pdfwrite - Fix missing shading case

Bug #706453 "Element missing while converting pdf from rgb to cmyk using Ghostscript"

When converting a shading to an image (for ps2write or certain cases of
colour converting shadings with pdfwrite) it is possible that the
shading functions only call fill_trapezoid() and not fill_path().

This can result in us reaching dump_converted_image with
cvd->mask_is_empty and cvd->path_is_empty both true and we would then not
write out the rendered image.

Because of the convoluted nature of this code (which I am convinced is
sub-optimal!) simply setting either mask_is_empty and/or path_is_empty
to false results in incorrect rendering. So here we add a new boolean
'filled_trap' and add a case to deal with that after testing all the
other conditions.

This results in distinct progressions with ps2write and thee files:
/tests_private/comparefiles/Bug689189.pdf
/tests_private/comparefiles/Clarke-Tate-Manns-Chinese.ai

The file /tests_private/comparefiles/Bug691775.pdf shows diffs that
are imperceptible by eye.

2 files changed, 22 insertions, 1 deletions

diff --git a/devices/vector/gdevpdfd.c b/devices/vector/gdevpdfd.c
index 96d60ea13..fb55655c9 100644
--- a/devices/vector/gdevpdfd.c
+++ b/devices/vector/gdevpdfd.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 2001-2022 Artifex Software, Inc.
+/* Copyright (C) 2001-2023 Artifex Software, Inc.
    All Rights Reserved.
 
    This software is provided AS-IS with no warranty, either express or
@@ -1397,7 +1397,12 @@ pdf_dump_converted_image(gx_device_pdf *pdev, pdf_lcvd_t *cvd, int for_pattern)
         stream_puts(pdev->strm, "q\n");
         code = write_image_with_clip(pdev, cvd, for_pattern);
         stream_puts(pdev->strm, "Q\n");
+    } else if (cvd->filled_trap){
+        stream_puts(pdev->strm, "q\n");
+        code = write_image_with_clip(pdev, cvd, for_pattern);
+        stream_puts(pdev->strm, "Q\n");
     }
+    cvd->filled_trap = false;
     cvd->mdev.width += cvd->mdev.mapped_x;
     cvd->mdev.height += cvd->mdev.mapped_y;
     if (code > 0)
@@ -1406,6 +1411,19 @@ pdf_dump_converted_image(gx_device_pdf *pdev, pdf_lcvd_t *cvd, int for_pattern)
     return code;
 }
 static int
+lcvd_fill_trapezoid(gx_device * dev, const gs_fixed_edge * left,
+    const gs_fixed_edge * right, fixed ybot, fixed ytop, bool swap_axes,
+    const gx_device_color * pdevc, gs_logical_operation_t lop)
+{
+    int code = 0;
+    pdf_lcvd_t *cvd = (pdf_lcvd_t *)dev;
+
+    if (cvd->mask != NULL)
+        cvd->filled_trap = true;
+    return gx_default_fill_trapezoid(dev, left, right, ybot, ytop, swap_axes, pdevc, lop);
+}
+
+static int
 lcvd_handle_fill_path_as_shading_coverage(gx_device *dev,
     const gs_gstate *pgs, gx_path *ppath,
     const gx_fill_params *params,
@@ -1539,6 +1557,7 @@ pdf_setup_masked_image_converter(gx_device_pdf *pdev, gs_memory_t *mem, const gs
     cvd->path_is_empty = true;
     cvd->mask_is_empty = true;
     cvd->mask_is_clean = false;
+    cvd->filled_trap = false;
     cvd->has_background = false;
     cvd->mask = 0;
     cvd->write_matrix = true;
@@ -1588,6 +1607,7 @@ pdf_setup_masked_image_converter(gx_device_pdf *pdev, gs_memory_t *mem, const gs
     dev_proc(&cvd->mdev, dev_spec_op) = lcvd_dev_spec_op;
     dev_proc(&cvd->mdev, fill_path) = lcvd_handle_fill_path_as_shading_coverage;
     dev_proc(&cvd->mdev, transform_pixel_region) = lcvd_transform_pixel_region;
+    dev_proc(&cvd->mdev, fill_trapezoid) = lcvd_fill_trapezoid;
     cvd->m = *m;
     if (write_on_close) {
         cvd->mdev.is_open = true;
diff --git a/devices/vector/gdevpdfx.h b/devices/vector/gdevpdfx.h
index e2b7c98d5..c7d564f96 100644
--- a/devices/vector/gdevpdfx.h
+++ b/devices/vector/gdevpdfx.h
@@ -1282,6 +1282,7 @@ typedef struct pdf_lcvd_s {
     bool mask_is_empty;
     bool path_is_empty;
     bool mask_is_clean;
+    bool filled_trap;
     bool write_matrix;
     bool has_background;
     gs_matrix m;