Skip to content

Ghostscript (gstoraster): new halftone/dithering algorithms #160

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
tillkamppeter merged 3 commits into from
Jun 23, 2026
Merged

Ghostscript (gstoraster): new halftone/dithering algorithms #160

Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
d0bea89
Ghostscript filters (gsto...): 8x8 ordered dithering algorithm
ValdikSS Jun 22, 2026
8eaea0a
Ghostscript filters (gsto...): genordered halftone+dithering algorithm
ValdikSS Jun 22, 2026
17409b7
Ghostscript filters (gsto...): spot halftone algorithm from PDF
ValdikSS Jun 22, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
156 changes: 155 additions & 1 deletion cupsfilters/ghostscript.c
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -49,9 +49,51 @@ typedef enum cups_halftone_type_e
HALFTONE_DEFAULT,
HALFTONE_STOCHASTIC,
HALFTONE_FOO2ZJS,
HALFTONE_BI_LEVEL
HALFTONE_BI_LEVEL,
HALFTONE_DITHERING,
HALFTONE_GENORDERED,
HALFTONE_SPOT
} cups_halftone_type_t;

static const char *ht_spot_functions[] =
{
"{dup mul exch dup mul add 1 exch sub}", /* 0 SimpleDot */
"{dup mul exch dup mul add 1 sub}", /* 1 InvertedSimpleDot */
"{360 mul sin 2 div exch 360 mul sin 2 div add}",/* 2 DoubleDot */
"{360 mul sin 2 div exch 360 mul sin 2 div add neg}",/* 3 InvertedDoubleDot */
"{180 mul cos exch 180 mul cos add 2 div}", /* 4 CosineDot */
"{360 mul sin 2 div exch 2 div 360 mul sin 2 div add}", /* 5 Double */
"{360 mul sin 2 div exch 2 div 360 mul sin 2 div add neg}",/* 6 InvertedDouble */
"{exch pop abs neg}", /* 7 Line */
"{pop}", /* 8 LineX */
"{exch pop}", /* 9 LineY */
"{abs exch abs 2 copy add 1.0 le"
" {dup mul exch dup mul add 1 exch sub}"
" {1 sub dup mul exch 1 sub dup mul add 1 sub}"
" ifelse}", /* 10 Round */
"{abs exch abs 2 copy 3 mul exch 4 mul add 3 sub dup 0 lt"
" {pop dup mul exch 0.75 div dup mul add 4 div 1 exch sub}"
" {dup 1 gt"
" {pop 1 exch sub dup mul exch 1 exch sub 0.75 div dup mul add 4 div 1 sub}"
" {0.5 exch sub exch pop exch pop}}"
" ifelse}ifelse}", /* 11 Ellipse */
"{dup mul 0.9 mul exch dup mul add 1 exch sub}", /* 12 EllipseA */
"{dup mul 0.9 mul exch dup mul add 1 sub}", /* 13 InvertedEllipseA */
"{dup 5 mul 8 div mul exch dup mul exch add sqrt 1 exch sub}", /* 14 EllipseB */
"{dup mul exch dup mul 0.9 mul add 1 exch sub}", /* 15 EllipseC */
"{dup mul exch dup mul 0.9 mul add 1 sub}", /* 16 InvertedEllipseC */
"{abs exch abs 2 copy lt {exch} if pop neg}", /* 17 Square */
"{abs exch abs 2 copy gt {exch} if pop neg}", /* 18 Cross */
"{abs exch abs 0.9 mul add 2 div}", /* 19 Rhomboid */
"{abs exch abs 2 copy add 0.75 le"
" {dup mul exch dup mul add 1 exch sub}"
" {2 copy add 1.23 le"
" {0.85 mul add 1 exch sub}"
" {1 sub dup mul exch 1 sub dup mul add 1 sub}"
" ifelse} ifelse}" /* 20 Diamond */
};
#define HT_SPOT_FUNCTIONS_COUNT ((int)(sizeof(ht_spot_functions)/sizeof(ht_spot_functions[0])))

static gs_doc_t
parse_doc_type(FILE *fp)
{
Expand Down Expand Up @@ -825,6 +867,7 @@ cfFilterGhostscript(int inputfd, // I - File descriptor input
cf_cm_calibration_t cm_calibrate;
int pxlcolor = 0; // 1 if printer is color printer otherwise 0.
cups_halftone_type_t halftonetype = HALFTONE_DEFAULT;
int ht_frequency = 133, ht_angle = 45, ht_dotshape = 0;
ipp_attribute_t *ipp_attr;
cf_logfunc_t log = data->logfunc;
void *ld = data->logdata;
Expand Down Expand Up @@ -1636,6 +1679,58 @@ cfFilterGhostscript(int inputfd, // I - File descriptor input
halftonetype = HALFTONE_FOO2ZJS;
else if (!strcasecmp(t, "bi-level"))
halftonetype = HALFTONE_BI_LEVEL;
else if (!strcasecmp(t, "dithering"))
halftonetype = HALFTONE_DITHERING;
else if (!strncasecmp(t, "genordered", 10) &&
(t[10] == '-' || t[10] == '\0'))
{
halftonetype = HALFTONE_GENORDERED;
if (t[10] == '-')
{
const char *p = t + 11;
char *endp;
long v;
v = strtol(p, &endp, 10);
if (endp != p) { ht_frequency = (int)v; p = endp; }
if (*p == '-')
{
p++;
v = strtol(p, &endp, 10);
if (endp != p) { ht_angle = (int)v; p = endp; }
if (*p == '-')
{
p++;
v = strtol(p, &endp, 10);
if (endp != p) ht_dotshape = (int)v;
}
}
}
}
else if (!strncasecmp(t, "spot", 4) &&
(t[4] == '-' || t[4] == '\0'))
{
halftonetype = HALFTONE_SPOT;
if (t[4] == '-')
{
const char *p = t + 5;
char *endp;
long v;
v = strtol(p, &endp, 10);
if (endp != p) { ht_frequency = (int)v; p = endp; }
if (*p == '-')
{
p++;
v = strtol(p, &endp, 10);
if (endp != p) { ht_angle = (int)v; p = endp; }
if (*p == '-')
{
p++;
v = strtol(p, &endp, 10);
if (endp != p) ht_dotshape = (int)v;
}
}
}
}
}

//
Expand Down Expand Up @@ -1771,6 +1866,65 @@ cfFilterGhostscript(int inputfd, // I - File descriptor input
cupsArrayAdd(gs_args, strdup("{ .5 gt { 1 } { 0 } ifelse} settransfer"));
}

//
// Use 8x8 ordered dithering.
//
// This uses .setloresscreen Ghostscript setup function which is a part
// of dithering vs halftoning automatic selection code and is used only
// if DPI of the output device is low (< 150).
// To correctly force it, we need to call it in Install function of
// Page Device.
// DPI is auto-detected and passed for screen frequency calculation.
//
// Particularly useful for printing images on label printers (203 DPI).
//
if (halftonetype == HALFTONE_DITHERING)
{
if (log) log(ld, CF_LOGLEVEL_DEBUG,
"cfFilterGhostscript: Ghostscript using 8x8 ordered dithering.");
cupsArrayAdd(gs_args, strdup("<< /Install { 72 72 matrix defaultmatrix dtransform abs exch abs .min .setloresscreen } >> setpagedevice"));
}

//
// Ghostscript .genordered advanced ordered dithering
// PostScript HalftoneType 3 (threshold array based).
//
// This uses the .genordered operator to generate an ordered dither
// halftone screen with configurable frequency, angle and dot shape.
//
// Dot shapes:
// 0=CIRCLE, 1=REDBOOK, 2=INVERTED, 3=RHOMBOID, 4=LINE_X, 5=LINE_Y,
// 6=DIAMOND1, 7=DIAMOND2, 8=ROUNDSPOT */
//
if (halftonetype == HALFTONE_GENORDERED) {
if (log) log(ld, CF_LOGLEVEL_DEBUG,
"cfFilterGhostscript: Ghostscript using .genordered halftone (frequency=%d angle=%d dotshape=%d).",
ht_frequency, ht_angle, ht_dotshape);
snprintf(tmpstr, sizeof(tmpstr),
"<< /Frequency %d /Angle %d /DotShape %d >> .genordered /Default exch /Halftone defineresource sethalftone { } settransfer 0.003 setsmoothness",
ht_frequency, ht_angle, ht_dotshape);
cupsArrayAdd(gs_args, strdup(tmpstr));
}

//
// PostScript HalftoneType 1 spot-function halftone algorithm.
//
// Spot functions are from PDF specification, see ht_spot_functions array.
//
if (halftonetype == HALFTONE_SPOT)
{
int shape = ht_dotshape;
if (shape < 0) shape = 0;
if (shape >= HT_SPOT_FUNCTIONS_COUNT) shape = HT_SPOT_FUNCTIONS_COUNT - 1;
if (log) log(ld, CF_LOGLEVEL_DEBUG,
"cfFilterGhostscript: Ghostscript using Spot halftone (frequency=%d angle=%d dotshape=%d).\n",
ht_frequency, ht_angle, shape);
snprintf(tmpstr, sizeof(tmpstr),
"<< /HalftoneType 1 /Frequency %d /Angle %d /SpotFunction %s >> /Default exch /Halftone defineresource sethalftone",
ht_frequency, ht_angle, ht_spot_functions[shape]);
cupsArrayAdd(gs_args, strdup(tmpstr));
}

// 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"));
Expand Down
Loading