Merge changes from JPEG 8c (released Jan 16, 2011)

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@8426 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

jpeg/README

@@ -1,7 +1,7 @@
 The Independent JPEG Group's JPEG software
 ==========================================
 
-README for release 8b of 16-May-2010
+README for release 8c of 16-Jan-2011
 ====================================
 
 This distribution contains the eighth public release of the Independent JPEG
@@ -114,7 +114,7 @@ with respect to this software, its quality, accuracy, merchantability, or
 fitness for a particular purpose.  This software is provided "AS IS", and you,
 its user, assume the entire risk as to its quality and accuracy.
 
-This software is copyright (C) 1991-2010, Thomas G. Lane, Guido Vollbeding.
+This software is copyright (C) 1991-2011, Thomas G. Lane, Guido Vollbeding.
 All Rights Reserved except as specified below.
 
 Permission is hereby granted to use, copy, modify, and distribute this
@@ -254,8 +254,8 @@ ARCHIVE LOCATIONS
 The "official" archive site for this software is www.ijg.org.
 The most recent released version can always be found there in
 directory "files".  This particular version will be archived as
-http://www.ijg.org/files/jpegsrc.v8b.tar.gz, and in Windows-compatible
+http://www.ijg.org/files/jpegsrc.v8c.tar.gz, and in Windows-compatible
-"zip" archive format as http://www.ijg.org/files/jpegsr8b.zip.
+"zip" archive format as http://www.ijg.org/files/jpegsr8c.zip.
 
 The JPEG FAQ (Frequently Asked Questions) article is a source of some
 general information about JPEG.
@@ -285,8 +285,8 @@ Thank to John Korejwa and Massimo Ballerini for inviting me to
 fruitful consultations in Boston, MA and Milan, Italy.
 
 Thank to Hendrik Elstner, Roland Fassauer, Simone Zuck, Guenther
-Maier-Gerber, Walter Stoeber, and Fred Schmitz for corresponding
+Maier-Gerber, Walter Stoeber, Fred Schmitz, and Norbert Braunagel
-business development.
+for corresponding business development.
 
 Thank to Nico Zschach and Dirk Stelling of the technical support team
 at the Digital Images company in Halle for providing me with extra

jpeg/change.log

@@ -1,6 +1,17 @@
 CHANGE LOG for Independent JPEG Group's JPEG software
 
 
+Version 8c  16-Jan-2011
+-----------------------
+
+Add option to compression library and cjpeg (-block N) to use
+different DCT block size.
+All N from 1 to 16 are possible.  Default is 8 (baseline format).
+Larger values produce higher compression,
+smaller values produce higher quality.
+SmartScale capable decoder (introduced with IJG JPEG 8) required.
+
+
 Version 8b  16-May-2010
 -----------------------
 

jpeg/jcmarker.c

@@ -2,7 +2,7 @@
  * jcmarker.c
  *
  * Copyright (C) 1991-1998, Thomas G. Lane.
- * Modified 2003-2009 by Guido Vollbeding.
+ * Modified 2003-2010 by Guido Vollbeding.
  * This file is part of the Independent JPEG Group's software.
  * For conditions of distribution and use, see the accompanying README file.
  *
@@ -249,18 +249,20 @@ emit_dac (j_compress_ptr cinfo)
   for (i = 0; i < NUM_ARITH_TBLS; i++)
     length += dc_in_use[i] + ac_in_use[i];
 
-  emit_marker(cinfo, M_DAC);
+  if (length) {
+    emit_marker(cinfo, M_DAC);
 
-  emit_2bytes(cinfo, length*2 + 2);
+    emit_2bytes(cinfo, length*2 + 2);
 
-  for (i = 0; i < NUM_ARITH_TBLS; i++) {
+    for (i = 0; i < NUM_ARITH_TBLS; i++) {
-    if (dc_in_use[i]) {
+      if (dc_in_use[i]) {
-      emit_byte(cinfo, i);
+	emit_byte(cinfo, i);
-      emit_byte(cinfo, cinfo->arith_dc_L[i] + (cinfo->arith_dc_U[i]<<4));
+	emit_byte(cinfo, cinfo->arith_dc_L[i] + (cinfo->arith_dc_U[i]<<4));
-    }
+      }
-    if (ac_in_use[i]) {
+      if (ac_in_use[i]) {
-      emit_byte(cinfo, i + 0x10);
+	emit_byte(cinfo, i + 0x10);
-      emit_byte(cinfo, cinfo->arith_ac_K[i]);
+	emit_byte(cinfo, cinfo->arith_ac_K[i]);
+      }
     }
   }
 #endif /* C_ARITH_CODING_SUPPORTED */

jpeg/jcmaster.c

@@ -2,7 +2,7 @@
  * jcmaster.c
  *
  * Copyright (C) 1991-1997, Thomas G. Lane.
- * Modified 2003-2010 by Guido Vollbeding.
+ * Modified 2003-2011 by Guido Vollbeding.
  * This file is part of the Independent JPEG Group's software.
  * For conditions of distribution and use, see the accompanying README file.
  *
@@ -55,125 +55,140 @@ jpeg_calc_jpeg_dimensions (j_compress_ptr cinfo)
 {
 #ifdef DCT_SCALING_SUPPORTED
 
+  /* Sanity check on input image dimensions to prevent overflow in
+   * following calculation.
+   * We do check jpeg_width and jpeg_height in initial_setup below,
+   * but image_width and image_height can come from arbitrary data,
+   * and we need some space for multiplication by block_size.
+   */
+  if (((long) cinfo->image_width >> 24) || ((long) cinfo->image_height >> 24))
+    ERREXIT1(cinfo, JERR_IMAGE_TOO_BIG, (unsigned int) JPEG_MAX_DIMENSION);
+
   /* Compute actual JPEG image dimensions and DCT scaling choices. */
-  if (cinfo->scale_num >= cinfo->scale_denom * 8) {
+  if (cinfo->scale_num >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/1 scaling */
+    /* Provide block_size/1 scaling */
-    cinfo->jpeg_width = cinfo->image_width << 3;
+    cinfo->jpeg_width = cinfo->image_width * cinfo->block_size;
-    cinfo->jpeg_height = cinfo->image_height << 3;
+    cinfo->jpeg_height = cinfo->image_height * cinfo->block_size;
     cinfo->min_DCT_h_scaled_size = 1;
     cinfo->min_DCT_v_scaled_size = 1;
-  } else if (cinfo->scale_num >= cinfo->scale_denom * 4) {
+  } else if (cinfo->scale_num * 2 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 4/1 scaling */
+    /* Provide block_size/2 scaling */
-    cinfo->jpeg_width = cinfo->image_width << 2;
+    cinfo->jpeg_width = (JDIMENSION)
-    cinfo->jpeg_height = cinfo->image_height << 2;
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 2L);
+    cinfo->jpeg_height = (JDIMENSION)
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 2L);
     cinfo->min_DCT_h_scaled_size = 2;
     cinfo->min_DCT_v_scaled_size = 2;
-  } else if (cinfo->scale_num * 3 >= cinfo->scale_denom * 8) {
+  } else if (cinfo->scale_num * 3 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/3 scaling */
+    /* Provide block_size/3 scaling */
-    cinfo->jpeg_width = (cinfo->image_width << 1) + (JDIMENSION)
+    cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 2, 3L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 3L);
-    cinfo->jpeg_height = (cinfo->image_height << 1) + (JDIMENSION)
+    cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 2, 3L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 3L);
     cinfo->min_DCT_h_scaled_size = 3;
     cinfo->min_DCT_v_scaled_size = 3;
-  } else if (cinfo->scale_num >= cinfo->scale_denom * 2) {
+  } else if (cinfo->scale_num * 4 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 2/1 scaling */
+    /* Provide block_size/4 scaling */
-    cinfo->jpeg_width = cinfo->image_width << 1;
+    cinfo->jpeg_width = (JDIMENSION)
-    cinfo->jpeg_height = cinfo->image_height << 1;
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 4L);
+    cinfo->jpeg_height = (JDIMENSION)
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 4L);
     cinfo->min_DCT_h_scaled_size = 4;
     cinfo->min_DCT_v_scaled_size = 4;
-  } else if (cinfo->scale_num * 5 >= cinfo->scale_denom * 8) {
+  } else if (cinfo->scale_num * 5 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/5 scaling */
+    /* Provide block_size/5 scaling */
-    cinfo->jpeg_width = cinfo->image_width + (JDIMENSION)
+    cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 3, 5L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 5L);
-    cinfo->jpeg_height = cinfo->image_height + (JDIMENSION)
+    cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 3, 5L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 5L);
     cinfo->min_DCT_h_scaled_size = 5;
     cinfo->min_DCT_v_scaled_size = 5;
-  } else if (cinfo->scale_num * 3 >= cinfo->scale_denom * 4) {
+  } else if (cinfo->scale_num * 6 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 4/3 scaling */
+    /* Provide block_size/6 scaling */
-    cinfo->jpeg_width = cinfo->image_width + (JDIMENSION)
+    cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width, 3L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 6L);
-    cinfo->jpeg_height = cinfo->image_height + (JDIMENSION)
+    cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height, 3L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 6L);
     cinfo->min_DCT_h_scaled_size = 6;
     cinfo->min_DCT_v_scaled_size = 6;
-  } else if (cinfo->scale_num * 7 >= cinfo->scale_denom * 8) {
+  } else if (cinfo->scale_num * 7 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/7 scaling */
+    /* Provide block_size/7 scaling */
-    cinfo->jpeg_width = cinfo->image_width + (JDIMENSION)
+    cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width, 7L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 7L);
-    cinfo->jpeg_height = cinfo->image_height + (JDIMENSION)
+    cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height, 7L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 7L);
     cinfo->min_DCT_h_scaled_size = 7;
     cinfo->min_DCT_v_scaled_size = 7;
-  } else if (cinfo->scale_num >= cinfo->scale_denom) {
+  } else if (cinfo->scale_num * 8 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 1/1 scaling */
+    /* Provide block_size/8 scaling */
-    cinfo->jpeg_width = cinfo->image_width;
+    cinfo->jpeg_width = (JDIMENSION)
-    cinfo->jpeg_height = cinfo->image_height;
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 8L);
+    cinfo->jpeg_height = (JDIMENSION)
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 8L);
     cinfo->min_DCT_h_scaled_size = 8;
     cinfo->min_DCT_v_scaled_size = 8;
-  } else if (cinfo->scale_num * 9 >= cinfo->scale_denom * 8) {
+  } else if (cinfo->scale_num * 9 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/9 scaling */
+    /* Provide block_size/9 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 8, 9L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 9L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 8, 9L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 9L);
     cinfo->min_DCT_h_scaled_size = 9;
     cinfo->min_DCT_v_scaled_size = 9;
-  } else if (cinfo->scale_num * 5 >= cinfo->scale_denom * 4) {
+  } else if (cinfo->scale_num * 10 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 4/5 scaling */
+    /* Provide block_size/10 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 4, 5L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 10L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 4, 5L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 10L);
     cinfo->min_DCT_h_scaled_size = 10;
     cinfo->min_DCT_v_scaled_size = 10;
-  } else if (cinfo->scale_num * 11 >= cinfo->scale_denom * 8) {
+  } else if (cinfo->scale_num * 11 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/11 scaling */
+    /* Provide block_size/11 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 8, 11L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 11L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 8, 11L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 11L);
     cinfo->min_DCT_h_scaled_size = 11;
     cinfo->min_DCT_v_scaled_size = 11;
-  } else if (cinfo->scale_num * 3 >= cinfo->scale_denom * 2) {
+  } else if (cinfo->scale_num * 12 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 2/3 scaling */
+    /* Provide block_size/12 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 2, 3L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 12L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 2, 3L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 12L);
     cinfo->min_DCT_h_scaled_size = 12;
     cinfo->min_DCT_v_scaled_size = 12;
-  } else if (cinfo->scale_num * 13 >= cinfo->scale_denom * 8) {
+  } else if (cinfo->scale_num * 13 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/13 scaling */
+    /* Provide block_size/13 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 8, 13L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 13L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 8, 13L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 13L);
     cinfo->min_DCT_h_scaled_size = 13;
     cinfo->min_DCT_v_scaled_size = 13;
-  } else if (cinfo->scale_num * 7 >= cinfo->scale_denom * 4) {
+  } else if (cinfo->scale_num * 14 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 4/7 scaling */
+    /* Provide block_size/14 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 4, 7L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 14L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 4, 7L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 14L);
     cinfo->min_DCT_h_scaled_size = 14;
     cinfo->min_DCT_v_scaled_size = 14;
-  } else if (cinfo->scale_num * 15 >= cinfo->scale_denom * 8) {
+  } else if (cinfo->scale_num * 15 >= cinfo->scale_denom * cinfo->block_size) {
-    /* Provide 8/15 scaling */
+    /* Provide block_size/15 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width * 8, 15L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 15L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height * 8, 15L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 15L);
     cinfo->min_DCT_h_scaled_size = 15;
     cinfo->min_DCT_v_scaled_size = 15;
   } else {
-    /* Provide 1/2 scaling */
+    /* Provide block_size/16 scaling */
     cinfo->jpeg_width = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_width, 2L);
+      jdiv_round_up((long) cinfo->image_width * cinfo->block_size, 16L);
     cinfo->jpeg_height = (JDIMENSION)
-      jdiv_round_up((long) cinfo->image_height, 2L);
+      jdiv_round_up((long) cinfo->image_height * cinfo->block_size, 16L);
     cinfo->min_DCT_h_scaled_size = 16;
     cinfo->min_DCT_v_scaled_size = 16;
   }
@@ -193,25 +208,11 @@ jpeg_calc_jpeg_dimensions (j_compress_ptr cinfo)
 LOCAL(void)
 jpeg_calc_trans_dimensions (j_compress_ptr cinfo)
 {
-  if (cinfo->min_DCT_h_scaled_size < 1 || cinfo->min_DCT_h_scaled_size > 16
+  if (cinfo->min_DCT_h_scaled_size != cinfo->min_DCT_v_scaled_size)
-      || cinfo->min_DCT_h_scaled_size != cinfo->min_DCT_v_scaled_size)
     ERREXIT2(cinfo, JERR_BAD_DCTSIZE,
 	     cinfo->min_DCT_h_scaled_size, cinfo->min_DCT_v_scaled_size);
 
   cinfo->block_size = cinfo->min_DCT_h_scaled_size;
-
-  switch (cinfo->block_size) {
-  case 2: cinfo->natural_order = jpeg_natural_order2; break;
-  case 3: cinfo->natural_order = jpeg_natural_order3; break;
-  case 4: cinfo->natural_order = jpeg_natural_order4; break;
-  case 5: cinfo->natural_order = jpeg_natural_order5; break;
-  case 6: cinfo->natural_order = jpeg_natural_order6; break;
-  case 7: cinfo->natural_order = jpeg_natural_order7; break;
-  default: cinfo->natural_order = jpeg_natural_order; break;
-  }
-
-  cinfo->lim_Se = cinfo->block_size < DCTSIZE ?
-    cinfo->block_size * cinfo->block_size - 1 : DCTSIZE2-1;
 }
 
 
@@ -229,6 +230,25 @@ initial_setup (j_compress_ptr cinfo, boolean transcode_only)
   else
     jpeg_calc_jpeg_dimensions(cinfo);
 
+  /* Sanity check on block_size */
+  if (cinfo->block_size < 1 || cinfo->block_size > 16)
+    ERREXIT2(cinfo, JERR_BAD_DCTSIZE, cinfo->block_size, cinfo->block_size);
+
+  /* Derive natural_order from block_size */
+  switch (cinfo->block_size) {
+  case 2: cinfo->natural_order = jpeg_natural_order2; break;
+  case 3: cinfo->natural_order = jpeg_natural_order3; break;
+  case 4: cinfo->natural_order = jpeg_natural_order4; break;
+  case 5: cinfo->natural_order = jpeg_natural_order5; break;
+  case 6: cinfo->natural_order = jpeg_natural_order6; break;
+  case 7: cinfo->natural_order = jpeg_natural_order7; break;
+  default: cinfo->natural_order = jpeg_natural_order; break;
+  }
+
+  /* Derive lim_Se from block_size */
+  cinfo->lim_Se = cinfo->block_size < DCTSIZE ?
+    cinfo->block_size * cinfo->block_size - 1 : DCTSIZE2-1;
+
   /* Sanity check on image dimensions */
   if (cinfo->jpeg_height <= 0 || cinfo->jpeg_width <= 0 ||
       cinfo->num_components <= 0 || cinfo->input_components <= 0)

jpeg/jpeglib.h

@@ -2,7 +2,7 @@
  * jpeglib.h
  *
  * Copyright (C) 1991-1998, Thomas G. Lane.
- * Modified 2002-2009 by Guido Vollbeding.
+ * Modified 2002-2010 by Guido Vollbeding.
  * This file is part of the Independent JPEG Group's software.
  * For conditions of distribution and use, see the accompanying README file.
  *
@@ -33,11 +33,13 @@ extern "C" {
 #endif
 #endif
 
-/* Version ID for the JPEG library.
+/* Version IDs for the JPEG library.
  * Might be useful for tests like "#if JPEG_LIB_VERSION >= 80".
  */
 
-#define JPEG_LIB_VERSION  80	/* Version 8.0 */
+#define JPEG_LIB_VERSION        80	/* Compatibility version 8.0 */
+#define JPEG_LIB_VERSION_MAJOR  8
+#define JPEG_LIB_VERSION_MINOR  3
 
 
 /* Various constants determining the sizes of things.

jpeg/jversion.h

@@ -1,7 +1,7 @@
 /*
  * jversion.h
  *
- * Copyright (C) 1991-2010, Thomas G. Lane, Guido Vollbeding.
+ * Copyright (C) 1991-2011, Thomas G. Lane, Guido Vollbeding.
  * This file is part of the Independent JPEG Group's software.
  * For conditions of distribution and use, see the accompanying README file.
  *
@@ -9,6 +9,6 @@
  */
 
 
-#define JVERSION	"8b  16-May-2010"
+#define JVERSION	"8c  16-Jan-2011"
 
-#define JCOPYRIGHT	"Copyright (C) 2010, Thomas G. Lane, Guido Vollbeding"
+#define JCOPYRIGHT	"Copyright (C) 2011, Thomas G. Lane, Guido Vollbeding"

jpeg/libjpeg.txt

@@ -1,6 +1,6 @@
 USING THE IJG JPEG LIBRARY
 
-Copyright (C) 1994-2009, Thomas G. Lane, Guido Vollbeding.
+Copyright (C) 1994-2010, Thomas G. Lane, Guido Vollbeding.
 This file is part of the Independent JPEG Group's software.
 For conditions of distribution and use, see the accompanying README file.
 
@@ -876,6 +876,18 @@ jpeg_simple_progression (j_compress_ptr cinfo)
 
 Compression parameters (cinfo fields) include:
 
+int block_size
+	Set DCT block size.  All N from 1 to 16 are possible.
+	Default is 8 (baseline format).
+	Larger values produce higher compression,
+	smaller values produce higher quality.
+	An exact DCT stage is possible with 1 or 2.
+	With the default quality of 75 and default Luminance qtable
+	the DCT+Quantization stage is lossless for value 1.
+	Note that values other than 8 require a SmartScale capable decoder,
+	introduced with IJG JPEG 8.  Setting the block_size parameter for
+	compression works with version 8c and later.
+
 J_DCT_METHOD dct_method
 	Selects the algorithm used for the DCT step.  Choices are:
 		JDCT_ISLOW: slow but accurate integer algorithm
@@ -895,8 +907,10 @@ J_DCT_METHOD dct_method
 unsigned int scale_num, scale_denom
 	Scale the image by the fraction scale_num/scale_denom.  Default is
 	1/1, or no scaling.  Currently, the supported scaling ratios are
-	8/N with all N from 1 to 16.  (The library design allows for arbitrary
+	M/N with all N from 1 to 16, where M is the destination DCT size,
-	scaling ratios but this is not likely to be implemented any time soon.)
+	which is 8 by default (see block_size parameter above).
+	(The library design allows for arbitrary scaling ratios but this
+	is not likely to be implemented any time soon.)
 
 J_COLOR_SPACE jpeg_color_space
 int num_components

jpeg/usage.txt

@@ -87,8 +87,9 @@ The basic command line switches for cjpeg are:
 	-progressive	Create progressive JPEG file (see below).
 
 	-scale M/N	Scale the output image by a factor M/N.  Currently
-			supported scale factors are 8/N with all N from 1 to
+			supported scale factors are M/N with all N from 1 to
-			16.
+			16, where M is the destination DCT size, which is 8 by
+			default (see -block N switch below).
 
 	-targa		Input file is Targa format.  Targa files that contain
 			an "identification" field will not be automatically
@@ -150,6 +151,19 @@ file size is about the same --- often a little smaller.
 
 Switches for advanced users:
 
+	-block N	Set DCT block size.  All N from 1 to 16 are possible.
+			Default is 8 (baseline format).
+			Larger values produce higher compression,
+			smaller values produce higher quality
+			(exact DCT stage possible with 1 or 2; with the
+			default quality of 75 and default Luminance qtable
+			the DCT+Quantization stage is lossless for N=1).
+			CAUTION: An implementation of the JPEG SmartScale
+			extension is required for this feature.  SmartScale
+			enabled JPEG is not yet widely implemented, so many
+			decoders will be unable to view a SmartScale extended
+			JPEG file at all.
+
 	-dct int	Use integer DCT method (default).
 	-dct fast	Use fast integer DCT (less accurate).
 	-dct float	Use floating-point DCT method.