Add macOS backdrop blur and preserve PDF ink opacity

1 files changed, 19 insertions, 2 deletions

diff --git a/src/pdf_bridge.c b/src/pdf_bridge.c
index bdc7b891..976ed838 100644
--- a/src/pdf_bridge.c
+++ b/src/pdf_bridge.c
@@ -588,7 +588,8 @@ pardes_pdf_render_into(
     int height,
     int stride,
     int band_y,
-    int band_height)
+    int band_height,
+    int transparent_paper)
 {
     fz_context *ctx;
     fz_pixmap *pixmap = NULL;
@@ -654,7 +655,8 @@ pardes_pdf_render_into(
          * paid either way by the glyph spans and the tint pass that walk the
          * same buffer immediately afterwards. Leave it alone.
          */
-        fz_clear_pixmap_with_value(ctx, pixmap, 0xFF);
+        if (transparent_paper) fz_clear_pixmap(ctx, pixmap);
+        else fz_clear_pixmap_with_value(ctx, pixmap, 0xFF);
 
         if (document->cached_display_list != NULL ||
             document->display_list_candidate_page_number == page_number) {
@@ -719,6 +721,21 @@ pardes_pdf_render_into(
         return PARDES_PDF_ERROR;
     }
 
+    /* MuPDF draws premultiplied samples. The public RGBA contract and tint
+     * pass use straight alpha; preserve coverage and undo premultiplication. */
+    if (transparent_paper) {
+        size_t pixel;
+        for (pixel = 0; pixel < samples_len; pixel += 4) {
+            unsigned int a = samples[pixel + 3];
+            int channel;
+            if (a == 0 || a == 255) continue;
+            for (channel = 0; channel < 3; ++channel) {
+                unsigned int straight = (samples[pixel + channel] * 255u + a / 2) / a;
+                samples[pixel + channel] = (unsigned char)(straight > 255 ? 255 : straight);
+            }
+        }
+    }
+
     if (document->cached_display_list == NULL)
         document->display_list_candidate_page_number = page_number;