meta-crtc-xrandr.c: use nearest neighbor filter for integer randr sca…

…les (#692)

Use the nearest neighbor filter if the scaling factor is an integer.
This provides a crisper, less blurry scale when the scale is an integer.

Note that this change only affects fractional scaling when the
"scale-up" mode is used.  Scaling down is unaffected by this change, and
non-integer scaling up is also unaffected by this change.

This behavior was already in Cinnamon 5.4, but it was lost at some point
in between 5.4 and now.

Note finally that the "nearest" filter is a filter guaranteed to exist
by the RENDER protocol [1].

[1] https://cgit.freedesktop.org/xorg/proto/renderproto/tree/renderproto.txt

src/backends/x11/meta-crtc-xrandr.c

@@ -135,6 +135,7 @@ meta_crtc_xrandr_set_scale (MetaCrtc         *crtc,
     DOUBLE_TO_FIXED (0), DOUBLE_TO_FIXED (1), DOUBLE_TO_FIXED (0),
     DOUBLE_TO_FIXED (0), DOUBLE_TO_FIXED (0), DOUBLE_TO_FIXED (1)
   };
+  float integer_scale;
 
   if (!(meta_monitor_manager_get_capabilities (monitor_manager) &
         META_MONITOR_MANAGER_CAPABILITY_NATIVE_OUTPUT_SCALING))
@@ -145,9 +146,19 @@ meta_crtc_xrandr_set_scale (MetaCrtc         *crtc,
 
   if (fabsf (scale - 1.0f) > 0.001)
     {
-      scale_filter = FilterGood;
-      transformation.matrix11 = DOUBLE_TO_FIXED (1.0 / scale);
-      transformation.matrix22 = DOUBLE_TO_FIXED (1.0 / scale);
+      integer_scale = roundf (scale);
+      if (fabsf (scale - integer_scale) > 0.001)
+        {
+          scale_filter = FilterGood;
+          transformation.matrix11 = DOUBLE_TO_FIXED (1.0 / scale);
+          transformation.matrix22 = DOUBLE_TO_FIXED (1.0 / scale);
+        }
+      else /* if integer multiple then use nearest neighbor filter */
+        {
+          scale_filter = "nearest";
+          transformation.matrix11 = DOUBLE_TO_FIXED (1.0 / integer_scale);
+          transformation.matrix22 = DOUBLE_TO_FIXED (1.0 / integer_scale);
+        }
     }
   else
     scale_filter = FilterFast;