[gegl] median-blur: change bin distribution; perform in perceptual

[N/A <ell src gnome org>]

commit acf896712606de88504d8ea553ac532ba4c689f0
Author: Ell <ell_se yahoo com>
Date:   Thu Sep 14 09:12:51 2017 -0400

    median-blur: change bin distribution; perform in perceptual
    
    Change the histogram bin distribution such that, for an n-bin
    histogram, the bins are centered around multiples of 1 / (n - 1).
    This means that the width of the lowest and highest bins is half of
    that of the rest of the bins, but also that they result in 0 and 1,
    respectively, exactly.
    
    Change the working space of the op from RGB(A) to R'G'B'(A).  Note
    that, ideally, since the median depends only on the relative order
    of the values, this is insignificant.  However, since we quantize
    the value range into bins, this affects the distribution of the
    bins, and, in particular, creates visible banding when done in RGB,
    given the current bin count (even when the op in a nop, i.e., when
    radius == 0).

 operations/workshop/median-blur.c |   10 +++++-----
 1 files changed, 5 insertions(+), 5 deletions(-)
---
diff --git a/operations/workshop/median-blur.c b/operations/workshop/median-blur.c
index 30eb12b..a756295 100644
--- a/operations/workshop/median-blur.c
+++ b/operations/workshop/median-blur.c
@@ -111,7 +111,7 @@ histogram_get_median (Histogram *hist,
   comp->last_median     = i;
   comp->last_median_sum = sum;
 
-  return ((gfloat) i + .5f) / (gfloat) N_BINS;
+  return (gfloat) i / (gfloat) (N_BINS - 1);
 }
 
 static inline void
@@ -398,8 +398,8 @@ convert_values_to_bins (gint32   *src,
           gfloat value = ((gfloat *) src)[c];
           gint   bin;
 
-          bin = (gint) (CLAMP (value, 0.0f, 1.0f) * N_BINS);
-          bin = MIN (bin, N_BINS - 1);
+          bin = (gint) (value * (N_BINS - 1) + 0.5f);
+          bin = CLAMP (bin, 0, N_BINS - 1);
 
           src[c] = bin;
         }
@@ -414,7 +414,7 @@ prepare (GeglOperation *operation)
   GeglOperationAreaFilter *area      = GEGL_OPERATION_AREA_FILTER (operation);
   GeglProperties          *o         = GEGL_PROPERTIES (operation);
   const Babl              *in_format = gegl_operation_get_source_format (operation, "input");
-  const Babl              *format    = babl_format ("RGB float");
+  const Babl              *format    = babl_format ("R'G'B' float");
 
   area->left   =
   area->right  =
@@ -427,7 +427,7 @@ prepare (GeglOperation *operation)
   if (in_format)
     {
       if (babl_format_has_alpha (in_format))
-        format = babl_format ("RGBA float");
+        format = babl_format ("R'G'B'A float");
     }
 
   gegl_operation_set_format (operation, "input", format);