From accb0261b8fd4c1bd32f58b3ecc7a0b732d6006e Mon Sep 17 00:00:00 2001
From: Rokas Kupstys <rokups@zoho.com>
Date: Sat, 31 Aug 2019 18:44:20 +0300
Subject: [PATCH] ColorPicker / ColorEdit: restore Hue when zeroing Saturation.
 (#2722, #2770) Issue is fixed by storing last active color picker color and
 last hue value when active color picker takes rgb as input. Then if current
 color picker color matches last active color - hue value will be restored.
 IDs are not used because ColorEdit4() and ColorWidget4() may call each other
 in hard-to-predict ways and they both push their own IDs on to the stack. We
 need hue restoration to happen in entire stack of these widgets if topmost
 widget used hue restoration. Since these widgets operate on exact same color
 value - color was chosen as a factor deciding which widgets should restore
 hue.

---
 imgui_internal.h  |  2 ++
 imgui_widgets.cpp | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/imgui_internal.h b/imgui_internal.h
index 47da60abc..8fc03a03c 100644
--- a/imgui_internal.h
+++ b/imgui_internal.h
@@ -1012,6 +1012,8 @@ struct ImGuiContext
     ImFont                  InputTextPasswordFont;
     ImGuiID                 TempInputTextId;                    // Temporary text input when CTRL+clicking on a slider, etc.
     ImGuiColorEditFlags     ColorEditOptions;                   // Store user options for color edit widgets
+    float                   ColorEditLastHue;
+    float                   ColorEditLastActiveColor[3];
     ImVec4                  ColorPickerRef;
     bool                    DragCurrentAccumDirty;
     float                   DragCurrentAccum;                   // Accumulator for dragging modification. Always high-precision, not rounded by end-user precision settings
diff --git a/imgui_widgets.cpp b/imgui_widgets.cpp
index 3adfc892f..7466acff6 100644
--- a/imgui_widgets.cpp
+++ b/imgui_widgets.cpp
@@ -4200,7 +4200,12 @@ bool ImGui::ColorEdit4(const char* label, float col[4], ImGuiColorEditFlags flag
     if ((flags & ImGuiColorEditFlags_InputHSV) && (flags & ImGuiColorEditFlags_DisplayRGB))
         ColorConvertHSVtoRGB(f[0], f[1], f[2], f[0], f[1], f[2]);
     else if ((flags & ImGuiColorEditFlags_InputRGB) && (flags & ImGuiColorEditFlags_DisplayHSV))
+    {
         ColorConvertRGBtoHSV(f[0], f[1], f[2], f[0], f[1], f[2]);
+        // Hue is lost when converting from greyscale rgb (saturation=0). Restore it.
+        if (f[1] == 0 && memcmp(g.ColorEditLastActiveColor, col, sizeof(float) * 3) == 0)
+            f[0] = g.ColorEditLastHue;
+    }
     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]) };
 
     bool value_changed = false;
@@ -4327,7 +4332,11 @@ bool ImGui::ColorEdit4(const char* label, float col[4], ImGuiColorEditFlags flag
             for (int n = 0; n < 4; n++)
                 f[n] = i[n] / 255.0f;
         if ((flags & ImGuiColorEditFlags_DisplayHSV) && (flags & ImGuiColorEditFlags_InputRGB))
+        {
+            g.ColorEditLastHue = f[0];
             ColorConvertHSVtoRGB(f[0], f[1], f[2], f[0], f[1], f[2]);
+            memcpy(g.ColorEditLastActiveColor, f, sizeof(float) * 3);
+        }
         if ((flags & ImGuiColorEditFlags_DisplayRGB) && (flags & ImGuiColorEditFlags_InputHSV))
             ColorConvertRGBtoHSV(f[0], f[1], f[2], f[0], f[1], f[2]);
 
@@ -4504,7 +4513,12 @@ bool ImGui::ColorPicker4(const char* label, float col[4], ImGuiColorEditFlags fl
     float H = col[0], S = col[1], V = col[2];
     float R = col[0], G = col[1], B = col[2];
     if (flags & ImGuiColorEditFlags_InputRGB)
+    {
         ColorConvertRGBtoHSV(R, G, B, H, S, V);
+        // Hue is lost when converting from greyscale rgb (saturation=0). Restore it.
+        if (S == 0 && memcmp(g.ColorEditLastActiveColor, col, sizeof(float) * 3) == 0)
+            H = g.ColorEditLastHue;
+    }
     else if (flags & ImGuiColorEditFlags_InputHSV)
         ColorConvertHSVtoRGB(H, S, V, R, G, B);
 
@@ -4628,6 +4642,8 @@ bool ImGui::ColorPicker4(const char* label, float col[4], ImGuiColorEditFlags fl
         if (flags & ImGuiColorEditFlags_InputRGB)
         {
             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;
+            memcpy(g.ColorEditLastActiveColor, col, sizeof(float) * 3);
         }
         else if (flags & ImGuiColorEditFlags_InputHSV)
         {
@@ -4680,7 +4696,9 @@ bool ImGui::ColorPicker4(const char* label, float col[4], ImGuiColorEditFlags fl
             R = col[0];
             G = col[1];
             B = col[2];
+            float preserve_hue = H;
             ColorConvertRGBtoHSV(R, G, B, H, S, V);
+            H = preserve_hue;                           // Avoids picker losing hue value for 1 frame glitch.
         }
         else if (flags & ImGuiColorEditFlags_InputHSV)
         {