gimp r26984 - in trunk: . app/gegl

[simon svn gnome org]

Author: simon
Date: Wed Sep 17 22:55:55 2008
New Revision: 26984
URL: http://svn.gnome.org/viewvc/gimp?rev=26984&view=rev

Log:
2008-09-18  Simon Budig  <simon gimp org>

	* app/gegl/gimpbrightnesscontrastconfig.c: fix the
	brightness/contrast --> levels step by doing proper math (TM).



Modified:
   trunk/ChangeLog
   trunk/app/gegl/gimpbrightnesscontrastconfig.c

Modified: trunk/app/gegl/gimpbrightnesscontrastconfig.c
==============================================================================
--- trunk/app/gegl/gimpbrightnesscontrastconfig.c	(original)
+++ trunk/app/gegl/gimpbrightnesscontrastconfig.c	Wed Sep 17 22:55:55 2008
@@ -162,7 +162,7 @@
 {
   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 @@
   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;
 
-  levels->low_output[GIMP_HISTOGRAM_VALUE] = value;
+          /* this slightly convoluted math follows by inverting the
+           * calculation of the brightness/contrast LUT in base/lut-funcs.h */
 
-  value = 0.5 * contrast + brightness + 0.5;
+          levels->low_input[GIMP_HISTOGRAM_VALUE] =
+            (brightness * slant + 0.5 * slant - 0.5) / (slant - brightness * slant);
+        }
 
-  if (value > 1.0)
-    {
-      value = 1.0;
+      levels->low_output[GIMP_HISTOGRAM_VALUE] = value;
+
+      value = 0.5 * slant + 0.5;
 
-      levels->high_input[GIMP_HISTOGRAM_VALUE] =
-        (0.5 - brightness) / contrast + 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;
     }
+  else
+    {
+      value = 0.5 - 0.5 * slant;
 
-  levels->high_output[GIMP_HISTOGRAM_VALUE] = value;
+      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;
+    }
 
   return levels;
 }