Unit example;

{ This file illustrates how to use the IJG code as a subroutine library
  to read or write JPEG image files.  You should look at this code in
  conjunction with the documentation file libjpeg.doc.

  This code will not do anything useful as-is, but it may be helpful as a
  skeleton for constructing routines that call the JPEG library. }

{ Original: example.c }

Interface

{ Include file for users of JPEG library.
  You will need to have included system headers that define at least
  the typedefs FILE and size_t before you can include jpeglib.h.
  (stdio.h is sufficient on ANSI-conforming systems.)
  You may also wish to include "jerror.h". }

uses
  jmorecfg, jerror, jpeglib,
  jdatadst, jcparam, jcapimin, jcapistd, jdapimin, jdatasrc, jdapistd,
  test;


{ Sample routine for JPEG compression.  We assume that the target file name
  and a compression quality factor are passed in. }

{GLOBAL}
procedure write_JPEG_file (filename : string; quality : int);

{ Sample routine for JPEG decompression.  We assume that the source file name
  is passed in.  We want to return TRUE on success, FALSE on error. }

{GLOBAL}
function read_JPEG_file (filename : string) : boolean;

implementation

{$IFOPT I+} {$DEFINE IoCheck} {$ENDIF}

{ <setjmp.h> is used for the optional error recovery mechanism shown in
  the second part of the example. }


{******************* JPEG COMPRESSION SAMPLE INTERFACE ******************}

{ This half of the example shows how to feed data into the JPEG compressor.
  We present a minimal version that does not worry about refinements such
  as error recovery (the JPEG code will just exit() if it gets an error). }


{ IMAGE DATA FORMATS:

  The standard input image format is a rectangular array of pixels, with
  each pixel having the same number of "component" values (color channels).
  Each pixel row is an array of JSAMPLEs (which typically are unsigned chars).
  If you are working with color data, then the color values for each pixel
  must be adjacent in the row; for example, R,G,B,R,G,B,R,G,B,... for 24-bit
  RGB color.

  For this example, we'll assume that this data structure matches the way
  our application has stored the image in memory, so we can just pass a
  pointer to our image buffer.  In particular, let's say that the image is
  RGB color and is described by: }

{$IFDEF TEST}
{extern}
var
  image_buffer : JSAMPROW;	{ Points to large array of R,G,B-order data }
  image_height : int;	        { Number of rows in image }
  image_width : int;		{ Number of columns in image }

{$ENDIF}

{ Sample routine for JPEG compression.  We assume that the target file name
  and a compression quality factor are passed in. }

{GLOBAL}
procedure write_JPEG_file (filename : string;  quality : int);
var
  { This struct contains the JPEG compression parameters and pointers to
    working space (which is allocated as needed by the JPEG library).
    It is possible to have several such structures, representing multiple
    compression/decompression processes, in existence at once.  We refer
    to any one struct (and its associated working data) as a "JPEG object". }
  cinfo : jpeg_compress_struct;
  { This struct represents a JPEG error handler.  It is declared separately
    because applications often want to supply a specialized error handler
    (see the second half of this file for an example).  But here we just
    take the easy way out and use the standard error handler, which will
    print a message on stderr and call exit() if compression fails.
    Note that this struct must live as long as the main JPEG parameter
    struct, to avoid dangling-pointer problems. }

  jerr : jpeg_error_mgr;
  { More stuff }
  outfile : FILE;		{ target file }
  row_pointer : array[0..0] of JSAMPROW ;	{ pointer to JSAMPLE row[s] }
  row_stride : int;		{ physical row width in image buffer }
begin
  { Step 1: allocate and initialize JPEG compression object }

  { We have to set up the error handler first, in case the initialization
    step fails.  (Unlikely, but it could happen if you are out of memory.)
    This routine fills in the contents of struct jerr, and returns jerr's
    address which we place into the link field in cinfo. }

  cinfo.err := jpeg_std_error(jerr);
  { msg_level that will be displayed. (Nomssi) }
  jerr.trace_level := 3;
  { Now we can initialize the JPEG compression object. }
  jpeg_create_compress(@cinfo);

  { Step 2: specify data destination (eg, a file) }
  { Note: steps 2 and 3 can be done in either order. }

  { Here we use the library-supplied code to send compressed data to a
    stdio stream.  You can also write your own code to do something else.
    VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
    requires it in order to write binary files. }

  Assign(outfile, filename);
  {$I-}
  ReWrite(outfile, 1);
  {$IFDEF IoCheck} {$I+} {$ENDIF}
  if (IOresult <> 0) then
  begin
    WriteLn(output, 'can''t open ', filename);
    Halt(1);
  end;
  jpeg_stdio_dest(@cinfo, @outfile);

  { Step 3: set parameters for compression }

  { First we supply a description of the input image.
    Four fields of the cinfo struct must be filled in: }

  cinfo.image_width := image_width; 	{ image width and height, in pixels }
  cinfo.image_height := image_height;
  cinfo.input_components := 3;		{ # of color components per pixel }
  cinfo.in_color_space := JCS_RGB; 	{ colorspace of input image }
  { Now use the library's routine to set default compression parameters.
    (You must set at least cinfo.in_color_space before calling this,
    since the defaults depend on the source color space.) }

  jpeg_set_defaults(@cinfo);
  { Now you can set any non-default parameters you wish to.
    Here we just illustrate the use of quality (quantization table) scaling: }

  jpeg_set_quality(@cinfo, quality, TRUE { limit to baseline-JPEG values });

  { Step 4: Start compressor }

  { TRUE ensures that we will write a complete interchange-JPEG file.
    Pass TRUE unless you are very sure of what you're doing. }

  jpeg_start_compress(@cinfo, TRUE);

  { Step 5: while (scan lines remain to be written) }
  {           jpeg_write_scanlines(...); }

  { Here we use the library's state variable cinfo.next_scanline as the
    loop counter, so that we don't have to keep track ourselves.
    To keep things simple, we pass one scanline per call; you can pass
    more if you wish, though. }

  row_stride := image_width * 3;	{ JSAMPLEs per row in image_buffer }

  while (cinfo.next_scanline < cinfo.image_height) do
  begin
    { jpeg_write_scanlines expects an array of pointers to scanlines.
      Here the array is only one element long, but you could pass
      more than one scanline at a time if that's more convenient. }

    row_pointer[0] := JSAMPROW(@image_buffer^[cinfo.next_scanline * row_stride]);
    {void} jpeg_write_scanlines(@cinfo, JSAMPARRAY(@row_pointer), 1);
  end;

  { Step 6: Finish compression }

  jpeg_finish_compress(@cinfo);
  { After finish_compress, we can close the output file. }
  system.close(outfile);

  { Step 7: release JPEG compression object }

  { This is an important step since it will release a good deal of memory. }
  jpeg_destroy_compress(@cinfo);

  { And we're done! }
end;


{ SOME FINE POINTS:

  In the above loop, we ignored the return value of jpeg_write_scanlines,
  which is the number of scanlines actually written.  We could get away
  with this because we were only relying on the value of cinfo.next_scanline,
  which will be incremented correctly.  If you maintain additional loop
  variables then you should be careful to increment them properly.
  Actually, for output to a stdio stream you needn't worry, because
  then jpeg_write_scanlines will write all the lines passed (or else exit
  with a fatal error).  Partial writes can only occur if you use a data
  destination module that can demand suspension of the compressor.
  (If you don't know what that's for, you don't need it.)

  If the compressor requires full-image buffers (for entropy-coding
  optimization or a multi-scan JPEG file), it will create temporary
  files for anything that doesn't fit within the maximum-memory setting.
  (Note that temp files are NOT needed if you use the default parameters.)
  On some systems you may need to set up a signal handler to ensure that
  temporary files are deleted if the program is interrupted.  See libjpeg.doc.

  Scanlines MUST be supplied in top-to-bottom order if you want your JPEG
  files to be compatible with everyone else's.  If you cannot readily read
  your data in that order, you'll need an intermediate array to hold the
  image.  See rdtarga.c or rdbmp.c for examples of handling bottom-to-top
  source data using the JPEG code's internal virtual-array mechanisms. }




{******************* JPEG DECOMPRESSION SAMPLE INTERFACE ******************}

{ This half of the example shows how to read data from the JPEG decompressor.
  It's a bit more refined than the above, in that we show:
    (a) how to modify the JPEG library's standard error-reporting behavior;
    (b) how to allocate workspace using the library's memory manager.

  Just to make this example a little different from the first one, we'll
  assume that we do not intend to put the whole image into an in-memory
  buffer, but to send it line-by-line someplace else.  We need a one-
  scanline-high JSAMPLE array as a work buffer, and we will let the JPEG
  memory manager allocate it for us.  This approach is actually quite useful
  because we don't need to remember to deallocate the buffer separately: it
  will go away automatically when the JPEG object is cleaned up. }