cjpeg -verbose: Print PBMPLUS input file precision

This gives the user a hint as to whether converting the input file into
a JPEG file with a particular data precision will result in a loss of
precision.  Also modify usage.txt and the cjpeg man page to warn the
user about that possibility.

cderror.h

@@ -5,7 +5,7 @@
  * Copyright (C) 1994-1997, Thomas G. Lane.
  * Modified 2009-2017 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2021, D. R. Commander.
+ * Copyright (C) 2021, 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -76,10 +76,10 @@ JMESSAGE(JERR_PPM_COLORSPACE, "PPM output must be grayscale or RGB")
 JMESSAGE(JERR_PPM_NONNUMERIC, "Nonnumeric data in PPM file")
 JMESSAGE(JERR_PPM_NOT, "Not a PPM/PGM file")
 JMESSAGE(JERR_PPM_OUTOFRANGE, "Numeric value out of range in PPM file")
-JMESSAGE(JTRC_PGM, "%ux%u PGM image")
+JMESSAGE(JTRC_PGM, "%ux%u PGM image (maximum color value = %u)")
-JMESSAGE(JTRC_PGM_TEXT, "%ux%u text PGM image")
+JMESSAGE(JTRC_PGM_TEXT, "%ux%u text PGM image (maximum color value = %u)")
-JMESSAGE(JTRC_PPM, "%ux%u PPM image")
+JMESSAGE(JTRC_PPM, "%ux%u PPM image (maximum color value = %u)")
-JMESSAGE(JTRC_PPM_TEXT, "%ux%u text PPM image")
+JMESSAGE(JTRC_PPM_TEXT, "%ux%u text PPM image (maximum color value = %u)")
 
 JMESSAGE(JERR_TGA_BADCMAP, "Unsupported Targa colormap format")
 JMESSAGE(JERR_TGA_BADPARMS, "Invalid or unsupported Targa file")

cjpeg.1

@@ -157,7 +157,13 @@ must also be specified.  Note that only the PBMPLUS input file format supports
 data precisions other than 8.  (For historical reasons,
 .B cjpeg
 allows GIF input files to be converted into 12-bit-per-sample JPEG files, but
-this is not a useful conversion.)
+this is not a useful conversion.)  Note also that PBMPLUS input files are
+silently scaled to the target data precision, even if it is lower than the
+precision of the input file.  Passing an argument of
+.B \-verbose
+to
+.B cjpeg
+will cause it to print information about the precision of the input file.
 .B Caution:
 12-bit and 16-bit data precision is not yet widely implemented, so many
 decoders will be unable to handle a 12-bit-per-sample or 16-bit-per-sample JPEG

rdppm.c

@@ -5,7 +5,7 @@
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * Modified 2009 by Bill Allombert, Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2015-2017, 2020-2023, D. R. Commander.
+ * Copyright (C) 2015-2017, 2020-2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -707,7 +707,7 @@ start_input_ppm(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
     if (cinfo->in_color_space == JCS_UNKNOWN ||
         cinfo->in_color_space == JCS_RGB)
       cinfo->in_color_space = JCS_GRAYSCALE;
-    TRACEMS2(cinfo, 1, JTRC_PGM_TEXT, w, h);
+    TRACEMS3(cinfo, 1, JTRC_PGM_TEXT, w, h, maxval);
     if (cinfo->in_color_space == JCS_GRAYSCALE)
       source->pub.get_pixel_rows = get_text_gray_row;
     else if (IsExtRGB(cinfo->in_color_space))
@@ -722,7 +722,7 @@ start_input_ppm(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
   case '3':                     /* it's a text-format PPM file */
     if (cinfo->in_color_space == JCS_UNKNOWN)
       cinfo->in_color_space = JCS_EXT_RGB;
-    TRACEMS2(cinfo, 1, JTRC_PPM_TEXT, w, h);
+    TRACEMS3(cinfo, 1, JTRC_PPM_TEXT, w, h, maxval);
     if (IsExtRGB(cinfo->in_color_space))
       source->pub.get_pixel_rows = get_text_rgb_row;
     else if (cinfo->in_color_space == JCS_CMYK)
@@ -736,7 +736,7 @@ start_input_ppm(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
     if (cinfo->in_color_space == JCS_UNKNOWN ||
         cinfo->in_color_space == JCS_RGB)
       cinfo->in_color_space = JCS_GRAYSCALE;
-    TRACEMS2(cinfo, 1, JTRC_PGM, w, h);
+    TRACEMS3(cinfo, 1, JTRC_PGM, w, h, maxval);
     if (maxval > 255) {
       if (cinfo->in_color_space == JCS_GRAYSCALE)
         source->pub.get_pixel_rows = get_word_gray_row;
@@ -766,7 +766,7 @@ start_input_ppm(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
   case '6':                     /* it's a raw-format PPM file */
     if (cinfo->in_color_space == JCS_UNKNOWN)
       cinfo->in_color_space = JCS_EXT_RGB;
-    TRACEMS2(cinfo, 1, JTRC_PPM, w, h);
+    TRACEMS3(cinfo, 1, JTRC_PPM, w, h, maxval);
     if (maxval > 255) {
       if (IsExtRGB(cinfo->in_color_space))
         source->pub.get_pixel_rows = get_word_rgb_row;

usage.txt

@@ -171,10 +171,15 @@ Switches for advanced users:
                        PBMPLUS input file format supports data precisions other
                        than 8.  (For historical reasons, cjpeg allows GIF input
                        files to be converted into 12-bit-per-sample JPEG files,
-                       but this is not a useful conversion.)  CAUTION: 12-bit
+                       but this is not a useful conversion.)  Note also that
-                       and 16-bit data precision is not yet widely implemented,
+                       PBMPLUS input files are silently scaled to the target
-                       so many decoders will be unable to handle a
+                       data precision, even if it is lower than the precision
-                       12-bit-per-sample or 16-bit-per-sample JPEG file at all.
+                       of the input file.  Passing an argument of -verbose to
+                       cjpeg will cause it to print information about the
+                       precision of the input file.  CAUTION: 12-bit and 16-bit
+                       data precision is not yet widely implemented, so many
+                       decoders will be unable to handle a 12-bit-per-sample or
+                       16-bit-per-sample JPEG file at all.
 
         -lossless psv[,Pt] Create a lossless JPEG file using the specified
                         predictor selection value (1 - 7) and optional point