# HG changeset patch
# User alex@thinkpad
# Date 1386872498 -7200
# Node ID 518491f4fa3046733ec1f095dff8ce9b12cd3904
# Parent  2003390ed062ad333e0d15e4984fdc4e2de459bf
Enhancement: softer "soft film" highlight handling
1) new curve formula, with higher slope near white point: (1 - 1/(1+a*x)) / (1 - 1/(1+a)) => little or no blown highlights at large EV
2) soft film is applied starting from -2 EV instead of 0 (the picture is developed linearly underexposed by 2 EV, then it's brightened back by the soft film curve)
3) a little refactoring in develop_linear needed for fixing a small bug in highlight recovery (without it you may get some pink highlights)

diff -r 2003390ed062 -r 518491f4fa30 ufraw_developer.c
--- a/ufraw_developer.c	Sat Apr 20 00:36:54 2013 +0300
+++ b/ufraw_developer.c	Thu Dec 12 20:21:38 2013 +0200
@@ -302,24 +302,6 @@
     return hICC;
 }
 
-/* Find a for which (1-exp(-a x)/(1-exp(-a)) has derivative b at x=0 */
-/* In other words, solve a/(1-exp(-a))==b */
-static double findExpCoeff(double b)
-{
-    double a, bg;
-    int try;
-    if (b <= 1) return 0;
-    if (b < 2) a = (b - 1) / 2;
-    else a = b;
-    bg = a / (1 - exp(-a));
-    /* The limit on try is just to be sure there is no infinite loop. */
-    for (try = 0; abs(bg - b) > 0.001 || try < 100; try++) {
-                    a = a + (b - bg);
-                    bg = a / (1 - exp(-a));
-                }
-    return a;
-}
-
 static void developer_create_transform(developer_data *d, DeveloperMode mode)
 {
     if (!d->updateTransform)
@@ -474,8 +456,17 @@
     /* Handle the exposure normalization for Canon EOS cameras. */
     if (conf->ExposureNorm > 0)
         exposure = (guint64)exposure * d->rgbMax / conf->ExposureNorm;
-    if (exposure >= 0x10000) d->restoreDetails = clip_details;
-    if (exposure <= 0x10000) clipHighlights = digital_highlights;
+    
+    /* film exposures are developed linearly at -2 EV and brightened by +2 EV using the film curve */
+    /* at 0 EV (that is, -2 on the GUI), the film curve is linear, and below the formula doesn't make sense */
+    unsigned exposure_limit_for_film = 0x10000 / 4;
+    if (exposure <= exposure_limit_for_film) clipHighlights = digital_highlights;
+    
+    /* highlight recovery for negative EV is only done when exposure is... well... negative */
+    /* note that film exposures are developed linearly at -2 EV, so this threhold on the GUI becomes +2 EV */
+    unsigned exposure_limit_for_negative_restore = (clipHighlights == film_highlights ? 0x10000 * 4 : 0x10000);
+    if (exposure >= exposure_limit_for_negative_restore) d->restoreDetails = clip_details;
+    
     /* Check if gamma curve data has changed. */
     if (in->gamma != d->gamma || in->linear != d->linear ||
             exposure != d->exposure || clipHighlights != d->clipHighlights ||
@@ -498,10 +489,12 @@
         guint16 FilmCurve[0x10000];
         if (d->clipHighlights == film_highlights) {
             /* Exposure is set by FilmCurve[].
-             * Set initial slope to d->exposuse/0x10000 */
-            double a = findExpCoeff((double)d->exposure / 0x10000);
-            for (i = 0; i < 0x10000; i++) FilmCurve[i] =
-                    (1 - exp(-a * i / 0x10000)) / (1 - exp(-a)) * 0xFFFF;
+             * Set initial slope to d->exposure/0x10000 */
+            double a = MAX((double)d->exposure * 4 / 0x10000 - 1, 1e-5);
+            for (i = 0; i < 0x10000; i++) {
+                double x = (double) i / 0x10000;
+                FilmCurve[i] = (1 - 1/(1+a*x)) / (1 - 1/(1+a)) * 0xFFFF;
+            }
         } else { /* digital highlights */
             for (i = 0; i < 0x10000; i++) FilmCurve[i] = i;
         }
@@ -861,6 +854,14 @@
     unsigned c;
     gint64 tmppix[4];
     gboolean clipped = FALSE;
+    unsigned exposure = d->exposure;
+
+    if (d->clipHighlights == film_highlights) {
+        /* develop the image at -2 EV to recover highlights blown at 0 and small negative EV */
+        /* the film curve will brighten the image back */
+        exposure = 0x10000 / 4;
+    }
+
     for (c = 0; c < d->colors; c++) {
         /* Set WB, normalizing tmppix[c]<0x10000 */
         tmppix[c] = in[c];
@@ -872,12 +873,7 @@
         } else {
             tmppix[c] = MIN(tmppix[c], d->max);
         }
-        /* We are counting on the fact that film_highlights
-         * and !clip_highlights cannot be set simultaneously. */
-        if (d->clipHighlights == film_highlights)
-            tmppix[c] = tmppix[c] * 0x10000 / d->max;
-        else
-            tmppix[c] = tmppix[c] * d->exposure / d->max;
+        tmppix[c] = tmppix[c] * exposure / d->max;
     }
     if (clipped) {
         /* At this point a value of d->exposure in tmppix[c] corresponds
@@ -885,7 +881,7 @@
          * d->exposure * 0x10000 / d->max */
         gint64 unclippedPix[3], clippedPix[3];
         cond_apply_matrix(d, tmppix, unclippedPix);
-        for (c = 0; c < 3; c++) tmppix[c] = MIN(tmppix[c], d->exposure);
+        for (c = 0; c < 3; c++) tmppix[c] = MIN(tmppix[c], exposure);
         cond_apply_matrix(d, tmppix, clippedPix);
         if (d->restoreDetails == restore_lch_details) {
             float lch[3], clippedLch[3], unclippedLch[3];