wayland: Unify integer and fractional output scaling

Previously, scale values used by the displays and surfaces were always integers, with fractional scale values only being calculated when the backbuffer and viewport sizes were being determined. Now, if xdg-output is available, the fractional scale of output displays is calculated when the displays are enumerated and the true scale values of the output devices are used whenever possible.

This unifies the integer and fractional scaling systems, allows for the use of more accurate scale values that minimize overdraw when windows straddle multiple outputs, and lays the groundwork for the pending Wayland scaling protocols that will report the preferred scale values per-surface instead of per-output.

src/video/wayland/SDL_waylandvideo.c

@@ -578,8 +578,9 @@ display_handle_done(void *data,
     SDL_zero(desktop_mode);
     desktop_mode.format = SDL_PIXELFORMAT_RGB888;
 
-    /* Scale the desktop coordinates, if xdg-output isn't present */
-    if (!driverdata->has_logical_size) {
+    if (driverdata->has_logical_size) { /* If xdg-output is present, calculate the true scale of the desktop */
+        driverdata->scale_factor = (float)native_mode.w / (float)driverdata->width;
+    } else  { /* Scale the desktop coordinates, if xdg-output isn't present */
         driverdata->width /= driverdata->scale_factor;
         driverdata->height /= driverdata->scale_factor;
     }

src/video/wayland/SDL_waylandwindow.c

@@ -44,6 +44,15 @@
 #include <libdecor.h>
 #endif
 
+SDL_FORCE_INLINE SDL_bool
+FloatEqual(float a, float b)
+{
+    const float diff    = SDL_fabsf(a - b);
+    const float largest = SDL_max(SDL_fabsf(a), SDL_fabsf(b));
+
+    return diff <= largest * SDL_FLT_EPSILON;
+}
+
 static void
 GetFullScreenDimensions(SDL_Window *window, int *width, int *height, int *drawable_width, int *drawable_height)
 {
@@ -104,17 +113,10 @@ GetFullScreenDimensions(SDL_Window *window, int *width, int *height, int *drawab
 }
 
 SDL_FORCE_INLINE SDL_bool
-DesktopIsFractionalScaled(SDL_Window *window)
+SurfaceScaleIsFractional(SDL_Window *window)
 {
-    SDL_WindowData        *data   = window->driverdata;
-    SDL_WaylandOutputData *output = (SDL_WaylandOutputData *)SDL_GetDisplayForWindow(window)->driverdata;
-
-    if ((output->native_width != (int)(output->width * data->scale_factor) ||
-         output->native_height != (int)(output->height * data->scale_factor))) {
-        return SDL_TRUE;
-    }
-
-    return SDL_FALSE;
+    SDL_WindowData *data = window->driverdata;
+    return !FloatEqual(SDL_roundf(data->scale_factor), data->scale_factor);
 }
 
 SDL_FORCE_INLINE SDL_bool
@@ -143,7 +145,7 @@ NeedViewport(SDL_Window *window)
             if (fs_width != output->width || fs_height != output->height) {
                 return SDL_TRUE;
             }
-        } else if (DesktopIsFractionalScaled(window) && (window->flags & SDL_WINDOW_ALLOW_HIGHDPI)) {
+        } else if (SurfaceScaleIsFractional(window) && (window->flags & SDL_WINDOW_ALLOW_HIGHDPI)) {
             return SDL_TRUE;
         }
     }
@@ -169,22 +171,26 @@ GetWindowHeight(SDL_Window *window)
 static void
 GetBufferSize(SDL_Window *window, int *width, int *height)
 {
-    SDL_WindowData        *data   = window->driverdata;
-    SDL_WaylandOutputData *output = (SDL_WaylandOutputData *)SDL_GetDisplayForWindow(window)->driverdata;
-    int                    buf_width;
-    int                    buf_height;
-
+    SDL_WindowData *data = window->driverdata;
+    int             buf_width;
+    int             buf_height;
 
     if (FullscreenModeEmulation(window)) {
         GetFullScreenDimensions(window, NULL, NULL, &buf_width, &buf_height);
     } else if (NeedViewport(window)) {
-        const double frac_scale = (double)output->native_width / (double)output->width;
+        buf_width  = (int)SDL_ceil(window->w * data->scale_factor);
+        buf_height = (int)SDL_ceil(window->h * data->scale_factor);
+    } else {
+        /*
+         * Integer scaled windowed or fullscreen with no viewport
+         *
+         * Round the scale factor up in the unlikely scenario of a compositor
+         * that supports fractional scaling, but not viewports.
+         */
+        int scale_factor = (int)SDL_ceilf(data->scale_factor);
 
-        buf_width  = (int)SDL_ceil(window->w * frac_scale);
-        buf_height = (int)SDL_ceil(window->h * frac_scale);
-    } else { /* Windowed or fullscreen with no viewport */
-        buf_width  = window->w * data->scale_factor;
-        buf_height = window->h * data->scale_factor;
+        buf_width  = window->w * scale_factor;
+        buf_height = window->h * scale_factor;
     }
 
     if (width) {
@@ -230,7 +236,7 @@ ConfigureWindowGeometry(SDL_Window *window)
     SDL_WaylandOutputData *output  = (SDL_WaylandOutputData *)SDL_GetDisplayForWindow(window)->driverdata;
     struct wl_region      *region;
 
-    /* Set the drawable backbuffer size */
+    /* Set the drawable backbuffer size. */
     GetBufferSize(window, &data->drawable_width, &data->drawable_height);
 
     if (data->egl_window) {
@@ -269,7 +275,9 @@ ConfigureWindowGeometry(SDL_Window *window)
             SetDrawSurfaceViewport(window, data->drawable_width, data->drawable_height, window->w, window->h);
         } else {
             UnsetDrawSurfaceViewport(window);
-            wl_surface_set_buffer_scale(data->surface, (int32_t)data->scale_factor);
+
+            /* Round to the next integer in case of a fractional value. */
+            wl_surface_set_buffer_scale(data->surface, (int32_t)SDL_ceilf(data->scale_factor));
         }
 
         SDL_zero(data->viewport_rect);
@@ -507,7 +515,7 @@ handle_configure_xdg_toplevel(void *data,
             }
 
             /* This part is good though. */
-            if ((window->flags & SDL_WINDOW_ALLOW_HIGHDPI) && wind->scale_factor != driverdata->scale_factor) {
+            if ((window->flags & SDL_WINDOW_ALLOW_HIGHDPI) && !FloatEqual(wind->scale_factor, driverdata->scale_factor)) {
                 wind->scale_factor = driverdata->scale_factor;
                 wind->needs_resize_event = SDL_TRUE;
             }
@@ -586,7 +594,7 @@ handle_configure_xdg_toplevel(void *data,
         }
 
         /* This part is good though. */
-        if ((window->flags & SDL_WINDOW_ALLOW_HIGHDPI) && wind->scale_factor != driverdata->scale_factor) {
+        if ((window->flags & SDL_WINDOW_ALLOW_HIGHDPI) && !FloatEqual(wind->scale_factor, driverdata->scale_factor)) {
             wind->scale_factor = driverdata->scale_factor;
             wind->needs_resize_event = SDL_TRUE;
         }
@@ -896,13 +904,11 @@ update_scale_factor(SDL_WindowData *window)
         new_factor = 0.0f;
         for (i = 0; i < window->num_outputs; i++) {
             SDL_WaylandOutputData* driverdata = window->outputs[i];
-            if (driverdata->scale_factor > new_factor) {
-                new_factor = driverdata->scale_factor;
-            }
+            new_factor = SDL_max(new_factor, driverdata->scale_factor);
         }
     }
 
-    if (new_factor != old_factor) {
+    if (!FloatEqual(new_factor, old_factor)) {
         Wayland_HandleResize(window->sdlwindow, window->sdlwindow->w, window->sdlwindow->h, new_factor);
     }
 }
@@ -1800,9 +1806,7 @@ int Wayland_CreateWindow(_THIS, SDL_Window *window)
         int i;
         for (i=0; i < SDL_GetVideoDevice()->num_displays; i++) {
             float scale = ((SDL_WaylandOutputData*)SDL_GetVideoDevice()->displays[i].driverdata)->scale_factor;
-            if (scale > data->scale_factor) {
-                data->scale_factor = scale;
-            }
+            data->scale_factor = SDL_max(data->scale_factor, scale);
         }
     }
 
@@ -1841,8 +1845,8 @@ int Wayland_CreateWindow(_THIS, SDL_Window *window)
     }
 #endif /* SDL_VIDEO_DRIVER_WAYLAND_QT_TOUCH */
 
-    data->drawable_width = window->w * data->scale_factor;
-    data->drawable_height = window->h * data->scale_factor;
+    data->drawable_width = SDL_ceilf(window->w * data->scale_factor);
+    data->drawable_height = SDL_ceilf(window->h * data->scale_factor);
 
     if (window->flags & SDL_WINDOW_OPENGL) {
         data->egl_window = WAYLAND_wl_egl_window_create(data->surface, data->drawable_width, data->drawable_height);
@@ -1906,7 +1910,7 @@ Wayland_HandleResize(SDL_Window *window, int width, int height, float scale)
     SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
     SDL_VideoData *viddata = data->waylandData;
 
-    if (data->needs_resize_event || window->w != width || window->h != height || data->scale_factor != scale) {
+    if (data->needs_resize_event || window->w != width || window->h != height || !FloatEqual(data->scale_factor, scale)) {
         /* We may have already updated window w/h (or only adjusted scale factor),
          * so we must override the deduplication logic in the video core */
         window->w = 0;
@@ -1914,6 +1918,7 @@ Wayland_HandleResize(SDL_Window *window, int width, int height, float scale)
         SDL_SendWindowEvent(window, SDL_WINDOWEVENT_RESIZED, width, height);
         window->w = width;
         window->h = height;
+        data->scale_factor = scale;
         data->needs_resize_event = SDL_FALSE;
     }