compositor: q&d solution for surface drift

When a transformed (rotated) surface is continuously resized from its
top-left corner, its location will drift. This is due to accumulating
rounding errors in transforming an offset from surface-local to global
coordinates in surface_attach().

Diminish the drift down to unobservable level by changing the
weston_surface global position from integer to float.

The offset transformation is now done without rounding. To preserve the
precision, wl_shell::configure() interface must use floats, and so does
weston_surface_configure(), too.

The con of this patch is that it adds inconsistency to the surface
position coordinates: sometimes they are floats, sometimes integers.

diff --git a/src/compositor.c b/src/compositor.c
index 6b15a54..bd5d2e7 100644 (file)
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -311,12 +311,10 @@ weston_surface_update_transform(struct weston_surface *surface)
                             &surface->transform.boundingbox);
 }
 
-WL_EXPORT void
-weston_surface_to_global(struct weston_surface *surface,
-                        int32_t sx, int32_t sy, int32_t *x, int32_t *y)
+static void
+surface_to_global_float(struct weston_surface *surface,
+                       int32_t sx, int32_t sy, GLfloat *x, GLfloat *y)
 {
-       weston_surface_update_transform(surface);
-
        if (surface->transform.enabled) {
                struct weston_vector v = { { sx, sy, 0.0f, 1.0f } };
 
@@ -331,14 +329,27 @@ weston_surface_to_global(struct weston_surface *surface,
                        return;
                }
 
-               *x = floorf(v.f[0] / v.f[3]);
-               *y = floorf(v.f[1] / v.f[3]);
+               *x = v.f[0] / v.f[3];
+               *y = v.f[1] / v.f[3];
        } else {
                *x = sx + surface->geometry.x;
                *y = sy + surface->geometry.y;
        }
 }
 
+WL_EXPORT void
+weston_surface_to_global(struct weston_surface *surface,
+                        int32_t sx, int32_t sy, int32_t *x, int32_t *y)
+{
+       GLfloat xf, yf;
+
+       weston_surface_update_transform(surface);
+
+       surface_to_global_float(surface, sx, sy, &xf, &yf);
+       *x = floorf(xf);
+       *y = floorf(yf);
+}
+
 static void
 surface_from_global_float(struct weston_surface *surface,
                          int32_t x, int32_t y, GLfloat *sx, GLfloat *sy)
@@ -449,7 +460,7 @@ weston_surface_flush_damage(struct weston_surface *surface)
 
 WL_EXPORT void
 weston_surface_configure(struct weston_surface *surface,
-                        int x, int y, int width, int height)
+                        GLfloat x, GLfloat y, int width, int height)
 {
        weston_surface_damage_below(surface);
 
@@ -1100,12 +1111,11 @@ surface_attach(struct wl_client *client,
        } else if (sx != 0 || sy != 0 ||
                   es->geometry.width != buffer->width ||
                   es->geometry.height != buffer->height) {
-               int32_t from_x, from_y;
-               int32_t to_x, to_y;
+               GLfloat from_x, from_y;
+               GLfloat to_x, to_y;
 
-               /* FIXME: this has serious cumulating rounding errors */
-               weston_surface_to_global(es, 0, 0, &from_x, &from_y);
-               weston_surface_to_global(es, sx, sy, &to_x, &to_y);
+               surface_to_global_float(es, 0, 0, &from_x, &from_y);
+               surface_to_global_float(es, sx, sy, &to_x, &to_y);
                shell->configure(shell, es,
                                 es->geometry.x + to_x - from_x,
                                 es->geometry.y + to_y - from_y,

diff --git a/src/compositor.h b/src/compositor.h
index 5d12d43..4495412 100644 (file)
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -128,7 +128,7 @@ struct weston_shell {
                    int32_t width, int32_t height);
        void (*configure)(struct weston_shell *shell,
                          struct weston_surface *surface,
-                         int32_t x, int32_t y, int32_t width, int32_t height);
+                         GLfloat x, GLfloat y, int32_t width, int32_t height);
        void (*destroy)(struct weston_shell *shell);
 };
 
@@ -228,7 +228,7 @@ struct weston_surface {
         * That includes the transformations referenced from the list.
         */
        struct {
-               int32_t x, y; /* surface translation on display */
+               GLfloat x, y; /* surface translation on display */
                int32_t width, height;
 
                /* struct weston_transform */
@@ -372,7 +372,7 @@ weston_surface_create(struct weston_compositor *compositor);
 
 void
 weston_surface_configure(struct weston_surface *surface,
-                        int x, int y, int width, int height);
+                        GLfloat x, GLfloat y, int width, int height);
 
 void
 weston_surface_assign_output(struct weston_surface *surface);

diff --git a/src/shell.c b/src/shell.c
index d15c8e2..78120ef 100644 (file)
--- a/src/shell.c
+++ b/src/shell.c
@@ -1397,7 +1397,7 @@ map(struct weston_shell *base,
 
 static void
 configure(struct weston_shell *base, struct weston_surface *surface,
-         int32_t x, int32_t y, int32_t width, int32_t height)
+         GLfloat x, GLfloat y, int32_t width, int32_t height)
 {
        struct wl_shell *shell = container_of(base, struct wl_shell, shell);
        int do_configure = !shell->locked;

diff --git a/src/tablet-shell.c b/src/tablet-shell.c
index 14ac889..bcaba16 100644 (file)
--- a/src/tablet-shell.c
+++ b/src/tablet-shell.c
@@ -141,7 +141,7 @@ tablet_shell_map(struct weston_shell *base, struct weston_surface *surface,
 static void
 tablet_shell_configure(struct weston_shell *base,
                             struct weston_surface *surface,
-                            int32_t x, int32_t y,
+                            GLfloat x, GLfloat y,
                             int32_t width, int32_t height)
 {
        weston_surface_configure(surface, x, y, width, height);