Widgets: ColorEdit: Fix multiple issues.

* Change g.ColorEditLastColor type to ImU32 and store RGB color value.
  - Fixes inability to change hue when saturation is 0. (ocornut#4014)
  - Fixes edgecases where lossy color conversion prevent restoration of hue/saturation.
  - Fixes hue value jitter when modifying color using SV square.
* Fix hue resetting to 0 when it is set to 255 by explicitly restoring hue if it is 0 and previous value was 1.
* Further reduce hue jitter by restoring hue when color is modified using SV square.

docs/CHANGELOG.txt

@@ -50,6 +50,9 @@ Other Changes:
   consistent with the compile-time default. (#3922)
 - DragScalar: Add default value for v_speed argument to match higher-level functions. (#3922) [@eliasdaler]
 - ColorEdit4: Alpha default to 255 (instead of 0) when omitted in hex input. (#3973) [@squadack]
+- ColorEdit4: Fixed not being able to change hue when saturation is 0. (#4014) [@rokups]
+- ColorEdit4: Fixed hue resetting to 0 when it is set to 255. [@rokups]
+- ColorEdit4: Fixed hue value jitter when source color is stored as rgb in 32bit integer. [@rokups]
 - InputText: Do not filter private unicode codepoints (e.g. icons) when pasted from clipboard. (#4005) [@dougbinks]
 - LabelText: Fixed clipping of multi-line value text when label is single-line. (#4004)
 - LabelText: Fixed vertical alignment of single-line value text when label is multi-line. (#4004)

imgui_internal.h

@@ -1481,9 +1481,9 @@ struct ImGuiContext
     ImFont                  InputTextPasswordFont;
     ImGuiID                 TempInputId;                        // Temporary text input when CTRL+clicking on a slider, etc.
     ImGuiColorEditFlags     ColorEditOptions;                   // Store user options for color edit widgets
-    float                   ColorEditLastHue;                   // Backup of last Hue associated to LastColor[3], so we can restore Hue in lossy RGB<>HSV round trips
-    float                   ColorEditLastSat;                   // Backup of last Saturation associated to LastColor[3], so we can restore Saturation in lossy RGB<>HSV round trips
-    float                   ColorEditLastColor[3];
+    float                   ColorEditLastHue;                   // Backup of last Hue associated to LastColor, so we can restore Hue in lossy RGB<>HSV round trips
+    float                   ColorEditLastSat;                   // Backup of last Saturation associated to LastColor, so we can restore Saturation in lossy RGB<>HSV round trips
+    ImU32                   ColorEditLastColor;                 // RGB value with alpha set to 0.
     ImVec4                  ColorPickerRef;                     // Initial/reference color at the time of opening the color picker.
     float                   SliderCurrentAccum;                 // Accumulated slider delta when using navigation controls.
     bool                    SliderCurrentAccumDirty;            // Has the accumulated slider delta changed since last time we tried to apply it?
@@ -1647,7 +1647,7 @@ struct ImGuiContext
         TempInputId = 0;
         ColorEditOptions = ImGuiColorEditFlags__OptionsDefault;
         ColorEditLastHue = ColorEditLastSat = 0.0f;
-        ColorEditLastColor[0] = ColorEditLastColor[1] = ColorEditLastColor[2] = FLT_MAX;
+        ColorEditLastColor = 0;
         SliderCurrentAccum = 0.0f;
         SliderCurrentAccumDirty = false;
         DragCurrentAccumDirty = false;

imgui_widgets.cpp

@@ -4683,6 +4683,35 @@ bool ImGui::ColorEdit3(const char* label, float col[3], ImGuiColorEditFlags flag
     return ColorEdit4(label, col, flags | ImGuiColorEditFlags_NoAlpha);
 }
 
+// ColorEdit supports RGB and HSV inputs. In case of RGB input resulting color may have undefined hue and/or saturation.
+// Since widget displays both RGB and HSV values we must preserve hue and saturation to prevent these values resetting
+// in some cases. Last color pointer is saved along with hue and saturation values. We use color pointer comparison to
+// determine whether we should restore hue/saturation. Pointer value is enough to validate source of backed up
+// hue/saturation values. Saving and comparing color value is unreliable when source color is stored in int32 as RGB
+// since float<->int32 conversions are lossy.
+static void RestoreHS(const float* col, float& H, float& S, float& V)
+{
+    ImGuiContext& g = *GImGui;
+
+    // This check is optional. Suppose we have two color widgets side by side, both widgets display different colors, but both colors have hue and/or saturation undefined.
+    // With color check: hue/saturation is preserved in one widget. Editing color in one widget would reset  hue/saturation in another one.
+    // Without color check: common hue/saturation would be displayed in all widgets that have hue/saturation undefined.
+    // g.ColorEditLastColor was changed to store ImU32 RGB value. This essentially gives us color equality check with reduced precision.
+    // Tiny external color change would not be detected and this check would still pass. This is OK, since we only restore hue/saturation _only_ if they are undefined,
+    // therefore this change flipping hue/saturation from undefined to a very tiny value would still be represented in color picker.
+    if (g.ColorEditLastColor != ImGui::ColorConvertFloat4ToU32(ImVec4(col[0], col[1], col[2], 0)))
+        return;
+
+    // When S == 0, H is undefined.
+    // When H == 1 it wraps around to 0.
+    if (S == 0 || (H == 0 && g.ColorEditLastHue == 1))
+        H = g.ColorEditLastHue;
+
+    // When V == 0, S is undefined.
+    if (V == 0)
+        S = g.ColorEditLastSat;
+}
+
 // Edit colors components (each component in 0.0f..1.0f range).
 // See enum ImGuiColorEditFlags_ for available options. e.g. Only access 3 floats if ImGuiColorEditFlags_NoAlpha flag is set.
 // With typical options: Left-click on color square to open color picker. Right-click to open option menu. CTRL-Click over input fields to edit them and TAB to go to next item.
@@ -4738,13 +4767,7 @@ bool ImGui::ColorEdit4(const char* label, float col[4], ImGuiColorEditFlags flag
     {
         // Hue is lost when converting from greyscale rgb (saturation=0). Restore it.
         ColorConvertRGBtoHSV(f[0], f[1], f[2], f[0], f[1], f[2]);
-        if (memcmp(g.ColorEditLastColor, col, sizeof(float) * 3) == 0)
-        {
-            if (f[1] == 0)
-                f[0] = g.ColorEditLastHue;
-            if (f[2] == 0)
-                f[1] = g.ColorEditLastSat;
-        }
+        RestoreHS(col, f[0], f[1], f[2]);
     }
     int i[4] = { IM_F32_TO_INT8_UNBOUND(f[0]), IM_F32_TO_INT8_UNBOUND(f[1]), IM_F32_TO_INT8_UNBOUND(f[2]), IM_F32_TO_INT8_UNBOUND(f[3]) };
 
@@ -4877,7 +4900,7 @@ bool ImGui::ColorEdit4(const char* label, float col[4], ImGuiColorEditFlags flag
             g.ColorEditLastHue = f[0];
             g.ColorEditLastSat = f[1];
             ColorConvertHSVtoRGB(f[0], f[1], f[2], f[0], f[1], f[2]);
-            memcpy(g.ColorEditLastColor, f, sizeof(float) * 3);
+            g.ColorEditLastColor = ColorConvertFloat4ToU32(ImVec4(f[0], f[1], f[2], 0));
         }
         if ((flags & ImGuiColorEditFlags_DisplayRGB) && (flags & ImGuiColorEditFlags_InputHSV))
             ColorConvertRGBtoHSV(f[0], f[1], f[2], f[0], f[1], f[2]);
@@ -5012,13 +5035,7 @@ bool ImGui::ColorPicker4(const char* label, float col[4], ImGuiColorEditFlags fl
     {
         // Hue is lost when converting from greyscale rgb (saturation=0). Restore it.
         ColorConvertRGBtoHSV(R, G, B, H, S, V);
-        if (memcmp(g.ColorEditLastColor, col, sizeof(float) * 3) == 0)
-        {
-            if (S == 0)
-                H = g.ColorEditLastHue;
-            if (V == 0)
-                S = g.ColorEditLastSat;
-        }
+        RestoreHS(col, H, S, V);
     }
     else if (flags & ImGuiColorEditFlags_InputHSV)
     {
@@ -5071,6 +5088,10 @@ bool ImGui::ColorPicker4(const char* label, float col[4], ImGuiColorEditFlags fl
         {
             S = ImSaturate((io.MousePos.x - picker_pos.x) / (sv_picker_size - 1));
             V = 1.0f - ImSaturate((io.MousePos.y - picker_pos.y) / (sv_picker_size - 1));
+
+            // Greatly reduces hue jitter and reset to 0 when hue == 255 and color is rapidly modified using SV square.
+            if (g.ColorEditLastColor == ColorConvertFloat4ToU32(ImVec4(col[0], col[1], col[2], 0)))
+                H = g.ColorEditLastHue;
             value_changed = value_changed_sv = true;
         }
         if (!(flags & ImGuiColorEditFlags_NoOptions))
@@ -5147,7 +5168,7 @@ bool ImGui::ColorPicker4(const char* label, float col[4], ImGuiColorEditFlags fl
             ColorConvertHSVtoRGB(H >= 1.0f ? H - 10 * 1e-6f : H, S > 0.0f ? S : 10 * 1e-6f, V > 0.0f ? V : 1e-6f, col[0], col[1], col[2]);
             g.ColorEditLastHue = H;
             g.ColorEditLastSat = S;
-            memcpy(g.ColorEditLastColor, col, sizeof(float) * 3);
+            g.ColorEditLastColor = ColorConvertFloat4ToU32(ImVec4(col[0], col[1], col[2], 0));
         }
         else if (flags & ImGuiColorEditFlags_InputHSV)
         {
@@ -5201,13 +5222,7 @@ bool ImGui::ColorPicker4(const char* label, float col[4], ImGuiColorEditFlags fl
             G = col[1];
             B = col[2];
             ColorConvertRGBtoHSV(R, G, B, H, S, V);
-            if (memcmp(g.ColorEditLastColor, col, sizeof(float) * 3) == 0) // Fix local Hue as display below will use it immediately.
-            {
-                if (S == 0)
-                    H = g.ColorEditLastHue;
-                if (V == 0)
-                    S = g.ColorEditLastSat;
-            }
+            RestoreHS(col, H, S, V);   // Fix local Hue as display below will use it immediately.
         }
         else if (flags & ImGuiColorEditFlags_InputHSV)
         {