[gnumeric] xlsx: implement gamma/invGamma colour transformations

[Morten Welinder <mortenw src gnome org>]

commit e33e09e9625bd0ccb7318e77440aeb1953b50ba6
Author: Morten Welinder <terra gnome org>
Date:   Sat Apr 11 11:12:29 2015 -0400

    xlsx: implement gamma/invGamma colour transformations

 plugins/excel/ChangeLog         |    2 +
 plugins/excel/xlsx-read-color.c |   64 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 65 insertions(+), 1 deletions(-)
---
diff --git a/plugins/excel/ChangeLog b/plugins/excel/ChangeLog
index d08f1f3..1a2060a 100644
--- a/plugins/excel/ChangeLog
+++ b/plugins/excel/ChangeLog
@@ -1,5 +1,7 @@
 2015-04-11  Morten Welinder  <terra gnome org>
 
+       * xlsx-read-color.c (xlsx_draw_color_gamma): Implement.
+
        * xlsx-write.c (xlsx_write_workbook): Plug pivot leak.
 
 2015-04-10  Morten Welinder  <terra gnome org>
diff --git a/plugins/excel/xlsx-read-color.c b/plugins/excel/xlsx-read-color.c
index 7ac7a37..3175468 100644
--- a/plugins/excel/xlsx-read-color.c
+++ b/plugins/excel/xlsx-read-color.c
@@ -86,6 +86,61 @@ gnm_go_color_to_hsla (GOColor orig, int *ph, int *ps, int *pl, int *pa)
        *pa = a;
 }
 
+/* See http://en.wikipedia.org/wiki/SRGB#Specification_of_the_transformation */
+
+#define GAMMA1(c_) do {                                                \
+       double c = (c_) / 255.0;                                \
+       if (c < 0.04045)                                        \
+               c = c / 12.92;                                  \
+       else                                                    \
+               c = pow ((c + 0.055) / 1.055, (12.0 / 5.0));    \
+       c = MIN (255, c * 256);                                 \
+       (c_) = (int)c;                                          \
+} while (0)
+
+static GOColor
+gnm_go_color_gamma (GOColor col)
+{
+       int r = GO_COLOR_UINT_R (col);
+       int g = GO_COLOR_UINT_G (col);
+       int b = GO_COLOR_UINT_B (col);
+       int a = GO_COLOR_UINT_A (col);
+
+       GAMMA1 (r);
+       GAMMA1 (g);
+       GAMMA1 (b);
+
+       return GO_COLOR_FROM_RGBA (r, g, b, a);
+}
+#undef GAMMA1
+
+#define INVGAMMA1(c_) do {                                     \
+       double c = (c_) / 255.0;                                \
+       if (c < 0.0031308)                                      \
+               c = c * 12.92;                                  \
+       else                                                    \
+               c = 1.055 * pow (c, 5.0 / 12.0) - 0.055;        \
+       c = MIN (255, c * 256);                                 \
+       (c_) = (int)c;                                          \
+} while (0)
+
+static GOColor
+gnm_go_color_invgamma (GOColor col)
+{
+       int r = GO_COLOR_UINT_R (col);
+       int g = GO_COLOR_UINT_G (col);
+       int b = GO_COLOR_UINT_B (col);
+       int a = GO_COLOR_UINT_A (col);
+
+       INVGAMMA1 (r);
+       INVGAMMA1 (g);
+       INVGAMMA1 (b);
+
+       return GO_COLOR_FROM_RGBA (r, g, b, a);
+}
+#undef INVGAMMA1
+
+
 /*
  * Apply tinting or shading.
  *  0 <= tint <= 1: tinting -- l is increased
@@ -254,6 +309,7 @@ xlsx_draw_color_hsl_channel (GsfXMLIn *xin, xmlChar const **attrs)
 
                hsl[channel] = CLAMP (vf, 0, HSLMAX);
                state->color = gnm_go_color_from_hsla (hsl[2], hsl[1], hsl[0], a);
+               color_set_helper (state);
        }
 }
 
@@ -262,7 +318,13 @@ xlsx_draw_color_gamma (GsfXMLIn *xin, xmlChar const **attrs)
 {
        XLSXReadState *state = (XLSXReadState *)xin->user_state;
        gboolean inv = xin->node->user_data.v_int;
-       g_warning ("Unhandled colour %d gamma transformation of #%08x\n", inv, state->color);
+
+       if (inv)
+               state->color = gnm_go_color_invgamma (state->color);
+       else
+               state->color = gnm_go_color_gamma (state->color);
+
+       color_set_helper (state);
 }