Created attachment 331157 [details]
Pixmap with chess like mask pattern

When you call `gdk_pixbuf_render_threshold_alpha` to extract a bitmap mask of a PixBuf, the mask is incorrect when you have a one pixel of mask not surrounding by other pixels. The easiest way to see this is to use the attached png.

Not knowing much of the Mac OS drawing API, I found that the issue is caused by the implementation of `gdk_quartz_draw_segments` which calls to `CGContextAddLineToPoint` that doesn't seem to work when starting point and ending point are the same.

I found a workaround by calling CGContextFillRect for 1-pixel lines. Here is my patch.

diff --git a/gdk/quartz/gdkdrawable-quartz.c b/gdk/quartz/gdkdrawable-quartz.c
--- a/gdk/quartz/gdkdrawable-quartz.c
+++ b/gdk/quartz/gdkdrawable-quartz.c
@@ -572,7 +572,7 @@ gdk_quartz_draw_segments (GdkDrawable    *drawable,
   private = GDK_GC_QUARTZ (gc);
 
   if (!_gdk_quartz_gc_update_cg_context (gc, drawable, context,
-                                        GDK_QUARTZ_CONTEXT_STROKE))
+                                        GDK_QUARTZ_CONTEXT_STROKE | GDK_QUARTZ_CONTEXT_FILL))
     {
       gdk_quartz_drawable_release_context (drawable, context);
       return;
@@ -586,8 +586,14 @@ gdk_quartz_draw_segments (GdkDrawable    *drawable,
                                        segs[i].x2, segs[i].y2,
                                        &xfix, &yfix);
 
-      CGContextMoveToPoint (context, segs[i].x1 + 0.5, segs[i].y1 + 0.5);
-      CGContextAddLineToPoint (context, segs[i].x2 + 0.5 + xfix, segs[i].y2 + 0.5 + yfix);
+      // Drawing a one pixel line doesn't work, so instead we use CGContextFillRect
+      if ((segs[i].x1 == (segs[i].x2 + xfix)) && (segs[i].y1 == (segs[i].y2 + yfix))) {
+        CGRect rect = CGRectMake(segs[i].x1, segs[i].y1, 1, 1);
+        CGContextFillRect (context, rect);
+      } else {
+        CGContextMoveToPoint (context, segs[i].x1 + 0.5, segs[i].y1 + 0.5);
+        CGContextAddLineToPoint (context, segs[i].x2 + 0.5 + xfix, segs[i].y2 + 0.5 + yfix);
+      }
     }
   
   CGContextStrokePath (context);