bindct_revisions.doc~

来自「JPEG Image compression using IJG standar」· DOC~ 代码 · 共 120 行

1: cjpeg.c
	parse_switches, added options for bindct.

2: jpeglib.h
	Add bindct enum options in J_DCT_METHOD.

3: jmorecfg.h
	Define DCT_BIN_A1_SUPPORTED	etc.

4: jcdctmgr.c:
	jinit_forward_dct:
	added options for bindct.

5: Added new files:
	jfdct_bin_a1.c
	jfdct_bin_b1.c : not completed. Check minus sign!
	jfdct_bin_c1.c : not completed. Check minus sign !
	jfdct_bin_l1.c

6. jdct.h
	Declare functions jfdct_bin_a1,	jfdct_bin_b1, jfdct_bin_c1,	jfdct_bin_l1.
	and jpeg_idct_bin_a1 etc.

	Note in jdct.h:
	 * The DCT inputs are expected to be signed (range +-CENTERJSAMPLE).
	 * The DCT outputs are returned scaled up by a factor of 8; they therefore
	 * have a range of +-8K for 8-bit data, +-128K for 12-bit data.  This
	 * convention improves accuracy in integer implementations and saves some
	 * work in floating-point ones.
	 * Quantization of the output coefficients is done by jcdctmgr.c.
	 */

Is this scaling by 8 related to the scaling of Q table by 8 in jcdctmgr.c (for JDCT_ISLOW)?

Then the scaling in DCT coef and Q tabel cancel out with each other.

In jddctmgr.c, the dequantization table is the original one, without scling by 8. This justifies the above conclusion.

7.  Files involved in forward transform:

	1. cjpeg.c calls jpeg_write_scanlines(), which is defined in jcapistd.c;
	
	2. In jcapistd.c, jpeg_wtite_scanlines() call
	(*cinfo->main->process_data) (cinfo, scanlines, &row_ctr, num_lines);
	which points to the process_data_simple_main() in jcmainct.c;

	3. The jcmainct.c is the main buffer controller for compression.
	In  process_data_simple_main(), it call
	   (*cinfo->prep->pre_process_data), defined in jcprepct.c,
		which handles color conversion, downsamplign etc.
	It then check whether 8 rows have been copied to the buffer, 
	if no, it returns to the cjpeg.c to call write_scanlines again.
	If yes,  it calls 
	(*cinfo->coef->compress_data) (cinfo, main->buffer),
	this points to the function compress_data() in jccoefct.c.

	4. In the compress_data() of  jccoefct.c, 
	 (*cinfo->fdct->forward_DCT)
	is called, which is defined in jcdctmgr.c.

	5. In jcdctmgr.c, the function forward_DCT() does the followings:
		- subtract each pixel by 128.
		- call do (*do_dct) (workspace), which points to the actual 
			forward DCT code.
		- Quantize the DCT coefficients.


/************************************************
 *
 * 		Decoder
 *
 ************************************************
 */

jddctmgr.c :

/* The current scaled-IDCT routines require ISLOW-style multiplier tables,
 * so be sure to compile that code if either ISLOW or SCALING is requested.
 */
#ifdef DCT_ISLOW_SUPPORTED
#define PROVIDE_ISLOW_TABLES
#else
#ifdef IDCT_SCALING_SUPPORTED
#define PROVIDE_ISLOW_TABLES
#endif
#endif

What does this mean?


for JDCT_ISLOW:
Define the q table as type ISLOW_MULT_TYPE:
	ISLOW_MULT_TYPE * ismtbl = (ISLOW_MULT_TYPE *) compptr->dct_table;
The reason is discussed in jidctint.c.

Check this, do we need new type for binDCT?

1. djpeg.c call  (void) jpeg_start_decompress(&cinfo);

			start_input_pass in jdinput.c calls latech_quant_tables.
			latch_quant_tables in jdinput.c is called to get Q table.
			but where to scale it according to the Quality factor?

			and	jpeg_read_scanlines,
			both are defined in jdapistd.c.

2. In jdapistd.c, the jpeg_read_scanlines calls
		 (*cinfo->main->process_data),
	which points to the process_data_simple_main in jdmainct.c,

3: The jdmainct.c is the main buffer controller for decompression.
	In the process_data_simple_main, it calls
		(*cinfo->coef->decompress_data),
	which points to 
		decompress_onepass,
	which is defined in jdcoefct.c.

4. In jdcoefct.c, the (*cinfo->entropy->decode_mcu) is called,
	which