[PATCH v2 1/1] win32u: Use floats for map_monitor_rect computation.

[Rémi Bernon]

From: Rémi Bernon

Avoiding rounding errors caused by dpi_from and dpi * 2 denominators.
It's possible to tweak the operations to move the division up to the
end but it increases the risk of overflows. Let's just use floats for
this.

Based on a patch from Conor McCarthy.
---
 dlls/win32u/sysparams.c | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/dlls/win32u/sysparams.c b/dlls/win32u/sysparams.c
index 17432ef867d..d6ee5d643e2 100644
--- a/dlls/win32u/sysparams.c
+++ b/dlls/win32u/sysparams.c
@@ -1923,10 +1923,12 @@ static RECT map_monitor_rect( struct monitor *monitor, RECT rect, UINT dpi_from,

if (monitor->source)
     {
-        DEVMODEW current_mode = {.dmSize = sizeof(DEVMODEW)}, *mode_from, *mode_to;
+        float points[4] = {rect.left, rect.top, rect.right, rect.bottom}, from[2], to[2];
+        DEVMODEW current_mode = {.dmSize = sizeof(DEVMODEW)}, physical_mode;
         UINT num, den, dpi;

source_get_current_settings( monitor->source, &current_mode );
+        physical_mode = monitor->source->physical;

dpi = monitor_get_dpi( monitor, MDT_DEFAULT, &x, &y );
         if (!dpi_from) dpi_from = dpi;
@@ -1935,23 +1937,31 @@ static RECT map_monitor_rect( struct monitor *monitor, RECT rect, UINT dpi_from,
         if (type_from == MDT_RAW_DPI)
         {
             monitor_virt_to_raw_ratio( monitor, &den, &num );
-            mode_from = &monitor->source->physical;
-            mode_to = &current_mode;
+            from[0] = physical_mode.dmPosition.x + physical_mode.dmPelsWidth / 2.0;
+            from[1] = physical_mode.dmPosition.y + physical_mode.dmPelsHeight / 2.0;
+            to[0] = current_mode.dmPosition.x + current_mode.dmPelsWidth / 2.0;
+            to[1] = current_mode.dmPosition.y + current_mode.dmPelsHeight / 2.0;
         }
         else
         {
             monitor_virt_to_raw_ratio( monitor, &num, &den );
-            mode_from = &current_mode;
-            mode_to = &monitor->source->physical;
+            from[0] = current_mode.dmPosition.x + current_mode.dmPelsWidth / 2.0;
+            from[1] = current_mode.dmPosition.y + current_mode.dmPelsHeight / 2.0;
+            to[0] = physical_mode.dmPosition.x + physical_mode.dmPelsWidth / 2.0;
+            to[1] = physical_mode.dmPosition.y + physical_mode.dmPelsHeight / 2.0;
         }

-        rect = map_dpi_rect( rect, dpi_from, dpi * 2 );
-        OffsetRect( &rect, -mode_from->dmPosition.x * 2 - mode_from->dmPelsWidth,
-                    -mode_from->dmPosition.y * 2 - mode_from->dmPelsHeight );
-        rect = map_dpi_rect( rect, den, num );
-        OffsetRect( &rect, mode_to->dmPosition.x * 2 + mode_to->dmPelsWidth,
-                    mode_to->dmPosition.y * 2 + mode_to->dmPelsHeight );
-        return map_dpi_rect( rect, dpi * 2, dpi_to );
+        for (int i = 0; i < ARRAY_SIZE(points); i++)
+        {
+            points[i] *= (float)dpi / dpi_from;
+            points[i] -= from[i & 1];
+            points[i] *= (float)num / den;
+            points[i] += to[i & 1];
+            points[i] *= (float)dpi_to / dpi;
+        }
+
+        SetRect( &rect, round( points[0] ), round( points[1] ), round( points[2] ), round( points[3] ) );
+        return rect;
     }

if (!dpi_from) dpi_from = monitor_get_dpi( monitor, type_from, &x, &y );