/*

 * example.c

 *

 * 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.  

 *

 * We present these routines in the same coding style used in the JPEG code

 * (ANSI function definitions, etc); but you are of course free to code your

 * routines in a different style if you prefer.

 */



#include <stdio.h>



/*

 * 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".

 */



#include "jpeglib.h"



/*

 * <setjmp.h> is used for the optional error recovery mechanism shown in

 * the second part of the example.

 */



#include <setjmp.h>







/******************** 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:

 */



extern JSAMPLE * image_buffer;	/* Points to large array of R,G,B-order data */

extern int image_height;	/* Number of rows in image */

extern int image_width;		/* Number of columns in image */





/*

 * Sample routine for JPEG compression.  We assume that the target file name

 * and a compression quality factor are passed in.

 */



GLOBAL(void)

write_JPEG_file (char * filename, int quality)

{

  /* 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".

   */

  struct jpeg_compress_struct cinfo;

  /* 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.

   */

  struct jpeg_error_mgr jerr;

  /* More stuff */

  FILE * outfile;		/* target file */

  JSAMPROW row_pointer[1];	/* pointer to JSAMPLE row[s] */

  int row_stride;		/* physical row width in image buffer */



  /* 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);

  /* 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.

   */

  if ((outfile = fopen(filename, "wb")) == NULL) {

    fprintf(stderr, "can't open %s\n", filename);

    exit(1);

  }

  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) {

    /* 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] = & image_buffer[cinfo.next_scanline * row_stride];

    (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);

  }



  /* Step 6: Finish compression */



  jpeg_finish_compress(&cinfo);

  /* After finish_compress, we can close the output file. */

  fclose(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! */

}





/*

 * 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.

 */





/*

 * ERROR HANDLING:

 *

 * The JPEG library's standard error handler (jerror.c) is divided into

 * several "methods" which you can override individually.  This lets you

 * adjust the behavior without duplicating a lot of code, which you might

 * have to update with each future release.

 *

 * Our example here shows how to override the "error_exit" method so that

 * control is returned to the library's caller when a fatal error occurs,

 * rather than calling exit() as the standard error_exit method does.

 *

 * We use C's setjmp/longjmp facility to return control.  This means that the

 * routine which calls the JPEG library must first execute a setjmp() call to

 * establish the return point.  We want the replacement error_exit to do a

 * longjmp().  But we need to make the setjmp buffer accessible to the

 * error_exit routine.  To do this, we make a private extension of the

 * standard JPEG error handler object.  (If we were using C++, we'd say we

 * were making a subclass of the regular error handler.)

 *

 * Here's the extended error handler struct:

 */



struct my_error_mgr {

  struct jpeg_error_mgr pub;	/* "public" fields */



  jmp_buf setjmp_buffer;	/* for return to caller */

};



typedef struct my_error_mgr * my_error_ptr;



/*

 * Here's the routine that will replace the standard error_exit method:

 */



METHODDEF(void)

my_error_exit (j_common_ptr cinfo)

{

  /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */

  my_error_ptr myerr = (my_error_ptr) cinfo->err;



  /* Always display the message. */

  /* We could postpone this until after returning, if we chose. */

  (*cinfo->err->output_message) (cinfo);



  /* Return control to the setjmp point */

  longjmp(myerr->setjmp_buffer, 1);

}





/*

 * Sample routine for JPEG decompression.  We assume that the source file name

 * is passed in.  We want to return 1 on success, 0 on error.

 */





GLOBAL(int)

read_JPEG_file (char * filename)

{

  /* This struct contains the JPEG decompression parameters and pointers to

   * working space (which is allocated as needed by the JPEG library).

   */

  struct jpeg_decompress_struct cinfo;

  /* We use our private extension JPEG error handler.

   * Note that this struct must live as long as the main JPEG parameter

   * struct, to avoid dangling-pointer problems.

   */

  struct my_error_mgr jerr;

  /* More stuff */

  FILE * infile;		/* source file */

  JSAMPARRAY buffer;		/* Output row buffer */

  int row_stride;		/* physical row width in output buffer */



  /* In this example we want to open the input file before doing anything else,

   * so that the setjmp() error recovery below can assume the file is open.

   * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that

   * requires it in order to read binary files.

   */



  if ((infile = fopen(filename, "rb")) == NULL) {

    fprintf(stderr, "can't open %s\n", filename);

    return 0;

  }



  /* Step 1: allocate and initialize JPEG decompression object */



  /* We set up the normal JPEG error routines, then override error_exit. */

  cinfo.err = jpeg_std_error(&jerr.pub);

  jerr.pub.error_exit = my_error_exit;

  /* Establish the setjmp return context for my_error_exit to use. */

  if (setjmp(jerr.setjmp_buffer)) {

    /* If we get here, the JPEG code has signaled an error.

     * We need to clean up the JPEG object, close the input file, and return.

     */

    jpeg_destroy_decompress(&cinfo);

    fclose(infile);

    return 0;

  }

  /* Now we can initialize the JPEG decompression object. */

  jpeg_create_decompress(&cinfo);



  /* Step 2: specify data source (eg, a file) */



  jpeg_stdio_src(&cinfo, infile);



  /* Step 3: read file parameters with jpeg_read_header() */



  (void) jpeg_read_header(&cinfo, TRUE);

  /* We can ignore the return value from jpeg_read_header since

   *   (a) suspension is not possible with the stdio data source, and

   *   (b) we passed TRUE to reject a tables-only JPEG file as an error.

   * See libjpeg.doc for more info.

   */



  /* Step 4: set parameters for decompression */



  /* In this example, we don't need to change any of the defaults set by

   * jpeg_read_header(), so we do nothing here.

   */



  /* Step 5: Start decompressor */



  (void) jpeg_start_decompress(&cinfo);

  /* We can ignore the return value since suspension is not possible

   * with the stdio data source.

   */



  /* We may need to do some setup of our own at this point before reading

   * the data.  After jpeg_start_decompress() we have the correct scaled

   * output image dimensions available, as well as the output colormap

   * if we asked for color quantization.

   * In this example, we need to make an output work buffer of the right size.

   */ 

  /* JSAMPLEs per row in output buffer */

  row_stride = cinfo.output_width * cinfo.output_components;

  /* Make a one-row-high sample array that will go away when done with image */

  buffer = (*cinfo.mem->alloc_sarray)

		((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);



  /* Step 6: while (scan lines remain to be read) */

  /*           jpeg_read_scanlines(...); */



  /* Here we use the library's state variable cinfo.output_scanline as the

   * loop counter, so that we don't have to keep track ourselves.

   */

  while (cinfo.output_scanline < cinfo.output_height) {

    /* jpeg_read_scanlines expects an array of pointers to scanlines.

     * Here the array is only one element long, but you could ask for

     * more than one scanline at a time if that's more convenient.

     */

    (void) jpeg_read_scanlines(&cinfo, buffer, 1);

    /* Assume put_scanline_someplace wants a pointer and sample count. */

    put_scanline_someplace(buffer[0], row_stride);

  }



  /* Step 7: Finish decompression */



  (void) jpeg_finish_decompress(&cinfo);

  /* We can ignore the return value since suspension is not possible

   * with the stdio data source.

   */



  /* Step 8: Release JPEG decompression object */



  /* This is an important step since it will release a good deal of memory. */

  jpeg_destroy_decompress(&cinfo);



  /* After finish_decompress, we can close the input file.

   * Here we postpone it until after no more JPEG errors are possible,

   * so as to simplify the setjmp error logic above.  (Actually, I don't

   * think that jpeg_destroy can do an error exit, but why assume anything...)

   */

  fclose(infile);



  /* At this point you may want to check to see whether any corrupt-data

   * warnings occurred (test whether jerr.pub.num_warnings is nonzero).

   */



  /* And we're done! */

  return 1;

}





/*

 * SOME FINE POINTS:

 *

 * In the above code, we ignored the return value of jpeg_read_scanlines,

 * which is the number of scanlines actually read.  We could get away with

 * this because we asked for only one line at a time and we weren't using

 * a suspending data source.  See libjpeg.doc for more info.

 *

 * We cheated a bit by calling alloc_sarray() after jpeg_start_decompress();

 * we should have done it beforehand to ensure that the space would be

 * counted against the JPEG max_memory setting.  In some systems the above

 * code would risk an out-of-memory error.  However, in general we don't

 * know the output image dimensions before jpeg_start_decompress(), unless we

 * call jpeg_calc_output_dimensions().  See libjpeg.doc for more about this.

 *

 * Scanlines are returned in the same order as they appear in the JPEG file,

 * which is standardly top-to-bottom.  If you must emit data bottom-to-top,

 * you can use one of the virtual arrays provided by the JPEG memory manager

 * to invert the data.  See wrbmp.c for an example.

 *

 * As with compression, some operating modes may require temporary files.

 * 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.

 */

