destroy_write_struct (&png_ptr, &info_ptr);
}


#endif /* HAVE_LIBPNG */