-- pngparse.lua -- Simple example of parsing the main sections of a PNG file. -- -- This is mostly just an example. Not intended to be complete, -- robust, modular, or well tested. -- -- (c) 2008 David Manura. Licensed under the same terms as Lua (MIT license). -- Unpack 32-bit unsigned integer (most-significant-byte, MSB, first) -- from byte string. local function unpack_msb_uint32(s) local a,b,c,d = s:byte(1,#s) local num = (((a*256) + b) * 256 + c) * 256 + d return num end -- Read 32-bit unsigned integer (most-significant-byte, MSB, first) from file. local function read_msb_uint32(fh) return unpack_msb_uint32(fh:read(4)) end -- Read unsigned byte (integer) from file local function read_byte(fh) return fh:read(1):byte() end local function parse_zlib(fh, len) local byte1 = read_byte(fh) local byte2 = read_byte(fh) local compression_method = byte1 % 16 local compression_info = math.floor(byte1 / 16) local fcheck = byte2 % 32 local fdict = math.floor(byte2 / 32) % 1 local flevel = math.floor(byte2 / 64) print("compression_method=", compression_method) print("compression_info=", compression_info) print("fcheck=", fcheck) print("fdict=", fdict) print("flevel=", flevel) fh:read(len - 6) print("(deflate data not displayed)") local checksum = read_msb_uint32(fh) print("checksum=", checksum) end local function parse_IHDR(fh, len) assert(len == 13, 'format error') local width = read_msb_uint32(fh) local height = read_msb_uint32(fh) local bit_depth = read_byte(fh) local color_type = read_byte(fh) local compression_method = read_byte(fh) local filter_method = read_byte(fh) local interlace_method = read_byte(fh) print("width=", width) print("height=", height) print("bit_depth=", bit_depth) print("color_type=", color_type) print("compression_method=", compression_method) print("filter_method=", filter_method) print("interlace_method=", interlace_method) return compression_method end local function parse_sRGB(fh, len) assert(len == 1, 'format error') local rendering_intent = read_byte(fh) print("rendering_intent=", rendering_intent) end local function parse_gAMA(fh, len) assert(len == 4, 'format error') local rendering_intent = read_msb_uint32(fh) print("rendering_intent=", rendering_intent) end local function parse_cHRM(fh, len) assert(len == 32, 'format error') local white_x = read_msb_uint32(fh) local white_y = read_msb_uint32(fh) local red_x = read_msb_uint32(fh) local red_y = read_msb_uint32(fh) local green_x = read_msb_uint32(fh) local green_y = read_msb_uint32(fh) local blue_x = read_msb_uint32(fh) local blue_y = read_msb_uint32(fh) print('white_x=', white_x) print('white_y=', white_y) print('red_x=', red_x) print('red_y=', red_y) print('green_x=', green_x) print('green_y=', green_y) print('blue_x=', blue_x) print('blue_y=', blue_y) end local function parse_IDAT(fh, len, compression_method) if compression_method == 0 then -- fh:read(len) parse_zlib(fh, len) else print('(unrecognized compression method)') end end local function parse_png(fh) -- parse PNG header local bytes = fh:read(8) local expect = "137円080円078円071円013円010円026円010円" if bytes ~= expect then error 'not a PNG file' end -- parse chunks local compression_method while 1 do local len = read_msb_uint32(fh) local stype = fh:read(4) print("chunk:", "type=", stype, "len=", len) if stype == 'IHDR' then compression_method = parse_IHDR(fh, len) elseif stype == 'sRGB' then parse_sRGB(fh, len) elseif stype == 'gAMA' then parse_gAMA(fh, len) elseif stype == 'cHRM' then parse_cHRM(fh, len) elseif stype == 'IDAT' then parse_IDAT(fh, len, compression_method) else local data = fh:read(len) print("data=", len == 0 and "(empty)" or "(not displayed)") end local crc = read_msb_uint32(fh) print("crc=", crc) if stype == 'IEND' then break end end end local filename = arg[1] if not filename then io.stderr:write("usage: lua pngparse.lua <filename>") os.exit(1) end local fh = assert(io.open(filename, 'rb')) parse_png(fh)
Example output:
$ lua pngparse.lua myfile.png chunk: type= IHDR len= 13 width= 676 height= 647 bit_depth= 8 color_type= 2 compression_method= 0 filter_method= 0 interlace_method= 0 crc= 3625766246 chunk: type= sRGB len= 1 rendering_intent= 0 crc= 2932743401 chunk: type= gAMA len= 4 rendering_intent= 45455 crc= 201089285 chunk: type= cHRM len= 32 white_x= 31270 white_y= 32900 red_x= 64000 red_y= 33000 green_x= 30000 green_y= 60000 blue_x= 15000 blue_y= 6000 crc= 2629456188 chunk: type= IDAT len= 60975 compression_method= 8 compression_info= 7 fcheck= 30 fdict= 0 flevel= 1 (deflate data not displayed) checksum= 2411816191 crc= 1804717936 chunk: type= IEND len= 0 data= (empty) crc= 2923585666