cjpeg: auto. compr. gray BMP/GIF-->grayscale JPEG

aa7459050d was supposed to enable this for
BMP input images but didn't, due to a similar oversight to the one fixed
in the previous commit.

ChangeLog.md

@@ -7,6 +7,9 @@
 input files into full-color JPEG images unless the `-grayscale` option was
 used.
 
+2. cjpeg now automatically compresses GIF and 8-bit BMP input files into
+grayscale JPEG images if the input files contain only shades of gray.
+
 
 2.1.2
 =====

cjpeg.1

@@ -1,4 +1,4 @@
-.TH CJPEG 1 "18 November 2021"
+.TH CJPEG 1 "30 November 2021"
 .SH NAME
 cjpeg \- compress an image file to a JPEG file
 .SH SYNOPSIS
@@ -40,11 +40,7 @@ Scale quantization tables to adjust image quality.  Quality is 0 (worst) to
 100 (best); default is 75.  (See below for more info.)
 .TP
 .B \-grayscale
-Create monochrome JPEG file from color input.  Be sure to use this switch when
-compressing a grayscale BMP or GIF file, because
-.B cjpeg
-isn't bright enough to notice whether a BMP or GIF file uses only shades of
-gray.  By saying
+Create monochrome JPEG file from color input.  By saying
 .BR \-grayscale,
 you'll get a smaller JPEG file that takes less time to process.
 .TP

rdbmp.c

@@ -125,7 +125,8 @@ read_colormap(bmp_source_ptr sinfo, int cmaplen, int mapentrysize)
     break;
   }
 
-  if (sinfo->cinfo->in_color_space == JCS_UNKNOWN && gray)
+  if ((sinfo->cinfo->in_color_space == JCS_UNKNOWN ||
+       sinfo->cinfo->in_color_space == JCS_RGB) && gray)
     sinfo->cinfo->in_color_space = JCS_GRAYSCALE;
 
   if (sinfo->cinfo->in_color_space == JCS_GRAYSCALE && !gray)

rdgif.c

@@ -345,7 +345,7 @@ LOCAL(void)
 ReadColorMap(gif_source_ptr sinfo, int cmaplen, JSAMPARRAY cmap)
 /* Read a GIF colormap */
 {
-  int i;
+  int i, gray = 1;
 
   for (i = 0; i < cmaplen; i++) {
 #if BITS_IN_JSAMPLE == 8
@@ -356,6 +356,14 @@ ReadColorMap(gif_source_ptr sinfo, int cmaplen, JSAMPARRAY cmap)
     cmap[CM_RED][i]   = (JSAMPLE)UPSCALE(ReadByte(sinfo));
     cmap[CM_GREEN][i] = (JSAMPLE)UPSCALE(ReadByte(sinfo));
     cmap[CM_BLUE][i]  = (JSAMPLE)UPSCALE(ReadByte(sinfo));
+    if (cmap[CM_RED][i] != cmap[CM_GREEN][i] ||
+        cmap[CM_GREEN][i] != cmap[CM_BLUE][i])
+      gray = 0;
+  }
+
+  if (sinfo->cinfo->in_color_space == JCS_RGB && gray) {
+    sinfo->cinfo->in_color_space = JCS_GRAYSCALE;
+    sinfo->cinfo->input_components = 1;
   }
 }
 
@@ -516,10 +524,15 @@ start_input_gif(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
     source->pub.get_pixel_rows = get_pixel_rows;
   }
 
+  if (cinfo->in_color_space != JCS_GRAYSCALE) {
+    cinfo->in_color_space = JCS_RGB;
+    cinfo->input_components = NUMCOLORS;
+  }
+
   /* Create compressor input buffer. */
   source->pub.buffer = (*cinfo->mem->alloc_sarray)
-    ((j_common_ptr)cinfo, JPOOL_IMAGE, (JDIMENSION)width * NUMCOLORS,
-     (JDIMENSION)1);
+    ((j_common_ptr)cinfo, JPOOL_IMAGE,
+     (JDIMENSION)width * cinfo->input_components, (JDIMENSION)1);
   source->pub.buffer_height = 1;
 
   /* Pad colormap for safety. */
@@ -530,8 +543,6 @@ start_input_gif(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
   }
 
   /* Return info about the image. */
-  cinfo->in_color_space = JCS_RGB;
-  cinfo->input_components = NUMCOLORS;
   cinfo->data_precision = BITS_IN_JSAMPLE; /* we always rescale data to this */
   cinfo->image_width = width;
   cinfo->image_height = height;
@@ -556,11 +567,18 @@ get_pixel_rows(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
   register JSAMPARRAY colormap = source->colormap;
 
   ptr = source->pub.buffer[0];
-  for (col = cinfo->image_width; col > 0; col--) {
-    c = LZWReadByte(source);
-    *ptr++ = colormap[CM_RED][c];
-    *ptr++ = colormap[CM_GREEN][c];
-    *ptr++ = colormap[CM_BLUE][c];
+  if (cinfo->in_color_space == JCS_GRAYSCALE) {
+    for (col = cinfo->image_width; col > 0; col--) {
+      c = LZWReadByte(source);
+      *ptr++ = colormap[CM_RED][c];
+    }
+  } else {
+    for (col = cinfo->image_width; col > 0; col--) {
+      c = LZWReadByte(source);
+      *ptr++ = colormap[CM_RED][c];
+      *ptr++ = colormap[CM_GREEN][c];
+      *ptr++ = colormap[CM_BLUE][c];
+    }
   }
   return 1;
 }
@@ -646,11 +664,18 @@ get_interlaced_row(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
      FALSE);
   /* Scan the row, expand colormap, and output */
   ptr = source->pub.buffer[0];
-  for (col = cinfo->image_width; col > 0; col--) {
-    c = *sptr++;
-    *ptr++ = colormap[CM_RED][c];
-    *ptr++ = colormap[CM_GREEN][c];
-    *ptr++ = colormap[CM_BLUE][c];
+  if (cinfo->in_color_space == JCS_GRAYSCALE) {
+    for (col = cinfo->image_width; col > 0; col--) {
+      c = *sptr++;
+      *ptr++ = colormap[CM_RED][c];
+    }
+  } else {
+    for (col = cinfo->image_width; col > 0; col--) {
+      c = *sptr++;
+      *ptr++ = colormap[CM_RED][c];
+      *ptr++ = colormap[CM_GREEN][c];
+      *ptr++ = colormap[CM_BLUE][c];
+    }
   }
   source->cur_row_number++;     /* for next time */
   return 1;

usage.txt

@@ -72,12 +72,9 @@ The basic command line switches for cjpeg are:
                           Quality is 0 (worst) to 100 (best); default is 75.
                           (See below for more info.)
 
-        -grayscale      Create monochrome JPEG file from color input.
-                        Be sure to use this switch when compressing a grayscale
-                        BMP or GIF file, because cjpeg isn't bright enough to
-                        notice whether a BMP or GIF file uses only shades of
-                        gray.  By saying -grayscale, you'll get a smaller JPEG
-                        file that takes less time to process.
+        -grayscale      Create monochrome JPEG file from color input.  By
+                        saying -grayscale, you'll get a smaller JPEG file that
+                        takes less time to process.
 
         -rgb            Create RGB JPEG file.
                         Using this switch suppresses the conversion from RGB