Inconsistent width of PathArcTo strokes

[pixtur]

Version/Branch of Dear ImGui:

Version: 1.83
Branch: master

Back-end/Renderer/Compiler/OS

Back-ends imgui .net
Operating System: windows 10

My Issue/Question:
After upgrading from 1.77 to 1.86 there seem to appear rendering glitches for paths that include PathArcTo() with a small radius.

It's not unlikely that I'm not using the API correctly. But the effect only appeared recently and might be a regression of some optimization.

Screenshots/Video
As screenshot highlighting some issues.

Here is a capture of the code below...
https://user-images.githubusercontent.com/1732545/152852430-57253e47-46aa-4239-bcd4-939993eb1bef.mp4

Standalone, minimal, complete and verifiable example: (see #2261)

Sorry. c# only... 🙈

        private void DrawTest()
        {
            if (ImGui.Begin("test"))
            {
                ImGui.DragFloat("radius", ref _testRadius, 0.1f);
                ImGui.DragFloat("thickness", ref _thickness, 1f);
                
                const float Pi = 3.141592f;
                var drawList = ImGui.GetWindowDrawList();

                var center = new Vector2(300, 400);
                var max = 100;
                
                for (float i = _testRadius; i < max; i += 20)
                {
                    var radius = 10 + i;
                    drawList.PathClear();
                    drawList.PathArcTo(center + new Vector2(0, -100), max- radius,  1.5f * Pi,Pi );
                    drawList.PathArcTo(center + new Vector2(-max,10), radius, 0, Pi * 0.5f);
                    drawList.PathLineTo( center + new Vector2(-max - 100,10 + radius) );
                    
                    drawList.AddPolyline(ref drawList._Path[0], drawList._Path.Size, Color.White, ImDrawFlags.None, _thickness + 2);
                    drawList.PathStroke(Color.DarkGray, ImDrawFlags.None, _thickness);
                }

                ImGui.End();
            }
        }

[thedmd]

I got it fixed in 775b9e4.

Translated example:

        if (ImGui::Begin("test"))
        {
            static float _testRadius = 1.0f;
            static float _thickness = 1.0f;

            ImGui::DragFloat("radius", &_testRadius, 0.1f);
            ImGui::DragFloat("thickness", &_thickness, 1.0f);

            const float Pi = 3.141592f;
            auto drawList = ImGui::GetWindowDrawList();

            auto center = ImVec2(300, 400) + ImGui::GetCursorScreenPos();
            auto max = 100;

            //_testRadius = max - 20;

            for (float i = _testRadius; i < max; i += 20)
            {
                auto radius = 10 + i;
                drawList->PathClear();
                drawList->PathArcTo(center + ImVec2(0, -100), max - radius, 1.5f * Pi, Pi);
                drawList->PathArcTo(center + ImVec2(-max, 10), radius, 0, Pi * 0.5f);
                drawList->PathLineTo(center + ImVec2(-max - 100, 10 + radius));

                drawList->AddPolyline(drawList->_Path.Data, drawList->_Path.Size, IM_COL32_WHITE, ImDrawFlags_None, _thickness + 2.0);
                drawList->PathStroke(IM_COL32(169, 169, 169, 255), ImDrawFlags_None, _thickness);
            }

            ImGui::End();
        }

[pixtur]

Wow! That's incredible.

One more thing... I noticed that ImGui.net v1.84 can cause division/0 exceptions for PathArcTo() and DrawCicle() when called with small radii (e.g. 1e-7). I'm not sure if this is related do your fix. But these exceptions seem to be introduced the same time, as the drawing glitches.

[ocornut]

Merged @thedmd fix as 4691fa0

Thank you very much !