diff --git a/ChangeLog b/ChangeLog
index d7036d9dc9..ffbd644d4a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2008-09-18  Simon Budig  <simon@gimp.org>
+
+	* app/gegl/gimpbrightnesscontrastconfig.c: fix the
+	brightness/contrast --> levels step by doing proper math (TM).
+
 2008-09-18  Sven Neumann  <sven@gimp.org>
 
 	* tools/pdbgen/pdb/drawable.pdb: gimp_drawable_mask_intersect()
diff --git a/app/gegl/gimpbrightnesscontrastconfig.c b/app/gegl/gimpbrightnesscontrastconfig.c
index a637d18859..8a4b130262 100644
--- a/app/gegl/gimpbrightnesscontrastconfig.c
+++ b/app/gegl/gimpbrightnesscontrastconfig.c
@@ -162,7 +162,7 @@ gimp_brightness_contrast_config_to_levels_config (GimpBrightnessContrastConfig *
 {
   GimpLevelsConfig *levels;
   gdouble           brightness;
-  gdouble           contrast;
+  gdouble           slant;
   gdouble           value;
 
   g_return_val_if_fail (GIMP_IS_BRIGHTNESS_CONTRAST_CONFIG (config), NULL);
@@ -170,33 +170,63 @@ gimp_brightness_contrast_config_to_levels_config (GimpBrightnessContrastConfig *
   levels = g_object_new (GIMP_TYPE_LEVELS_CONFIG, NULL);
 
   brightness = config->brightness / 2.0;
-  contrast   = (config->contrast < 0 ?
-                (config->contrast + 1.0) :
-                config->contrast * 4.0 + 1.0);
+  slant = tan ((config->contrast + 1) * G_PI_4);
 
-  value = -0.5 * contrast + brightness + 0.5;
-
-  if (value < 0.0)
+  if (config->brightness >= 0)
     {
-      value = 0.0;
+      value = -0.5 * slant + brightness * slant + 0.5;
 
-      levels->low_input[GIMP_HISTOGRAM_VALUE] =
-        (-0.5 - brightness) / contrast + 0.5;
+      if (value < 0.0)
+        {
+          value = 0.0;
+
+          /* this slightly convoluted math follows by inverting the
+           * calculation of the brightness/contrast LUT in base/lut-funcs.h */
+
+          levels->low_input[GIMP_HISTOGRAM_VALUE] =
+            (brightness * slant + 0.5 * slant - 0.5) / (slant - brightness * slant);
+        }
+
+      levels->low_output[GIMP_HISTOGRAM_VALUE] = value;
+
+      value = 0.5 * slant + 0.5;
+
+      if (value > 1.0)
+        {
+          value = 1.0;
+
+          levels->high_input[GIMP_HISTOGRAM_VALUE] =
+            (brightness * slant + 0.5 * slant + 0.5) / (slant - brightness * slant);
+        }
+
+      levels->high_output[GIMP_HISTOGRAM_VALUE] = value;
     }
-
-  levels->low_output[GIMP_HISTOGRAM_VALUE] = value;
-
-  value = 0.5 * contrast + brightness + 0.5;
-
-  if (value > 1.0)
+  else
     {
-      value = 1.0;
+      value = 0.5 - 0.5 * slant;
 
-      levels->high_input[GIMP_HISTOGRAM_VALUE] =
-        (0.5 - brightness) / contrast + 0.5;
+      if (value < 0.0)
+        {
+          value = 0.0;
+
+          levels->low_input[GIMP_HISTOGRAM_VALUE] =
+            (0.5 * slant - 0.5) / (slant + brightness * slant);
+        }
+
+      levels->low_output[GIMP_HISTOGRAM_VALUE] = value;
+
+      value = slant * brightness + slant * 0.5 + 0.5;
+
+      if (value > 1.0)
+        {
+          value = 1.0;
+
+          levels->high_input[GIMP_HISTOGRAM_VALUE] =
+            (0.5 * slant + 0.5) / (slant + brightness * slant);
+        }
+
+      levels->high_output[GIMP_HISTOGRAM_VALUE] = value;
     }
 
-  levels->high_output[GIMP_HISTOGRAM_VALUE] = value;
-
   return levels;
 }