Make PathArcTo accept counter-clockwise angles (#4030, #3491)

docs/CHANGELOG.txt

@@ -54,6 +54,7 @@ Other Changes:
 - 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)
 - Popups: Added 'OpenPopup(ImGuiID id)' overload to facilitate calling from nested stacks. (#3993, #331) [@zlash]
+- ImDrawList: Fixed PathArcTo() regression from 1.82 preventing use of counter-clockwise angles. (#4030, #3491) [@thedmd]
 - Demo: Improved popups demo and comments.
 - Backends: SDL: Rework global mouse pos availability check listing supported platforms explicitly,
   effectively fixing mouse access on Raspberry Pi. (#2837, #3950) [@lethal-guitar, @hinxx]

imgui_draw.cpp

@@ -1037,7 +1037,6 @@ void ImDrawList::_PathArcToFastEx(const ImVec2& center, float radius, int a_min_
         _Path.push_back(center);
         return;
     }
-    IM_ASSERT(a_min_sample <= a_max_sample);
 
     // Calculate arc auto segment step size
     if (a_step <= 0)
@@ -1046,17 +1045,7 @@ void ImDrawList::_PathArcToFastEx(const ImVec2& center, float radius, int a_min_
     // Make sure we never do steps larger than one quarter of the circle
     a_step = ImClamp(a_step, 1, IM_DRAWLIST_ARCFAST_TABLE_SIZE / 4);
 
-    // Normalize a_min_sample to always start lie in [0..IM_DRAWLIST_ARCFAST_SAMPLE_MAX] range.
+    const int sample_range = ImAbs(a_max_sample - a_min_sample);
-    if (a_min_sample < 0)
-    {
-        int normalized_sample = a_min_sample % IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
-        if (normalized_sample < 0)
-            normalized_sample += IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
-        a_max_sample += (normalized_sample - a_min_sample);
-        a_min_sample = normalized_sample;
-    }
-
-    const int sample_range = a_max_sample - a_min_sample;
     const int a_next_step = a_step;
 
     int samples = sample_range + 1;
@@ -1082,6 +1071,15 @@ void ImDrawList::_PathArcToFastEx(const ImVec2& center, float radius, int a_min_
     ImVec2* out_ptr = _Path.Data + (_Path.Size - samples);
 
     int sample_index = a_min_sample;
+    if (sample_index < 0 || sample_index >= IM_DRAWLIST_ARCFAST_SAMPLE_MAX)
+    {
+        sample_index = sample_index % IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+        if (sample_index < 0)
+            sample_index += IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+    }
+
+    if (a_max_sample >= a_min_sample)
+    {
         for (int a = a_min_sample; a <= a_max_sample; a += a_step, sample_index += a_step, a_step = a_next_step)
         {
             // a_step is clamped to IM_DRAWLIST_ARCFAST_SAMPLE_MAX, so we have guaranteed that it will not wrap over range twice or more
@@ -1093,6 +1091,21 @@ void ImDrawList::_PathArcToFastEx(const ImVec2& center, float radius, int a_min_
             out_ptr->y = center.y + s.y * radius;
             out_ptr++;
         }
+    }
+    else
+    {
+        for (int a = a_min_sample; a >= a_max_sample; a -= a_step, sample_index -= a_step, a_step = a_next_step)
+        {
+            // a_step is clamped to IM_DRAWLIST_ARCFAST_SAMPLE_MAX, so we have guaranteed that it will not wrap over range twice or more
+            if (sample_index < 0)
+                sample_index += IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
+
+            const ImVec2 s = _Data->ArcFastVtx[sample_index];
+            out_ptr->x = center.x + s.x * radius;
+            out_ptr->y = center.y + s.y * radius;
+            out_ptr++;
+        }
+    }
 
     if (extra_max_sample)
     {
@@ -1116,7 +1129,6 @@ void ImDrawList::_PathArcToN(const ImVec2& center, float radius, float a_min, fl
         _Path.push_back(center);
         return;
     }
-    IM_ASSERT(a_min <= a_max);
 
     // Note that we are adding a point at both a_min and a_max.
     // If you are trying to draw a full closed circle you don't want the overlapping points!
@@ -1136,7 +1148,6 @@ void ImDrawList::PathArcToFast(const ImVec2& center, float radius, int a_min_of_
         _Path.push_back(center);
         return;
     }
-    IM_ASSERT(a_min_of_12 <= a_max_of_12);
     _PathArcToFastEx(center, radius, a_min_of_12 * IM_DRAWLIST_ARCFAST_SAMPLE_MAX / 12, a_max_of_12 * IM_DRAWLIST_ARCFAST_SAMPLE_MAX / 12, 0);
 }
 
@@ -1147,7 +1158,6 @@ void ImDrawList::PathArcTo(const ImVec2& center, float radius, float a_min, floa
         _Path.push_back(center);
         return;
     }
-    IM_ASSERT(a_min <= a_max);
 
     if (num_segments > 0)
     {
@@ -1158,28 +1168,33 @@ void ImDrawList::PathArcTo(const ImVec2& center, float radius, float a_min, floa
     // Automatic segment count
     if (radius <= _Data->ArcFastRadiusCutoff)
     {
+        const bool a_is_reverse = a_max < a_min;
+
         // We are going to use precomputed values for mid samples.
         // Determine first and last sample in lookup table that belong to the arc.
-        const int a_min_sample = (int)ImCeil(IM_DRAWLIST_ARCFAST_SAMPLE_MAX * a_min / (IM_PI * 2.0f));
+        const float a_min_sample_f = IM_DRAWLIST_ARCFAST_SAMPLE_MAX * a_min / (IM_PI * 2.0f);
-        const int a_max_sample = (int)(      IM_DRAWLIST_ARCFAST_SAMPLE_MAX * a_max / (IM_PI * 2.0f));
+        const float a_max_sample_f = IM_DRAWLIST_ARCFAST_SAMPLE_MAX * a_max / (IM_PI * 2.0f);
-        const int a_mid_samples = ImMax(a_max_sample - a_min_sample, 0);
+
+        const int a_min_sample = a_is_reverse ? (int)ImFloorSigned(a_min_sample_f) : (int)ImCeil(a_min_sample_f);
+        const int a_max_sample = a_is_reverse ? (int)ImCeil(a_max_sample_f) : (int)ImFloorSigned(a_max_sample_f);
+        const int a_mid_samples = a_is_reverse ? ImMax(a_min_sample - a_max_sample, 0) : ImMax(a_max_sample - a_min_sample, 0);
 
         const float a_min_segment_angle = a_min_sample * IM_PI * 2.0f / IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
         const float a_max_segment_angle = a_max_sample * IM_PI * 2.0f / IM_DRAWLIST_ARCFAST_SAMPLE_MAX;
-        const bool a_emit_start = (a_min_segment_angle - a_min) > 0.0f;
+        const bool a_emit_start = (a_min_segment_angle - a_min) != 0.0f;
-        const bool a_emit_end = (a_max - a_max_segment_angle) > 0.0f;
+        const bool a_emit_end = (a_max - a_max_segment_angle) != 0.0f;
 
         _Path.reserve(_Path.Size + (a_mid_samples + 1 + (a_emit_start ? 1 : 0) + (a_emit_end ? 1 : 0)));
         if (a_emit_start)
             _Path.push_back(ImVec2(center.x + ImCos(a_min) * radius, center.y + ImSin(a_min) * radius));
-        if (a_max_sample >= a_min_sample)
+        if (a_mid_samples > 0)
             _PathArcToFastEx(center, radius, a_min_sample, a_max_sample, 0);
         if (a_emit_end)
             _Path.push_back(ImVec2(center.x + ImCos(a_max) * radius, center.y + ImSin(a_max) * radius));
     }
     else
     {
-        const float arc_length = a_max - a_min;
+        const float arc_length = ImAbs(a_max - a_min);
         const int circle_segment_count = _CalcCircleAutoSegmentCount(radius);
         const int arc_segment_count = ImMax((int)ImCeil(circle_segment_count * arc_length / (IM_PI * 2.0f)), (int)(2.0f * IM_PI / arc_length));
         _PathArcToN(center, radius, a_min, a_max, arc_segment_count);

imgui_internal.h

@@ -371,6 +371,7 @@ static inline float  ImPow(float x, float y)    { return powf(x, y); }
 static inline double ImPow(double x, double y)  { return pow(x, y); }
 static inline float  ImLog(float x)             { return logf(x); }             // DragBehaviorT/SliderBehaviorT uses ImLog with either float/double and need the precision
 static inline double ImLog(double x)            { return log(x); }
+static inline int    ImAbs(int x)               { return x < 0 ? -x : x; }
 static inline float  ImAbs(float x)             { return fabsf(x); }
 static inline double ImAbs(double x)            { return fabs(x); }
 static inline float  ImSign(float x)            { return (x < 0.0f) ? -1.0f : ((x > 0.0f) ? 1.0f : 0.0f); } // Sign operator - returns -1, 0 or 1 based on sign of argument