ImQuadContainsPoint.cpp

static inline float  ImCross(const ImVec2& p0, const ImVec2& p1)                { return p0.x * p1.y - p0.y * p1.x; }
// Returns +1 if c is on the left side and -1 if it is on the right side of line (a, b).
#define ImOrient(a, b, c)   (ImCross((b) - (a), (c) - (a)) > 0 ? +1 : -1)
bool ImQuadContainsPoint(ImVec2 p1, ImVec2 p2, ImVec2 p3, ImVec2 p4, ImVec2 pt)
{
    if (ImAbs(ImOrient(p1, p2, pt) + ImOrient(p2, p3, pt) + ImOrient(p3, p1, pt)) == 3)    // Is inside triangle (p1, p2, p3)
        return true;
    if (ImAbs(ImOrient(p1, p3, pt) + ImOrient(p3, p4, pt) + ImOrient(p4, p1, pt)) == 3)    // Is inside triangle (p1, p3, p4)
        return true;
    return false;
}
#undef ImOrient


ImGuiContext& g = *GImGui;
ImDrawList* draw_list = ImGui::GetForegroundDrawList();
ImVec2 p1(100.0f + 16.0f * ImSin(g.Time * 5.0f), 100.0f + 11.0f * ImCos(g.Time * 7.0f));
ImVec2 p2(100.0f + 11.0f * ImSin(g.Time * 3.0f), 200.0f + 16.0f * ImCos(g.Time * 5.0f));
ImVec2 p3(200.0f + 16.0f * ImSin(g.Time * 5.0f), 200.0f + 11.0f * ImCos(g.Time * 3.0f));
ImVec2 p4(200.0f + 11.0f * ImSin(g.Time * 7.0f), 100.0f + 16.0f * ImCos(g.Time * 5.0f));
draw_list->AddCircleFilled(p1, 2.0f, IM_COL32_WHITE);
draw_list->AddCircleFilled(p2, 2.0f, IM_COL32_WHITE);
draw_list->AddCircleFilled(p3, 2.0f, IM_COL32_WHITE);
draw_list->AddCircleFilled(p4, 2.0f, IM_COL32_WHITE);
if (ImQuadContainsPoint(p1, p2, p3, p4, g.IO.MousePos))
    draw_list->AddQuadFilled(p1, p2, p3, p4, IM_COL32(0, 255, 0, 255));
else
    draw_list->AddQuad(p1, p2, p3, p4, IM_COL32(255, 0, 0, 255));