[gegl] Add a contrast input to negative-darkroom operation

[Øyvind "pippin" Kolås <ok src gnome org>]

commit d8f393137228d0450c2a412b38d02e410f4ef4b4
Author: JonnyRobbie <marcodv seznam cz>
Date:   Mon Jun 7 13:23:04 2021 +0200

    Add a contrast input to negative-darkroom operation

 operations/common/negative-darkroom.c | 42 ++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)
---
diff --git a/operations/common/negative-darkroom.c b/operations/common/negative-darkroom.c
index 96abd3f07..b7277ef47 100644
--- a/operations/common/negative-darkroom.c
+++ b/operations/common/negative-darkroom.c
@@ -53,6 +53,12 @@ property_double (boost, _("Density boost"), 1.0)
        description(_("Boost paper density to take advantage of increased dynamic range of a monitor compared 
to a photographic paper"))
        value_range (0.25, 4)
        ui_range (1, 2)
+       ui_gamma (2)
+property_double (contrast, _("Contrast boost"), 1.0)
+       description(_("Increase contrast for papers with fixed contrast (usually color papers)"))
+       value_range (0.25, 4)
+       ui_range (0.5, 2)
+       ui_gamma (2)
 
 property_double (dodge, _("Dodge/burn multiplier"), 1.0)
        description(_("The f-stop of dodge/burn for pure white/black auxillary input"))
@@ -159,6 +165,18 @@ array_min (gfloat * x, guint n)
        return(min);
 }
 
+static gfloat
+array_max (gfloat * x, guint n)
+{
+       gfloat max = x[0];
+       for (guint i = 1; i < n; i++)
+       {
+               if (x[i] > max)
+                       max = x[i];
+       }
+       return(max);
+}
+
 static inline gfloat
 clampE (gfloat x)
 {
@@ -209,6 +227,23 @@ process (GeglOperation       *operation,
                                  curves[o->curve].bn) * o->boost;
        }
 
+       // Calculate exposure for mid density
+       gfloat Dmaxc = array_max(curves[o->curve].ry, curves[o->curve].rn);
+       gfloat Dmaxm = array_max(curves[o->curve].gy, curves[o->curve].gn);
+       gfloat Dmaxy = array_max(curves[o->curve].by, curves[o->curve].bn);
+       gfloat rMid = curve_lerp(curves[o->curve].ry,
+                                curves[o->curve].rx,
+                                curves[o->curve].rn,
+                                Dmaxc / 2);
+       gfloat gMid = curve_lerp(curves[o->curve].gy,
+                                curves[o->curve].gx,
+                                curves[o->curve].gn,
+                                Dmaxm / 2);
+       gfloat bMid = curve_lerp(curves[o->curve].by,
+                                curves[o->curve].bx,
+                                curves[o->curve].bn,
+                                Dmaxy / 2);
+
        if (!aux)
        {
                rcomp = pow(2, (-o->expC) / 30);
@@ -296,6 +331,11 @@ process (GeglOperation       *operation,
                b -= Dfogy;
                /*printf("Adjusted RGB density %f %f %f\n", r, g, b);*/
 
+               // Adjust contrast
+               r = (r - rMid) * o->contrast + rMid;
+               g = (g - gMid) * o->contrast + gMid;
+               b = (b - bMid) * o->contrast + bMid;
+
                // Simulate dye density with exponentiation to get
                // the CIEXYZ tramsmittance back
                out[0] = (1 / pow(10, r * curves[o->curve].cdens.X)) *
@@ -334,7 +374,7 @@ gegl_op_class_init (GeglOpClass *klass)
        gegl_operation_class_set_keys (operation_class,
                "name",           "gegl:negative-darkroom",
                "title",          _("Negative Darkroom"),
-               "categories" ,    "color",
+               "categories",     "color",
                "reference-hash", "d492ac8ef38c336aa79e63b7f39f6139",
                "description",    _("Simulate a negative film enlargement in "
                                  "an analog darkroom."),