print: fix color management when using printer ICC profiles

When a printer ICC profile is selected, darktable applies the color
conversion via LittleCMS but fails to communicate this to the
downstream CUPS pipeline, causing incorrect color output.

This commit fixes the issue on both Linux and macOS:

1. The printer.profile field on the print info struct was never
   populated from the user's profile selection, so the cm-calibration
   CUPS option was always set to "false". Fixed by propagating the
   profile selection into printer.profile before calling dt_print_file().

2. On Linux, cm-calibration=true tells pdftoraster to skip its own
   color management, preventing double conversion.

3. On macOS, cgpdftoraster ignores cm-calibration entirely. Two
   additional fixes are needed:
   - Embed the ICC profile in the PDF so cgpdftoraster knows the
     pixel data is not sRGB (without this, it misinterprets the
     already-converted data).
   - Send AP_ColorMatchingMode=AP_ApplicationColorMatching (both
     dot and underscore variants) to tell cgpdftoraster and the
     printer driver that the application already handled color
     matching.

Note: on Linux, the ICC profile must NOT be embedded in the PDF —
Poppler (inside pdftoraster) would see the ICCBased color space and
convert the data back to sRGB, undoing the LittleCMS conversion.

Tested on macOS with Canon Pixma Pro-100 against Capture One output.

Author: brettnak

src/common/cups_print.c

@@ -547,12 +547,36 @@ void dt_print_file(const dt_imgid_t imgid,
 
     cupsFreeDests(num_dests, dests);
 
-    // if we have a profile, disable cm on CUPS, this is important as dt does the cm
+    // When a printer ICC profile is selected, tell CUPS to skip its
+    // own color management — darktable already did the conversion
+    // via LittleCMS, and the ICC profile is embedded in the PDF.
+    //
+    // Linux: cm-calibration=true causes pdftoraster to set cm_disabled=1,
+    //        skipping its own color management.
+    //
+    // macOS: cgpdftoraster ignores cm-calibration. Instead we send
+    //        AP_ColorMatchingMode=AP_ApplicationColorMatching which
+    //        tells both cgpdftoraster and the printer driver that the
+    //        application already handled color matching.
 
     num_options = cupsAddOption("cm-calibration",
                                 *pinfo->printer.profile ? "true" : "false",
                                 num_options, &options);
 
+#ifdef __APPLE__
+    if(*pinfo->printer.profile)
+    {
+      // dot-notation variant appears to be legacy but is observed in
+      // print jobs from other macOS applications
+      num_options = cupsAddOption("AP.ColorMatchingMode",
+                                  "AP_ApplicationColorMatching",
+                                  num_options, &options);
+      num_options = cupsAddOption("AP_ColorMatchingMode",
+                                  "AP_ApplicationColorMatching",
+                                  num_options, &options);
+    }
+#endif
+
     // media to print on
 
     num_options = cupsAddOption("media", pinfo->paper.name, num_options, &options);

src/libs/print_settings.c

@@ -445,7 +445,7 @@ static void _create_pdf(dt_job_t *job,
 
   const float page_width  = dt_pdf_mm_to_point(width);
   const float page_height = dt_pdf_mm_to_point(height);
-  const int icc_id = 0;
+  int icc_id = 0;
 
   dt_pdf_image_t *pdf_image[MAX_IMAGE_PER_PAGE];
 
@@ -454,11 +454,19 @@ static void _create_pdf(dt_job_t *job,
                                params->prt.printer.resolution,
                                DT_PDF_STREAM_ENCODER_FLATE);
 
-/*
-  // ??? should a profile be embedded here?
-  if(*printer_profile)
-    icc_id = dt_pdf_add_icc(pdf, printer_profile);
-*/
+#ifdef __APPLE__
+  // On macOS, embed the printer ICC profile in the PDF so the
+  // already-converted pixel data is correctly tagged with its actual
+  // color space. Without this, cgpdftoraster assumes DeviceRGB = sRGB
+  // and misinterprets the data.
+  //
+  // On Linux this must NOT be done — Poppler (inside pdftoraster) would
+  // see the ICCBased color space and convert the data back to sRGB,
+  // undoing the LittleCMS conversion. Linux uses cm-calibration instead.
+  if(params->p_icc_profile && *params->p_icc_profile)
+    icc_id = dt_pdf_add_icc(pdf, params->p_icc_profile);
+#endif
+
   int32_t count = 0;
 
   for(int k=0; k<imgs.count; k++)
@@ -779,6 +787,13 @@ static void _print_button_clicked(GtkWidget *widget, dt_lib_module_t *self)
   params->p_icc_intent = ps->v_pintent;
   params->black_point_compensation = ps->v_black_point_compensation;
 
+  // Propagate the printer profile selection so dt_print_file() can
+  // tell CUPS to skip color management when dt already did the
+  // ICC conversion. See cups_print.c for platform-specific details.
+  if(ps->v_piccprofile && *ps->v_piccprofile)
+    g_strlcpy(params->prt.printer.profile, ps->v_piccprofile,
+              sizeof(params->prt.printer.profile));
+
   dt_control_add_job(DT_JOB_QUEUE_USER_EXPORT, job);
 }