UI: add non-linear slider support

This patch introduces non linear sliders. That means, that the movement
of the mouse doesn't map linearly to the value of the slider.

The following changes have been made.

- Free logarithmic sliders with maximum range of (`0 <= x < inf`)
- Logarithmic sliders with correct value indication bar.
- Free cubic sliders with maximum range of (`-inf < x < inf`)
- Cubic sliders with correct value indication bar.

Cubic mapping has been added as well, because it's used for brush sizes
in other applications (Krita for e.g.).

To make a slider have a different scale type use following line in RNA:
 `RNA_def_property_ui_scale_type(prop, PROP_SCALE_LOGARITHMIC);`
or:
 `RNA_def_property_ui_scale_type(prop, PROP_SCALE_CUBIC);`

Test the precision, step size and soft-min if you change the scale type
of a property as it will feel very different and may need tweaking.

Ref D9074

1 files changed, 29 insertions, 6 deletions

diff --git a/source/blender/editors/interface/interface_widgets.c b/source/blender/editors/interface/interface_widgets.c
index c9c1f56dc98..d2e6165a20f 100644
--- a/source/blender/editors/interface/interface_widgets.c
+++ b/source/blender/editors/interface/interface_widgets.c
@@ -3752,12 +3752,35 @@ static void widget_numslider(
     float factor, factor_ui;
     float factor_discard = 1.0f; /* No discard. */
     const float value = (float)ui_but_value_get(but);
-
-    if (but->rnaprop && (RNA_property_subtype(but->rnaprop) == PROP_PERCENTAGE)) {
-      factor = value / but->softmax;
-    }
-    else {
-      factor = (value - but->softmin) / (but->softmax - but->softmin);
+    const float softmin = but->softmin;
+    const float softmax = but->softmax;
+    const float softrange = softmax - softmin;
+    const PropertyScaleType scale_type = ui_but_scale_type(but);
+
+    switch (scale_type) {
+      case PROP_SCALE_LINEAR: {
+        if (but->rnaprop && (RNA_property_subtype(but->rnaprop) == PROP_PERCENTAGE)) {
+          factor = value / softmax;
+        }
+        else {
+          factor = (value - softmin) / softrange;
+        }
+        break;
+      }
+      case PROP_SCALE_LOG: {
+        const float logmin = fmaxf(softmin, 0.5e-8f);
+        const float base = softmax / logmin;
+        factor = logf(value / logmin) / logf(base);
+        break;
+      }
+      case PROP_SCALE_CUBIC: {
+        const float cubicmin = cube_f(softmin);
+        const float cubicmax = cube_f(softmax);
+        const float cubicrange = cubicmax - cubicmin;
+        const float f = (value - softmin) * cubicrange / softrange + cubicmin;
+        factor = (cbrtf(f) - softmin) / softrange;
+        break;
+      }
     }
 
     const float width = (float)BLI_rcti_size_x(rect);