diff --git a/ChangeLog b/ChangeLog
index be6d84bd7d..99bb634fe8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2008-05-11  Sven Neumann  <sven@gimp.org>
+
+	* app/core/gimpcurve.[ch]: keep a boolean flag to identify an
+	identity mapping. Set it to TRUE when the curve is reset.
+
+	* app/core/gimpcurve-map.c (gimp_curve_map_value): optimize for
+	the case where the curve is an identity mapping.
+
 2008-05-12  Simon Budig  <simon@gimp.org>
 
 	* app/vectors/gimpvectors.[ch]
diff --git a/app/core/gimpcurve-map.c b/app/core/gimpcurve-map.c
index 8f1aa15c41..522a8c4381 100644
--- a/app/core/gimpcurve-map.c
+++ b/app/core/gimpcurve-map.c
@@ -34,6 +34,11 @@ gimp_curve_map_value (GimpCurve *curve,
 {
   g_return_val_if_fail (GIMP_IS_CURVE (curve), 0.0);
 
+  if (curve->identity)
+    {
+      return value;
+    }
+
   if (value < 0.0)
     {
       return curve->samples[0];
@@ -42,7 +47,7 @@ gimp_curve_map_value (GimpCurve *curve,
     {
       return curve->samples[curve->n_samples - 1];
     }
-  else /* interpolate the curve */
+  else  /* interpolate the curve */
     {
       gint    index = floor (value * (gdouble) (curve->n_samples - 1));
       gdouble f     = value * (gdouble) (curve->n_samples - 1) - index;
diff --git a/app/core/gimpcurve.c b/app/core/gimpcurve.c
index d314a93b71..9ff889762a 100644
--- a/app/core/gimpcurve.c
+++ b/app/core/gimpcurve.c
@@ -161,6 +161,7 @@ gimp_curve_class_init (GimpCurveClass *klass)
 static void
 gimp_curve_init (GimpCurve *curve)
 {
+  curve->identity = FALSE;
 }
 
 static void
@@ -316,7 +317,11 @@ gimp_curve_get_description (GimpViewable  *viewable,
 static void
 gimp_curve_dirty (GimpData *data)
 {
-  gimp_curve_calculate (GIMP_CURVE (data));
+  GimpCurve *curve = GIMP_CURVE (data);
+
+  curve->identity = FALSE;
+
+  gimp_curve_calculate (curve);
 
   GIMP_DATA_CLASS (parent_class)->dirty (data);
 }
@@ -410,6 +415,8 @@ gimp_curve_reset (GimpCurve *curve,
   g_object_thaw_notify (G_OBJECT (curve));
 
   gimp_data_dirty (GIMP_DATA (curve));
+
+  curve->identity = TRUE;
 }
 
 void
@@ -660,6 +667,23 @@ gimp_curve_set_curve (GimpCurve *curve,
   gimp_data_dirty (GIMP_DATA (curve));
 }
 
+/**
+ * gimp_curve_is_identity:
+ * @curve: a #GimpCurve object
+ *
+ * If this function returns %TRUE, then the curve maps each value to
+ * itself. If it returns %FALSE, then this assumption can not be made.
+ *
+ * Return value: %TRUE if the curve is an identity mapping, %FALSE otherwise.
+ **/
+gboolean
+gimp_curve_is_identity (GimpCurve *curve)
+{
+  g_return_val_if_fail (GIMP_IS_CURVE (curve), FALSE);
+
+  return curve->identity;
+}
+
 void
 gimp_curve_get_uchar (GimpCurve *curve,
                       gint       n_samples,
@@ -684,10 +708,10 @@ gimp_curve_get_uchar (GimpCurve *curve,
 static void
 gimp_curve_calculate (GimpCurve *curve)
 {
-  gint i;
   gint *points;
-  gint num_pts;
-  gint p1, p2, p3, p4;
+  gint  i;
+  gint  num_pts;
+  gint  p1, p2, p3, p4;
 
   if (GIMP_DATA (curve)->freeze_count > 0)
     return;
@@ -721,6 +745,10 @@ gimp_curve_calculate (GimpCurve *curve)
           for (i = boundary; i < curve->n_samples; i++)
             curve->samples[i] = point.y;
         }
+      else
+        {
+          curve->identity = TRUE;
+        }
 
       for (i = 0; i < num_pts - 1; i++)
         {
diff --git a/app/core/gimpcurve.h b/app/core/gimpcurve.h
index 38b1aa8692..8fce84b68e 100644
--- a/app/core/gimpcurve.h
+++ b/app/core/gimpcurve.h
@@ -45,6 +45,8 @@ struct _GimpCurve
 
   gint           n_samples;
   gdouble       *samples;
+
+  gboolean       identity;  /* whether the curve is an identiy mapping */
 };
 
 struct _GimpCurveClass
@@ -87,6 +89,8 @@ void            gimp_curve_set_curve         (GimpCurve     *curve,
                                               gdouble        x,
                                               gdouble        y);
 
+gboolean        gimp_curve_is_identity       (GimpCurve     *curve);
+
 void            gimp_curve_get_uchar         (GimpCurve     *curve,
                                               gint           n_samples,
                                               guchar        *samples);