sigmoid: allow target white above 1600% for HDR output

RELEASE_NOTES.md

@@ -244,6 +244,11 @@ changes (where available).
   light level metadata (MaxCLL and MaxFALL) when built with libavif 1.0.0
   or newer.
 
+- Sigmoid's target white can now be set up to 5000%, instead of 1600%,
+  to leave more headroom for PQ and HLG exports. 100% is the 203 nit HDR
+  reference white, so the new limit reaches about 10000 nits, the top of
+  the PQ range. The slider's default range is unchanged.
+
 - The external raster masks module imports its mask into the image's
   `.dtdata` sidecar instead of referencing a file in a root folder.
   Each instance holds one imported mask. Existing edits that point at

src/iop/sigmoid.c

@@ -58,7 +58,7 @@ typedef struct dt_iop_sigmoid_params_t
 {
   float middle_grey_contrast; // $MIN: 0.1  $MAX: 10.0 $DEFAULT: 1.5 $DESCRIPTION: "contrast"
   float contrast_skewness;    // $MIN: -1.0 $MAX: 1.0 $DEFAULT: 0.0 $DESCRIPTION: "skew"
-  float display_white_target; // $MIN: 20.0  $MAX: 1600.0 $DEFAULT: 100.0 $DESCRIPTION: "target white"
+  float display_white_target; // $MIN: 20.0  $MAX: 5000.0 $DEFAULT: 100.0 $DESCRIPTION: "target white"
   float display_black_target; // $MIN: 0.0  $MAX: 15.0 $DEFAULT: 0.0152 $DESCRIPTION: "target black"
   dt_iop_sigmoid_methods_type_t color_processing; // $DEFAULT: DT_SIGMOID_METHOD_PER_CHANNEL $DESCRIPTION: "color processing"
   float hue_preservation; // $MIN: 0.0 $MAX: 100.0 $DEFAULT: 100.0 $DESCRIPTION: "preserve hue"
@@ -969,7 +969,11 @@ void gui_init(dt_iop_module_t *self)
   dt_bauhaus_slider_set_soft_range(slider, 50.0f, 100.0f);
   dt_bauhaus_slider_set_format(slider, "%");
   gtk_widget_set_tooltip_text(slider, _("the white luminance of the target display or print.\n"
-                                        "can be used creatively for a faded look or blowing out whites earlier."));
+                                        "can be used creatively for a faded look or blowing out whites earlier.\n"
+                                        "for HDR (PQ or HLG) output, raise it above "
+                                        "100 % to give highlights headroom;\n"
+                                        "100 percent is the 203 nit reference white, "
+                                        "so 1000 nits is about 493 percent"));
 
   self->widget = main_box;
 }