PNGO(1) General Commands Manual PNGO(1)

pngo — PNG optimizer

pngo [-acgv] [-b depth] [-o file] [file ...]

pngo optimizes PNG files for size by performing the following:

  1. Discard ancillary chunks.
  2. Discard unnecessary alpha channel.
  3. Convert unnecessary truecolor to grayscale.
  4. Palletize color if possible.
  5. Reduce unnecessary bit depth.
  6. Apply a simple filter type heuristic.
  7. Apply zlib's best compression.

The arguments are as follows:

Always discard the alpha channel.
depth
Reduce bit depth to depth or lower.
Write to standard output.
Convert to grayscale.
file
Write to file.
Print header information and sizes to standard error.

glitch(1)

pngo does not support interlaced PNGs.

September 21, 2021 OpenBSD 7.4

pngo.c in git 

/* Copyright (C) 2018, 2021  June 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 <inttypes.h>
#include <limits.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <zlib.h>

#define ARRAY_LEN(a) (sizeof(a) / sizeof(a[0]))

static bool verbose;
static const char *path;
static FILE *file;
static uint32_t crc;

static void pngRead(void *ptr, size_t len, const char *desc) {
	size_t n = fread(ptr, len, 1, file);
	if (!n && ferror(file)) err(1, "%s", path);
	if (!n) errx(1, "%s: missing %s", path, desc);
	crc = crc32(crc, ptr, len);
}

static void pngWrite(const void *ptr, size_t len) {
	size_t n = fwrite(ptr, len, 1, file);
	if (!n) err(1, "%s", path);
	crc = crc32(crc, ptr, len);
}

static const uint8_t Sig[8] = "\x89PNG\r\n\x1A\n";

static void sigRead(void) {
	uint8_t sig[sizeof(Sig)];
	pngRead(sig, sizeof(sig), "signature");
	if (memcmp(sig, Sig, sizeof(sig))) {
		errx(1, "%s: invalid signature", path);
	}
}

static void sigWrite(void) {
	pngWrite(Sig, sizeof(Sig));
}

static uint32_t u32Read(const char *desc) {
	uint8_t b[4];
	pngRead(b, sizeof(b), desc);
	return (uint32_t)b[0] << 24 | (uint32_t)b[1] << 16
		| (uint32_t)b[2] << 8 | (uint32_t)b[3];
}

static void u32Write(uint32_t x) {
	uint8_t b[4] = { x >> 24 & 0xFF, x >> 16 & 0xFF, x >> 8 & 0xFF, x & 0xFF };
	pngWrite(b, sizeof(b));
}

struct Chunk {
	uint32_t len;
	char type[5];
};

static struct Chunk chunkRead(void) {
	struct Chunk chunk;
	chunk.len = u32Read("chunk length");
	crc = crc32(0, Z_NULL, 0);
	pngRead(chunk.type, 4, "chunk type");
	chunk.type[4] = 0;
	return chunk;
}

static void chunkWrite(struct Chunk chunk) {
	u32Write(chunk.len);
	crc = crc32(0, Z_NULL, 0);
	pngWrite(chunk.type, 4);
}

static void crcRead(void) {
	uint32_t expect = crc;
	uint32_t actual = u32Read("CRC32");
	if (actual == expect) return;
	errx(1, "%s: expected CRC32 %08X, found %08X", path, expect, actual);
}

static void crcWrite(void) {
	u32Write(crc);
}

static void chunkSkip(struct Chunk chunk) {
	if (!(chunk.type[0] & 0x20)) {
		errx(1, "%s: unsupported critical chunk %s", path, chunk.type);
	}
	uint8_t buf[4096];
	while (chunk.len > sizeof(buf)) {
		pngRead(buf, sizeof(buf), "chunk data");
		chunk.len -= sizeof(buf);
	}
	if (chunk.len) pngRead(buf, chunk.len, "chunk data");
	crcRead();
}

enum Color {
	Grayscale = 0,
	Truecolor = 2,
	Indexed = 3,
	GrayscaleAlpha = 4,
	TruecolorAlpha = 6,
};
enum Compression {
	Deflate,
};
enum FilterMethod {
	Adaptive,
};
enum Interlace {
	Progressive,
	Adam7,
};

enum { HeaderLen = 13 };
static struct {
	uint32_t width;
	uint32_t height;
	uint8_t depth;
	uint8_t color;
	uint8_t compression;
	uint8_t filter;
	uint8_t interlace;
} header;

static size_t pixelLen;
static size_t lineLen;
static size_t dataLen;

static void recalc(void) {
	size_t pixelBits = header.depth;
	switch (header.color) {
		break; case GrayscaleAlpha: pixelBits *= 2;
		break; case Truecolor: pixelBits *= 3;
		break; case TruecolorAlpha: pixelBits *= 4;
	}
	pixelLen = (pixelBits + 7) / 8;
	lineLen = (header.width * pixelBits + 7) / 8;
	dataLen = (1 + lineLen) * header.height;
}

static void headerPrint(void) {
	static const char *String[] = {
		[Grayscale] = "grayscale",
		[Truecolor] = "truecolor",
		[Indexed] = "indexed",
		[GrayscaleAlpha] = "grayscale alpha",
		[TruecolorAlpha] = "truecolor alpha",
	};
	fprintf(
		stderr, "%s: %" PRIu32 "x%" PRIu32 " %" PRIu8 "-bit %s\n",
		path, header.width, header.height, header.depth, String[header.color]
	);
}

static void headerRead(struct Chunk chunk) {
	if (chunk.len != HeaderLen) {
		errx(
			1, "%s: expected %s length %" PRIu32 ", found %" PRIu32,
			path, chunk.type, (uint32_t)HeaderLen, chunk.len
		);
	}
	header.width = u32Read("header width");
	header.height = u32Read("header height");
	pngRead(&header.depth, 1, "header depth");
	pngRead(&header.color, 1, "header color");
	pngRead(&header.compression, 1, "header compression");
	pngRead(&header.filter, 1, "header filter");
	pngRead(&header.interlace, 1, "header interlace");
	crcRead();
	recalc();

	if (!header.width) errx(1, "%s: invalid width 0", path);
	if (!header.height) errx(1, "%s: invalid height 0", path);
	static const struct {
		uint8_t color;
		uint8_t depth;
	} Valid[] = {
		{ Grayscale, 1 },
		{ Grayscale, 2 },
		{ Grayscale, 4 },
		{ Grayscale, 8 },
		{ Grayscale, 16 },
		{ Truecolor, 8 },
		{ Truecolor, 16 },
		{ Indexed, 1 },
		{ Indexed, 2 },
		{ Indexed, 4 },
		{ Indexed, 8 },
		{ Indexed, 16 },
		{ GrayscaleAlpha, 8 },
		{ GrayscaleAlpha, 16 },
		{ TruecolorAlpha, 8 },
		{ TruecolorAlpha, 16 },
	};
	bool valid = false;
	for (size_t i = 0; i < ARRAY_LEN(Valid); ++i) {
		valid = (
			header.color == Valid[i].color &&
			header.depth == Valid[i].depth
		);
		if (valid) break;
	}
	if (!valid) {
		errx(
			1, "%s: invalid color type %" PRIu8 " and bit depth %" PRIu8,
			path, header.color, header.depth
		);
	}
	if (header.compression != Deflate) {
		errx(
			1, "%s: invalid compression method %" PRIu8,
			path, header.compression
		);
	}
	if (header.filter != Adaptive) {
		errx(1, "%s: invalid filter method %" PRIu8, path, header.filter);
	}
	if (header.interlace > Adam7) {
		errx(1, "%s: invalid interlace method %" PRIu8, path, header.interlace);
	}

	if (verbose) headerPrint();
}

static void headerWrite(void) {
	if (verbose) headerPrint();

	struct Chunk ihdr = { HeaderLen, "IHDR" };
	chunkWrite(ihdr);
	u32Write(header.width);
	u32Write(header.height);
	pngWrite(&header.depth, 1);
	pngWrite(&header.color, 1);
	pngWrite(&header.compression, 1);
	pngWrite(&header.filter, 1);
	pngWrite(&header.interlace, 1);
	crcWrite();
}

static struct {
	uint32_t len;
	uint8_t rgb[256][3];
} pal;

static struct {
	uint32_t len;
	uint8_t a[256];
} trans;

static void palClear(void) {
	pal.len = 0;
	trans.len = 0;
}

static uint32_t palIndex(bool alpha, const uint8_t *rgba) {
	uint32_t i;
	for (i = 0; i < pal.len; ++i) {
		if (alpha && i < trans.len && trans.a[i] != rgba[3]) continue;
		if (!memcmp(pal.rgb[i], rgba, 3)) break;
	}
	return i;
}

static bool palAdd(bool alpha, const uint8_t *rgba) {
	uint32_t i = palIndex(alpha, rgba);
	if (i < pal.len) return true;
	if (i == 256) return false;
	memcpy(pal.rgb[i], rgba, 3);
	pal.len++;
	if (alpha) {
		trans.a[i] = rgba[3];
		trans.len++;
	}
	return true;
}

static void transCompact(void) {
	uint32_t i;
	for (i = 0; i < trans.len; ++i) {
		if (trans.a[i] == 0xFF) break;
	}
	if (i == trans.len) return;

	for (uint32_t j = i+1; j < trans.len; ++j) {
		if (trans.a[j] == 0xFF) continue;
		uint8_t a = trans.a[i];
		trans.a[i] = trans.a[j];
		trans.a[j] = a;
		uint8_t rgb[3];
		memcpy(rgb, pal.rgb[i], 3);
		memcpy(pal.rgb[i], pal.rgb[j], 3);
		memcpy(pal.rgb[j], rgb, 3);
		i++;
	}
	trans.len = i;
}

static void palRead(struct Chunk chunk) {
	if (chunk.len % 3) {
		errx(
			1, "%s: %s length %" PRIu32 " not divisible by 3",
			path, chunk.type, chunk.len
		);
	}
	pal.len = chunk.len / 3;
	if (pal.len > 256) {
		errx(1, "%s: %s length %" PRIu32 " > 256", path, chunk.type, pal.len);
	}
	pngRead(pal.rgb, chunk.len, "palette data");
	crcRead();
	if (verbose) {
		fprintf(stderr, "%s: palette length %" PRIu32 "\n", path, pal.len);
	}
}

static void palWrite(void) {
	if (verbose) {
		fprintf(stderr, "%s: palette length %" PRIu32 "\n", path, pal.len);
	}
	struct Chunk plte = { 3 * pal.len, "PLTE" };
	chunkWrite(plte);
	pngWrite(pal.rgb, plte.len);
	crcWrite();
}

static void transRead(struct Chunk chunk) {
	trans.len = chunk.len;
	if (trans.len > 256) {
		errx(1, "%s: %s length %" PRIu32 " > 256", path, chunk.type, trans.len);
	}
	pngRead(trans.a, chunk.len, "transparency data");
	crcRead();
	if (verbose) {
		fprintf(stderr, "%s: trans length %" PRIu32 "\n", path, trans.len);
	}
}

static void transWrite(void) {
	if (verbose) {
		fprintf(stderr, "%s: trans length %" PRIu32 "\n", path, trans.len);
	}
	struct Chunk trns = { trans.len, "tRNS" };
	chunkWrite(trns);
	pngWrite(trans.a, trns.len);
	crcWrite();
}

static uint8_t *data;

static void dataAlloc(void) {
	data = malloc(dataLen);
	if (!data) err(1, "malloc");
}

static const char *humanize(size_t n) {
	static char buf[64];
	if (n >> 10) {
		snprintf(buf, sizeof(buf), "%zuK", n >> 10);
	} else {
		snprintf(buf, sizeof(buf), "%zuB", n);
	}
	return buf;
}

static void dataRead(struct Chunk chunk) {
	if (verbose) {
		fprintf(stderr, "%s: data size %s\n", path, humanize(dataLen));
	}

	z_stream stream = { .next_out = data, .avail_out = dataLen };
	int error = inflateInit(&stream);
	if (error != Z_OK) errx(1, "inflateInit: %s", stream.msg);

	for (;;) {
		if (strcmp(chunk.type, "IDAT")) {
			errx(1, "%s: missing IDAT chunk", path);
		}

		uint8_t *idat = malloc(chunk.len);
		if (!idat) err(1, "malloc");

		pngRead(idat, chunk.len, "image data");
		crcRead();
		
		stream.next_in = idat;
		stream.avail_in = chunk.len;
		error = inflate(&stream, Z_SYNC_FLUSH);
		free(idat);

		if (error == Z_STREAM_END) break;
		if (error != Z_OK) {
			errx(1, "%s: inflate: %s", path, stream.msg);
		}

		chunk = chunkRead();
	}
	inflateEnd(&stream);
	if ((size_t)stream.total_out != dataLen) {
		errx(
			1, "%s: expected data length %zu, found %zu",
			path, dataLen, (size_t)stream.total_out
		);
	}

	if (verbose) {
		fprintf(
			stderr, "%s: deflate size %s\n",
			path, humanize(stream.total_in)
		);
	}
}

static void dataWrite(void) {
	if (verbose) {
		fprintf(stderr, "%s: data size %s\n", path, humanize(dataLen));
	}

	z_stream stream = {
		.next_in = data,
		.avail_in = dataLen,
	};
	int error = deflateInit2(
		&stream, Z_BEST_COMPRESSION, Z_DEFLATED, 15, 8, Z_FILTERED
	);
	if (error != Z_OK) errx(1, "deflateInit2: %s", stream.msg);

	uLong bound = deflateBound(&stream, dataLen);
	uint8_t *buf = malloc(bound);
	if (!buf) err(1, "malloc");

	stream.next_out = buf;
	stream.avail_out = bound;
	deflate(&stream, Z_FINISH);
	deflateEnd(&stream);

	struct Chunk idat = { stream.total_out, "IDAT" };
	chunkWrite(idat);
	pngWrite(buf, stream.total_out);
	crcWrite();
	free(buf);

	struct Chunk iend = { 0, "IEND" };
	chunkWrite(iend);
	crcWrite();

	if (verbose) {
		fprintf(
			stderr, "%s: deflate size %s\n",
			path, humanize(stream.total_out)
		);
	}
}

enum Filter {
	None,
	Sub,
	Up,
	Average,
	Paeth,
	FilterCap,
};

struct Bytes {
	uint8_t x, a, b, c;
};

static uint8_t paethPredictor(struct Bytes f) {
	int32_t p = (int32_t)f.a + (int32_t)f.b - (int32_t)f.c;
	int32_t pa = labs(p - (int32_t)f.a);
	int32_t pb = labs(p - (int32_t)f.b);
	int32_t pc = labs(p - (int32_t)f.c);
	if (pa <= pb && pa <= pc) return f.a;
	if (pb <= pc) return f.b;
	return f.c;
}

static uint8_t recon(enum Filter type, struct Bytes f) {
	switch (type) {
		case None:    return f.x;
		case Sub:     return f.x + f.a;
		case Up:      return f.x + f.b;
		case Average: return f.x + ((uint32_t)f.a + (uint32_t)f.b) / 2;
		case Paeth:   return f.x + paethPredictor(f);
		default: abort();
	}
}

static uint8_t filt(enum Filter type, struct Bytes f) {
	switch (type) {
		case None:    return f.x;
		case Sub:     return f.x - f.a;
		case Up:      return f.x - f.b;
		case Average: return f.x - ((uint32_t)f.a + (uint32_t)f.b) / 2;
		case Paeth:   return f.x - paethPredictor(f);
		default: abort();
	}
}

static uint8_t *lineType(uint32_t y) {
	return &data[y * (1 + lineLen)];
}
static uint8_t *lineData(uint32_t y) {
	return 1 + lineType(y);
}

static struct Bytes origBytes(uint32_t y, size_t i) {
	bool a = (i >= pixelLen), b = (y > 0), c = (a && b);
	return (struct Bytes) {
		.x = lineData(y)[i],
		.a = (a ? lineData(y)[i-pixelLen] : 0),
		.b = (b ? lineData(y-1)[i] : 0),
		.c = (c ? lineData(y-1)[i-pixelLen] : 0),
	};
}

static void dataRecon(void) {
	for (uint32_t y = 0; y < header.height; ++y) {
		for (size_t i = 0; i < lineLen; ++i) {
			lineData(y)[i] = recon(*lineType(y), origBytes(y, i));
		}
		*lineType(y) = None;
	}
}

static void dataFilter(void) {
	if (header.color == Indexed || header.depth < 8) return;
	uint8_t *filter[FilterCap];
	for (enum Filter i = None; i < FilterCap; ++i) {
		filter[i] = malloc(lineLen);
		if (!filter[i]) err(1, "malloc");
	}
	for (uint32_t y = header.height-1; y < header.height; --y) {
		uint32_t heuristic[FilterCap] = {0};
		enum Filter minType = None;
		for (enum Filter type = None; type < FilterCap; ++type) {
			for (size_t i = 0; i < lineLen; ++i) {
				filter[type][i] = filt(type, origBytes(y, i));
				heuristic[type] += abs((int8_t)filter[type][i]);
			}
			if (heuristic[type] < heuristic[minType]) minType = type;
		}
		*lineType(y) = minType;
		memcpy(lineData(y), filter[minType], lineLen);
	}
	for (enum Filter i = None; i < FilterCap; ++i) {
		free(filter[i]);
	}
}

static bool alphaUnused(void) {
	if (header.color != GrayscaleAlpha && header.color != TruecolorAlpha) {
		return false;
	}
	size_t sampleLen = header.depth / 8;
	size_t colorLen = pixelLen - sampleLen;
	for (uint32_t y = 0; y < header.height; ++y)
	for (uint32_t x = 0; x < header.width; ++x)
	for (size_t i = 0; i < sampleLen; ++i) {
		if (lineData(y)[x * pixelLen + colorLen + i] != 0xFF) return false;
	}
	return true;
}

static void alphaDiscard(void) {
	if (header.color != GrayscaleAlpha && header.color != TruecolorAlpha) {
		return;
	}
	size_t sampleLen = header.depth / 8;
	size_t colorLen = pixelLen - sampleLen;
	uint8_t *ptr = data;
	for (uint32_t y = 0; y < header.height; ++y) {
		*ptr++ = *lineType(y);
		for (uint32_t x = 0; x < header.width; ++x) {
			memmove(ptr, &lineData(y)[x * pixelLen], colorLen);
			ptr += colorLen;
		}
	}
	header.color = (header.color == GrayscaleAlpha ? Grayscale : Truecolor);
	recalc();
}

static bool depth16Unused(void) {
	if (header.color != Grayscale && header.color != Truecolor) return false;
	if (header.depth != 16) return false;
	for (uint32_t y = 0; y < header.height; ++y)
	for (size_t i = 0; i < lineLen; i += 2) {
		if (lineData(y)[i] != lineData(y)[i+1]) return false;
	}
	return true;
}

static void depth16Reduce(void) {
	if (header.depth != 16) return;
	uint8_t *ptr = data;
	for (uint32_t y = 0; y < header.height; ++y) {
		*ptr++ = *lineType(y);
		for (size_t i = 0; i < lineLen / 2; ++i) {
			*ptr++ = lineData(y)[i*2];
		}
	}
	header.depth = 8;
	recalc();
}

static bool colorUnused(void) {
	if (header.color != Truecolor && header.color != TruecolorAlpha) {
		return false;
	}
	if (header.depth != 8) return false;
	for (uint32_t y = 0; y < header.height; ++y)
	for (uint32_t x = 0; x < header.width; ++x) {
		uint8_t r = lineData(y)[x * pixelLen + 0];
		uint8_t g = lineData(y)[x * pixelLen + 1];
		uint8_t b = lineData(y)[x * pixelLen + 2];
		if (r != g || g != b) return false;
	}
	return true;
}

static void colorDiscard(void) {
	if (header.color != Truecolor && header.color != TruecolorAlpha) return;
	if (header.depth != 8) return;
	uint8_t *ptr = data;
	for (uint32_t y = 0; y < header.height; ++y) {
		*ptr++ = *lineType(y);
		for (uint32_t x = 0; x < header.width; ++x) {
			uint8_t r = lineData(y)[x * pixelLen + 0];
			uint8_t g = lineData(y)[x * pixelLen + 1];
			uint8_t b = lineData(y)[x * pixelLen + 2];
			*ptr++ = ((uint32_t)r + (uint32_t)g + (uint32_t)b) / 3;
			if (header.color == TruecolorAlpha) {
				*ptr++ = lineData(y)[x * pixelLen + 3];
			}
		}
	}
	header.color = (header.color == Truecolor ? Grayscale : GrayscaleAlpha);
	recalc();
}

static void colorIndex(void) {
	if (header.color != Truecolor && header.color != TruecolorAlpha) return;
	if (header.depth != 8) return;
	bool alpha = (header.color == TruecolorAlpha);
	for (uint32_t y = 0; y < header.height; ++y)
	for (uint32_t x = 0; x < header.width; ++x) {
		if (!palAdd(alpha, &lineData(y)[x * pixelLen])) return;
	}

	transCompact();
	uint8_t *ptr = data;
	for (uint32_t y = 0; y < header.height; ++y) {
		*ptr++ = *lineType(y);
		for (uint32_t x = 0; x < header.width; ++x) {
			*ptr++ = palIndex(alpha, &lineData(y)[x * pixelLen]);
		}
	}
	header.color = Indexed;
	recalc();
}

static bool depth8Unused(void) {
	if (header.depth != 8) return false;
	if (header.color == Indexed) return pal.len <= 16;
	if (header.color != Grayscale) return false;
	for (uint32_t y = 0; y < header.height; ++y)
	for (size_t i = 0; i < lineLen; ++i) {
		if ((lineData(y)[i] >> 4) != (lineData(y)[i] & 0x0F)) return false;
	}
	return true;
}

static void depth8Reduce(void) {
	if (header.color != Grayscale && header.color != Indexed) return;
	if (header.depth != 8) return;
	uint8_t *ptr = data;
	for (uint32_t y = 0; y < header.height; ++y) {
		*ptr++ = *lineType(y);
		for (size_t i = 0; i < lineLen; i += 2) {
			uint8_t a, b;
			uint8_t aa = lineData(y)[i];
			uint8_t bb = (i+1 < lineLen ? lineData(y)[i+1] : 0);
			if (header.color == Grayscale) {
				a = aa >> 4;
				b = bb >> 4;
			} else {
				a = aa & 0x0F;
				b = bb & 0x0F;
			}
			*ptr++ = a << 4 | b;
		}
	}
	header.depth = 4;
	recalc();
}

static bool depth4Unused(void) {
	if (header.depth != 4) return false;
	if (header.color == Indexed) return pal.len <= 4;
	if (header.color != Grayscale) return false;
	for (uint32_t y = 0; y < header.height; ++y)
	for (size_t i = 0; i < lineLen; ++i) {
		uint8_t a = lineData(y)[i] >> 4;
		uint8_t b = lineData(y)[i] & 0x0F;
		if ((a >> 2) != (a & 0x03)) return false;
		if ((b >> 2) != (b & 0x03)) return false;
	}
	return true;
}

static void depth4Reduce(void) {
	if (header.color != Grayscale && header.color != Indexed) return;
	if (header.depth != 4) return;
	uint8_t *ptr = data;
	for (uint32_t y = 0; y < header.height; ++y) {
		*ptr++ = *lineType(y);
		for (size_t i = 0; i < lineLen; i += 2) {
			uint8_t a, b, c, d;
			uint8_t aabb = lineData(y)[i];
			uint8_t ccdd = (i+1 < lineLen ? lineData(y)[i+1] : 0);
			if (header.color == Grayscale) {
				a = aabb >> 6;
				c = ccdd >> 6;
				b = aabb >> 2 & 0x03;
				d = ccdd >> 2 & 0x03;
			} else {
				a = aabb >> 4 & 0x03;
				c = ccdd >> 4 & 0x03;
				b = aabb & 0x03;
				d = ccdd & 0x03;
			}
			*ptr++ = a << 6 | b << 4 | c << 2 | d;
		}
	}
	header.depth = 2;
	recalc();
}

static bool depth2Unused(void) {
	if (header.depth != 2) return false;
	if (header.color == Indexed) return pal.len <= 2;
	if (header.color != Grayscale) return false;
	for (uint32_t y = 0; y < header.height; ++y)
	for (size_t i = 0; i < lineLen; ++i) {
		uint8_t a = lineData(y)[i] >> 6;
		uint8_t b = lineData(y)[i] >> 4 & 0x03;
		uint8_t c = lineData(y)[i] >> 2 & 0x03;
		uint8_t d = lineData(y)[i] & 0x03;
		if ((a >> 1) != (a & 1)) return false;
		if ((b >> 1) != (b & 1)) return false;
		if ((c >> 1) != (c & 1)) return false;
		if ((d >> 1) != (d & 1)) return false;
	}
	return true;
}

static void depth2Reduce(void) {
	if (header.color != Grayscale && header.color != Indexed) return;
	if (header.depth != 2) return;
	uint8_t *ptr = data;
	for (uint32_t y = 0; y < header.height; ++y) {
		*ptr++ = *lineType(y);
		for (size_t i = 0; i < lineLen; i += 2) {
			uint8_t a, b, c, d, e, f, g, h;
			uint8_t aabbccdd = lineData(y)[i];
			uint8_t eeffgghh = (i+1 < lineLen ? lineData(y)[i+1] : 0);
			if (header.color == Grayscale) {
				a = aabbccdd >> 7;
				b = aabbccdd >> 5 & 1;
				c = aabbccdd >> 3 & 1;
				d = aabbccdd >> 1 & 1;
				e = eeffgghh >> 7;
				f = eeffgghh >> 5 & 1;
				g = eeffgghh >> 3 & 1;
				h = eeffgghh >> 1 & 1;
			} else {
				a = aabbccdd >> 6 & 1;
				b = aabbccdd >> 4 & 1;
				c = aabbccdd >> 2 & 1;
				d = aabbccdd & 1;
				e = eeffgghh >> 6 & 1;
				f = eeffgghh >> 4 & 1;
				g = eeffgghh >> 2 & 1;
				h = eeffgghh & 1;
			}
			*ptr++ = 0
				| a << 7 | b << 6 | c << 5 | d << 4
				| e << 3 | f << 2 | g << 1 | h;
		}
	}
	header.depth = 1;
	recalc();
}

static bool discardAlpha;
static bool discardColor;
static uint8_t reduceDepth = 16;

static void optimize(const char *inPath, const char *outPath) {
	if (inPath) {
		path = inPath;
		file = fopen(path, "r");
		if (!file) err(1, "%s", path);
	} else {
		path = "stdin";
		file = stdin;
	}

	sigRead();
	struct Chunk ihdr = chunkRead();
	if (strcmp(ihdr.type, "IHDR")) {
		errx(1, "%s: expected IHDR, found %s", path, ihdr.type);
	}
	headerRead(ihdr);
	if (header.interlace != Progressive) {
		errx(1, "%s: unsupported interlacing", path);
	}

	palClear();
	dataAlloc();
	for (;;) {
		struct Chunk chunk = chunkRead();
		if (!strcmp(chunk.type, "PLTE")) {
			palRead(chunk);
		} else if (!strcmp(chunk.type, "tRNS")) {
			transRead(chunk);
		} else if (!strcmp(chunk.type, "IDAT")) {
			dataRead(chunk);
		} else if (!strcmp(chunk.type, "IEND")) {
			break;
		} else {
			chunkSkip(chunk);
		}
	}
	fclose(file);

	dataRecon();
	if (discardAlpha || alphaUnused()) alphaDiscard();
	if (reduceDepth < 16 || depth16Unused()) depth16Reduce();
	if (discardColor || colorUnused()) colorDiscard();
	colorIndex();
	if (reduceDepth < 8 || depth8Unused()) depth8Reduce();
	if (reduceDepth < 4 || depth4Unused()) depth4Reduce();
	if (reduceDepth < 2 || depth2Unused()) depth2Reduce();
	dataFilter();

	char buf[PATH_MAX];
	if (outPath) {
		path = outPath;
		if (outPath == inPath) {
			snprintf(buf, sizeof(buf), "%so", outPath);
			file = fopen(buf, "wx");
			if (!file) err(1, "%s", buf);
		} else {
			file = fopen(path, "w");
			if (!file) err(1, "%s", outPath);
		}
	} else {
		path = "stdout";
		file = stdout;
	}

	sigWrite();
	headerWrite();
	if (header.color == Indexed) {
		palWrite();
		if (trans.len) transWrite();
	}
	dataWrite();
	free(data);
	int error = fclose(file);
	if (error) err(1, "%s", path);

	if (outPath && outPath == inPath) {
		error = rename(buf, outPath);
		if (error) err(1, "%s", outPath);
	}
}

int main(int argc, char *argv[]) {
	bool stdio = false;
	char *outPath = NULL;

	for (int opt; 0 < (opt = getopt(argc, argv, "ab:cgo:v"));) {
		switch (opt) {
			break; case 'a': discardAlpha = true;
			break; case 'b': reduceDepth = strtoul(optarg, NULL, 10);
			break; case 'c': stdio = true;
			break; case 'g': discardColor = true;
			break; case 'o': outPath = optarg;
			break; case 'v': verbose = true;
			break; default:  return 1;
		}
	}

	if (optind < argc) {
		for (int i = optind; i < argc; ++i) {
			optimize(argv[i], (stdio ? NULL : outPath ? outPath : argv[i]));
		}
	} else {
		optimize(NULL, outPath);
	}
}