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, 26 insertions, 0 deletions

diff --git a/source/blender/makesrna/RNA_types.h b/source/blender/makesrna/RNA_types.h
index 6cd3678017e..4a6d6dddec7 100644
--- a/source/blender/makesrna/RNA_types.h
+++ b/source/blender/makesrna/RNA_types.h
@@ -95,6 +95,32 @@ typedef enum PropertyUnit {
   PROP_UNIT_TEMPERATURE = (11 << 16), /* C */
 } PropertyUnit;
 
+/**
+ * Use values besides #PROP_SCALE_LINEAR
+ * so the movement of the mouse doesn't map linearly to the value of the slider.
+ *
+ * For some settings it's useful to space motion in a non-linear way, see T77868.
+ *
+ * NOTE: The scale types are available for all float sliders.
+ * For integer sliders they are only available if they use the visible value bar.
+ * Sliders with logarithmic scale and value bar must have a range > 0
+ * while logarithmic sliders without the value bar can have a range of >= 0.
+ */
+typedef enum PropertyScaleType {
+  /** Linear scale (default). */
+  PROP_SCALE_LINEAR = 0,
+  /**
+   * Logarithmic scale
+   * - Maximum range: `0 <= x < inf`
+   */
+  PROP_SCALE_LOG = 1,
+  /**
+   * Cubic scale.
+   * - Maximum range: `-inf < x < inf`
+   */
+  PROP_SCALE_CUBIC = 2,
+} PropertyScaleType;
+
 #define RNA_SUBTYPE_UNIT(subtype) ((subtype)&0x00FF0000)
 #define RNA_SUBTYPE_VALUE(subtype) ((subtype) & ~0x00FF0000)
 #define RNA_SUBTYPE_UNIT_VALUE(subtype) ((subtype) >> 16)