cfFilterGhostscript(): Introduce cupsHalftoneType dithering algorithms (#92)

- Added new dithering (halftone) algorithms in addition to the default
  Ghostscript one
- Controlled either with `halftone-type` job option or `cupsHalftoneType`
  PPD option
- Stochastic algorithm is implemented in `stocht.ps` Ghostscript library,
  just include it if cupsHalftoneType is set to yes/true/on/stochastic
- foo2zjs algorithm is PostScript code taken from foo2zjs-pstops
  (don't know the name of the algorithm)
- Added bi-level threshold support, similar to pdftoraster implementation

Author: ValdikSS

cupsfilters/ghostscript.c

@@ -44,6 +44,14 @@ typedef enum gs_doc_e
 
 typedef cups_page_header_t gs_page_header;
 
+typedef enum cups_halftone_type_e
+{
+  HALFTONE_DEFAULT,
+  HALFTONE_STOCHASTIC,
+  HALFTONE_FOO2ZJS,
+  HALFTONE_BI_LEVEL
+} cups_halftone_type_t;
+
 static gs_doc_t
 parse_doc_type(FILE *fp)
 {
@@ -816,6 +824,7 @@ cfFilterGhostscript(int inputfd,            // I - File descriptor input
   struct sigaction sa;
   cf_cm_calibration_t cm_calibrate;
   int pxlcolor = 0; // 1 if printer is color printer otherwise 0.
+  cups_halftone_type_t halftonetype = HALFTONE_DEFAULT;
   ipp_attribute_t *ipp_attr;
   cf_logfunc_t log = data->logfunc;
   void          *ld = data->logdata;
@@ -1617,6 +1626,37 @@ cfFilterGhostscript(int inputfd,            // I - File descriptor input
   else if (!cm_disabled)
     cupsArrayAdd(gs_args, strdup("-sOutputICCProfile=srgb.icc"));
 
+  if ((t = cupsGetOption("cupsHalftoneType", num_options, options)) != NULL ||
+      (t = cupsGetOption("halftone-type", num_options, options)) != NULL)
+  {
+    if (!strcasecmp(t, "true") || !strcasecmp(t, "on") ||
+	!strcasecmp(t, "yes") || !strcasecmp(t, "stochastic"))
+      halftonetype = HALFTONE_STOCHASTIC;
+    else if (!strcasecmp(t, "foo2zjs"))
+      halftonetype = HALFTONE_FOO2ZJS;
+    else if (!strcasecmp(t, "bi-level"))
+      halftonetype = HALFTONE_BI_LEVEL;
+  }
+
+  //
+  // For bi-level type, also check print-color-mode, the way it is
+  // handled by Poppler pwgtoraster/pdftoraster/pclmtoraster utilities.
+  //
+  if ((t = cupsGetOption("print-color-mode", num_options, options)) != NULL &&
+      !strcasecmp(t, "bi-level"))
+    halftonetype = HALFTONE_BI_LEVEL;
+
+  //
+  // Use Stochastic Halftone dithering found in GhostScript stocht.ps library file.
+  // It is activated automatically.
+  //
+  if (halftonetype == HALFTONE_STOCHASTIC)
+  {
+    if (log) log(ld, CF_LOGLEVEL_DEBUG,
+		 "cfFilterGhostscript: Ghostscript using Stochastic Halftone dithering.");
+    cupsArrayAdd(gs_args, strdup("stocht.ps"));
+  }
+
   // Switch to taking PostScript commands on the Ghostscript command line
   cupsArrayAdd(gs_args, strdup("-c"));
 
@@ -1673,6 +1713,64 @@ cfFilterGhostscript(int inputfd,            // I - File descriptor input
 		 "cfFilterGhostscript: Ghostscript using Any-Part-of-Pixel method to "
 		 "fill paths.");
 
+  //
+  // Use halftone dithering algorithm found in foo2zjs-pstops file
+  //
+  if (halftonetype == HALFTONE_FOO2ZJS)
+  {
+    if (log) log(ld, CF_LOGLEVEL_DEBUG,
+		 "cfFilterGhostscript: Ghostscript using foo2zjs Halftone dithering.");
+    cupsArrayAdd(gs_args, strdup("/SpotDot { 180 mul cos exch 180 mul cos add 2 div } def\
+<<\
+    /HalftoneType 5\
+    /Cyan <<\
+        /HalftoneType 1\
+        /AccurateScreens true\
+        /Frequency 150\
+        /Angle 105\
+        /SpotFunction /SpotDot load\
+    >>\
+    /Magenta <<\
+        /HalftoneType 1\
+        /AccurateScreens true\
+        /Frequency 150\
+        /Angle 165\
+        /SpotFunction /SpotDot load\
+    >>\
+    /Yellow <<\
+        /HalftoneType 1\
+        /AccurateScreens true\
+        /Frequency 150\
+        /Angle 30\
+        /SpotFunction /SpotDot load\
+    >>\
+    /Black <<\
+        /HalftoneType 1\
+        /AccurateScreens true\
+        /Frequency 150\
+        /Angle 45\
+        /SpotFunction /SpotDot load\
+    >>\
+    /Default <<\
+        /HalftoneType 1\
+        /AccurateScreens true\
+        /Frequency 150\
+        /Angle 37\
+        /SpotFunction /SpotDot load\
+    >>\
+>> /Default exch /Halftone defineresource sethalftone"));
+  }
+
+  //
+  // Use simple threshold (bi-level) halftone algorithm
+  //
+  if (halftonetype == HALFTONE_BI_LEVEL)
+  {
+    if (log) log(ld, CF_LOGLEVEL_DEBUG,
+		 "cfFilterGhostscript: Ghostscript using Bi-Level Halftone dithering.");
+    cupsArrayAdd(gs_args, strdup("{ .5 gt { 1 } { 0 } ifelse} settransfer"));
+  }
+
   // Mark the end of PostScript commands supplied on the Ghostscript command
   // line (with the "-c" option), so that we can supply the input file name
   cupsArrayAdd(gs_args, strdup("-f"));