Program rdjpgcom; { This file contains a very simple stand-alone application that displays the text in COM (comment) markers in a JFIF file. This may be useful as an example of the minimum logic needed to parse JPEG markers. } { Original: Copyright (C) 1994-1995, Thomas G. Lane. } uses Objects; const EXIT_FAILURE = 1; EXIT_SUCCESS = 0; type int = integer; uInt = word; { These macros are used to read the input file. To reuse this code in another application, you might need to change these. } var infile : TBufStream; { input JPEG file } { Return next input byte, or EOF if no more } function NEXTBYTE : byte; var c : Byte; begin infile.Read(c, 1); NEXTBYTE := c; end; { Error exit handler } procedure ERREXIT(msg : string); begin WriteLn(output, msg); Halt(EXIT_FAILURE); end; { Read one byte, testing for EOF } function read_1_byte : int; var c : int; begin c := NEXTBYTE; if (infile.Status <> stOK) then ERREXIT('Premature EOF in JPEG file'); read_1_byte := c; end; { Read 2 bytes, convert to unsigned int } { All 2-byte quantities in JPEG markers are MSB first } function read_2_bytes : uInt; var c : word; begin infile.Read(c, 2); c := Swap(c); if (infile.Status <> stOK) then ERREXIT('Premature EOF in JPEG file'); read_2_bytes := c; end; { JPEG markers consist of one or more 0xFF bytes, followed by a marker code byte (which is not an FF). Here are the marker codes of interest in this program. (See jdmarker.c for a more complete list.) } const M_SOF0 = $C0; { Start Of Frame N } M_SOF1 = $C1; { N indicates which compression process } M_SOF2 = $C2; { Only SOF0-SOF2 are now in common use } M_SOF3 = $C3; M_SOF5 = $C5; { NB: codes C4 and CC are NOT SOF markers } M_SOF6 = $C6; M_SOF7 = $C7; M_SOF9 = $C9; M_SOF10 = $CA; M_SOF11 = $CB; M_SOF13 = $CD; M_SOF14 = $CE; M_SOF15 = $CF; M_SOI = $D8; { Start Of Image (beginning of datastream) } M_EOI = $D9; { End Of Image (end of datastream) } M_SOS = $DA; { Start Of Scan (begins compressed data) } M_COM = $FE; { COMment } { Find the next JPEG marker and return its marker code. We expect at least one FF byte, possibly more if the compressor used FFs to pad the file. There could also be non-FF garbage between markers. The treatment of such garbage is unspecified; we choose to skip over it but emit a warning msg. NB: this routine must not be used after seeing SOS marker, since it will not deal correctly with FF/00 sequences in the compressed image data... } function next_marker : int; var c : int; discarded_bytes : int; begin discarded_bytes := 0; { Find 0xFF byte; count and skip any non-FFs. } c := read_1_byte; while (c <> $FF) do begin Inc(discarded_bytes); c := read_1_byte; end; { Get marker code byte, swallowing any duplicate FF bytes. Extra FFs are legal as pad bytes, so don't count them in discarded_bytes. } repeat c := read_1_byte; until (c <> $FF); if (discarded_bytes <> 0) then begin WriteLn(output, 'Warning: garbage data found in JPEG file'); end; next_marker := c; end; { Read the initial marker, which should be SOI. For a JFIF file, the first two bytes of the file should be literally $FF M_SOI. To be more general, we could use next_marker, but if the input file weren't actually JPEG at all, next_marker might read the whole file and then return a misleading error message... } function first_marker : int; var c1, c2 : int; begin c1 := NEXTBYTE; c2 := NEXTBYTE; if (c1 <> $FF) or (c2 <> M_SOI) then ERREXIT('Not a JPEG file'); first_marker := c2; end; { Most types of marker are followed by a variable-length parameter segment. This routine skips over the parameters for any marker we don't otherwise want to process. Note that we MUST skip the parameter segment explicitly in order not to be fooled by $FF bytes that might appear within the parameter segment; such bytes do NOT introduce new markers. } procedure skip_variable; { Skip over an unknown or uninteresting variable-length marker } var length : uInt; begin { Get the marker parameter length count } length := read_2_bytes; { Length includes itself, so must be at least 2 } if (length < 2) then ERREXIT('Erroneous JPEG marker length'); Dec(length, 2); { Skip over the remaining bytes } while (length > 0) do begin read_1_byte; Dec(length); end; end; { Process a COM marker. We want to print out the marker contents as legible text; we must guard against random junk and varying newline representations. } procedure process_COM; const CR = 13; LF = 10; var length : uInt; comment : string; lastch : byte; begin comment := ''; { Get the marker parameter length count } length := read_2_bytes; { Length includes itself, so must be at least 2 } if (length < 2) then ERREXIT('Erroneous JPEG marker length'); Dec(length, 2); comment := ''; while (length > 0) do begin comment := comment + char(read_1_byte); Dec(length); end; WriteLn(comment); end; { Process a SOFn marker. This code is only needed if you want to know the image dimensions... } procedure process_SOFn (marker : int); var length : uInt; image_height, image_width : uInt; data_precision, num_components : int; process : string; ci: int; begin length := read_2_bytes; { usual parameter length count } data_precision := read_1_byte; image_height := read_2_bytes; image_width := read_2_bytes; num_components := read_1_byte; case marker of M_SOF0: process := 'Baseline'; M_SOF1: process := 'Extended sequential'; M_SOF2: process := 'Progressive'; M_SOF3: process := 'Lossless'; M_SOF5: process := 'Differential sequential'; M_SOF6: process := 'Differential progressive'; M_SOF7: process := 'Differential lossless'; M_SOF9: process := 'Extended sequential, arithmetic coding'; M_SOF10: process := 'Progressive, arithmetic coding'; M_SOF11: process := 'Lossless, arithmetic coding'; M_SOF13: process := 'Differential sequential, arithmetic coding'; M_SOF14: process := 'Differential progressive, arithmetic coding'; M_SOF15: process := 'Differential lossless, arithmetic coding'; else process := 'Unknown'; end; WriteLn('JPEG image is ',image_width,'w * ',image_height,'h, ', num_components, ' color components, ',data_precision, ' bits per sample'); WriteLn('JPEG process: ', process); if (length <> uInt(8 + num_components * 3)) then ERREXIT('Bogus SOF marker length'); for ci := 0 to pred(num_components) do begin read_1_byte; { Component ID code } read_1_byte; { H, V sampling factors } read_1_byte; { Quantization table number } end; end; { Parse the marker stream until SOS or EOI is seen; display any COM markers. While the companion program wrjpgcom will always insert COM markers before SOFn, other implementations might not, so we scan to SOS before stopping. If we were only interested in the image dimensions, we would stop at SOFn. (Conversely, if we only cared about COM markers, there would be no need for special code to handle SOFn; we could treat it like other markers.) } function scan_JPEG_header (verbose : boolean) : int; var marker : int; begin { Expect SOI at start of file } if (first_marker <> M_SOI) then ERREXIT('Expected SOI marker first'); { Scan miscellaneous markers until we reach SOS. } repeat marker := next_marker; case marker of M_SOF0, { Baseline } M_SOF1, { Extended sequential, Huffman } M_SOF2, { Progressive, Huffman } M_SOF3, { Lossless, Huffman } M_SOF5, { Differential sequential, Huffman } M_SOF6, { Differential progressive, Huffman } M_SOF7, { Differential lossless, Huffman } M_SOF9, { Extended sequential, arithmetic } M_SOF10, { Progressive, arithmetic } M_SOF11, { Lossless, arithmetic } M_SOF13, { Differential sequential, arithmetic } M_SOF14, { Differential progressive, arithmetic } M_SOF15: { Differential lossless, arithmetic } if (verbose) then process_SOFn(marker) else skip_variable; M_SOS: { stop before hitting compressed data } begin scan_JPEG_header := marker; exit; end; M_EOI: { in case it's a tables-only JPEG stream } begin scan_JPEG_header := marker; exit; end; M_COM: process_COM; else { Anything else just gets skipped } skip_variable; { we assume it has a parameter count... } end; until false; { end loop } end; { Command line parsing code } var progname : string[79]; { program name for error messages } procedure usage; { complain about bad command line } begin WriteLn(output, 'rdjpgcom displays any textual comments in a JPEG file.'); WriteLn(output, 'Usage: ',progname,' [switches] [inputfile]'); WriteLn(output, 'Switches (names may be abbreviated):'); WriteLn(output, ' -verbose Also display dimensions of JPEG image'); Halt(EXIT_FAILURE); end; { The main program. } var verbose : boolean; argn : int; arg : string; begin verbose := FALSE; progname := ParamStr(0); if (progname = '') then progname := 'rdjpgcom'; { in case C library doesn't provide it } { Parse switches, if any } for argn := 1 to Pred(ParamCount) do begin arg := ParamStr(argn); if arg[1] = '-' then if (Pos(arg, '-verbose') > 0) then begin verbose := TRUE; end else usage; end; { Open the input file. } arg := ParamStr(ParamCount); if not infile.Init(arg, stOpenRead, 4096) then begin WriteLn(output, 'can''t open ', arg); Halt(EXIT_FAILURE); end; { Scan the JPEG headers. } scan_JPEG_header(verbose); infile.done; { All done. } Halt(EXIT_SUCCESS); end.