png.h in git
PNG(3)                 FreeBSD Library Functions Manual                 PNG(3)

NAME
     png – basic PNG output

SYNOPSIS
     #include <png.h>

     void
     pngHead(FILE *file, uint32_t width, uint32_t height, uint8_t depth,
         uint8_t color);

     void
     pngPalette(FILE *file, const uint8_t *pal, uint32_t len);

     void
     pngData(FILE *file, const uint8_t *data, uint32_t len);

     void
     pngTail(FILE *file);

DESCRIPTION
     The pngHead() function writes the IHDR chunk to file.  The color
     parameter can be one of PNGGrayscale, PNGTruecolor optionally or'ed with
     PNGAlpha, or PNGIndexed.

     The pngPalette() function writes the PLTE chunk to file.

     The pngData() function writes the IDAT chunk to file without compression.
     The constants PNGNone, PNGSub, PNGUp, PNGAverage, PNGPaeth are defined
     for use in PNG data.

     The pngTail() function writes the IEND chunk to file.

ERRORS
     Any errors from writing to file are handled by calling err(3).

FreeBSD 12.0-RELEASE-p10         July 25, 2019        FreeBSD 12.0-RELEASE-p10
/* Copyright (C) 2018  C. McEnroe <june@causal.agency>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <err.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sysexits.h>

static inline uint32_t pngCRCTable(uint8_t n) {
	static uint32_t table[256];
	if (table[1]) return table[n];
	for (int i = 0; i < 256; ++i) {
		table[i] = i;
		for (int j = 0; j < 8; ++j) {
			table[i] = (table[i] >> 1) ^ (table[i] & 1 ? 0xEDB88320 : 0);
		}
	}
	return table[n];
}

static uint32_t pngCRC;

static inline void pngWrite(FILE *file, const uint8_t *ptr, uint32_t len) {
	if (!fwrite(ptr, len, 1, file)) err(EX_IOERR, "pngWrite");
	for (uint32_t i = 0; i < len; ++i) {
		pngCRC = pngCRCTable(pngCRC ^ ptr[i]) ^ (pngCRC >> 8);
	}
}
static inline void pngInt32(FILE *file, uint32_t n) {
	pngWrite(file, (uint8_t []) { n >> 24, n >> 16, n >> 8, n }, 4);
}
static inline void pngChunk(FILE *file, char type[static 4], uint32_t len) {
	pngInt32(file, len);
	pngCRC = ~0;
	pngWrite(file, (uint8_t *)type, 4);
}

enum {
	PNGGrayscale,
	PNGTruecolor = 2,
	PNGIndexed,
	PNGAlpha,
};

static inline void pngHead(
	FILE *file, uint32_t width, uint32_t height, uint8_t depth, uint8_t color
) {
	pngWrite(file, (uint8_t *)"\x89PNG\r\n\x1A\n", 8);
	pngChunk(file, "IHDR", 13);
	pngInt32(file, width);
	pngInt32(file, height);
	pngWrite(file, &depth, 1);
	pngWrite(file, &color, 1);
	pngWrite(file, (uint8_t []) { 0, 0, 0 }, 3);
	pngInt32(file, ~pngCRC);
}

static inline void pngPalette(FILE *file, const uint8_t *pal, uint32_t len) {
	pngChunk(file, "PLTE", len);
	pngWrite(file, pal, len);
	pngInt32(file, ~pngCRC);
}

enum {
	PNGNone,
	PNGSub,
	PNGUp,
	PNGAverage,
	PNGPaeth,
};

static inline void pngData(FILE *file, const uint8_t *data, uint32_t len) {
	uint32_t adler1 = 1, adler2 = 0;
	for (uint32_t i = 0; i < len; ++i) {
		adler1 = (adler1 + data[i]) % 65521;
		adler2 = (adler1 + adler2) % 65521;
	}
	uint32_t zlen = 2 + 5 * ((len + 0xFFFE) / 0xFFFF) + len + 4;
	pngChunk(file, "IDAT", zlen);
	pngWrite(file, (uint8_t []) { 0x08, 0x1D }, 2);
	for (; len > 0xFFFF; data += 0xFFFF, len -= 0xFFFF) {
		pngWrite(file, (uint8_t []) { 0x00, 0xFF, 0xFF, 0x00, 0x00 }, 5);
		pngWrite(file, data, 0xFFFF);
	}
	pngWrite(file, (uint8_t []) { 0x01, len, len >> 8, ~len, ~len >> 8 }, 5);
	pngWrite(file, data, len);
	pngInt32(file, adler2 << 16 | adler1);
	pngInt32(file, ~pngCRC);
}

static inline void pngTail(FILE *file) {
	pngChunk(file, "IEND", 0);
	pngInt32(file, ~pngCRC);
}