* Add cucul_get_dither_brightness(), _gamma() and _contrast(). There is no

reason to keep a copy of them in the calling program.
  * Deprecate cucul_set_dither_invert(). Inverting a dither's colours is now
    done by setting a negative gamma value.

cucul/cucul.h

@@ -208,9 +208,11 @@ int cucul_set_dither_palette(cucul_dither_t *,
                              unsigned int r[], unsigned int g[],
                              unsigned int b[], unsigned int a[]);
 int cucul_set_dither_brightness(cucul_dither_t *, float);
+float cucul_get_dither_brightness(cucul_dither_t *);
 int cucul_set_dither_gamma(cucul_dither_t *, float);
+float cucul_get_dither_gamma(cucul_dither_t *);
 int cucul_set_dither_contrast(cucul_dither_t *, float);
-int cucul_set_dither_invert(cucul_dither_t *, int);
+float cucul_get_dither_contrast(cucul_dither_t *);
 int cucul_set_dither_antialias(cucul_dither_t *, char const *);
 char const * const * cucul_get_dither_antialias_list(cucul_dither_t const *);
 int cucul_set_dither_color(cucul_dither_t *, char const *);
@@ -291,6 +293,7 @@ char const * const * cucul_get_export_list(void);
     cucul_canvas_t * cucul_import_canvas(cucul_buffer_t *,
                                          char const *) CUCUL_DEPRECATED;
     int cucul_rotate(cucul_canvas_t *) CUCUL_DEPRECATED;
+    int cucul_set_dither_invert(cucul_dither_t *, int) CUCUL_DEPRECATED;
 #   define CUCUL_COLOR_BLACK CUCUL_BLACK
 #   define CUCUL_COLOR_BLUE CUCUL_BLUE
 #   define CUCUL_COLOR_GREEN CUCUL_GREEN

cucul/dither.c

@@ -418,9 +418,25 @@ int cucul_set_dither_brightness(cucul_dither_t *d, float brightness)
     return 0;
 }
 
+/** \brief Get the brightness of a dither object.
+ *
+ *  Get the brightness of the given dither object.
+ *
+ *  This function never fails.
+ *
+ *  \param d Dither object.
+ *  \return Brightness value.
+ */
+float cucul_get_dither_brightness(cucul_dither_t *d)
+{
+    /* FIXME */
+    return 0.0;
+}
+
 /** \brief Set the gamma of a dither object.
  *
- *  Set the gamma of dither.
+ *  Set the gamma of the given dither object. A negative value causes
+ *  colour inversion.
  *
  *  If an error occurs, -1 is returned and \b errno is set accordingly:
  *  - \c EINVAL Gamma value was out of range.
@@ -432,11 +448,16 @@ int cucul_set_dither_brightness(cucul_dither_t *d, float brightness)
 int cucul_set_dither_gamma(cucul_dither_t *d, float gamma)
 {
     /* FIXME: we don't need 4096 calls to gammapow(), we could just compute
-     * 128 of them and do linear interpolation for the rest. This will
+     * a few of them and do linear interpolation for the rest. This will
      * probably speed up things a lot. */
     int i;
 
-    if(gamma <= 0.0)
+    if(gamma < 0.0)
+    {
+        d->invert = 1;
+        gamma = -gamma;
+    }
+    else if(gamma == 0.0)
     {
         seterrno(EINVAL);
         return -1;
@@ -450,21 +471,18 @@ int cucul_set_dither_gamma(cucul_dither_t *d, float gamma)
     return 0;
 }
 
-/** \brief Invert colors of dither
+/** \brief Get the gamma of a dither object.
  *
- *  Invert colors of dither.
+ *  Get the gamma of the given dither object.
  *
  *  This function never fails.
  *
  *  \param d Dither object.
- *  \param value 0 for normal behaviour, 1 for invert
- *  \return This function always returns 0.
+ *  \return Gamma value.
  */
-int cucul_set_dither_invert(cucul_dither_t *d, int value)
+float cucul_get_dither_gamma(cucul_dither_t *d)
 {
-    d->invert = value ? 1 : 0;
-
-    return 0;
+    return d->gamma;
 }
 
 /** \brief Set the contrast of a dither object.

cucul/legacy.c

@@ -62,6 +62,19 @@ int cucul_set_truecolor(cucul_canvas_t *cv, unsigned int fg, unsigned int bg)
     return cucul_set_color_argb(cv, fg, bg);
 }
 
+/*
+ * Functions from dither.c
+ */
+int cucul_set_dither_invert(cucul_dither_t *d, int value)
+{
+    float gamma = cucul_get_dither_gamma(d);
+
+    if(gamma * (value ? -1 : 1) < 0)
+        cucul_set_dither_gamma(d, -gamma);
+
+    return 0;
+}
+
 /*
  * Functions from import.c
  */