Floating point exception from slider in very small child window

[dbr]

Version/Branch of Dear ImGui:

Version: 1.86 via imgui-rs
Branch: docking

My Issue/Question:

We are using SetWindowFontScale to have a zoomable imnodes-style node graph. This all works well, except sometimes we get a floating point exception as follows:

#0  0x000055555d60405e in ImDrawList::_PathArcToFastEx (this=0x55555ebec2d8, center=..., radius=2.38418579e-07, a_min_sample=24, a_max_sample=36, a_step=0) at ./third-party/imgui-docking/imgui/imgui_draw.cpp:1076
#1  0x000055555d6044f3 in ImDrawList::PathArcToFast (this=0x55555ebec2d8, center=..., radius=2.38418579e-07, a_min_of_12=6, a_max_of_12=9) at ./third-party/imgui-docking/imgui/imgui_draw.cpp:1184
#2  0x000055555d6056a2 in ImDrawList::PathRect (this=0x55555ebec2d8, a=..., b=..., rounding=2.38418579e-07, flags=240) at ./third-party/imgui-docking/imgui/imgui_draw.cpp:1383
#3  0x000055555d605aa0 in ImDrawList::AddRectFilled (this=0x55555ebec2d8, p_min=..., p_max=..., col=4290559164, rounding=12, flags=0) at ./third-party/imgui-docking/imgui/imgui_draw.cpp:1423
#4  0x000055555d61316c in ImGui::ScrollbarEx (bb_frame=..., id=2833933471, axis=ImGuiAxis_Y, p_scroll_v=0x7ffffffe5308, size_avail_v=103, size_contents_v=119, flags=288) at ./third-party/imgui-docking/imgui/imgui_widgets.cpp:1002
#5  0x000055555d6128a8 in ImGui::Scrollbar (axis=ImGuiAxis_Y) at ./third-party/imgui-docking/imgui/imgui_widgets.cpp:906
#6  0x000055555d5a8411 in ImGui::RenderWindowDecorations (window=0x55555ec77d00, title_bar_rect=..., title_bar_is_highlight=true, handle_borders_and_resize_grips=true, resize_grip_count=2, resize_grip_col=0x7ffffffe55e0, resize_grip_draw_size=5)
    at ./third-party/imgui-docking/imgui/imgui.cpp:6119
#7  0x000055555d5abef5 in ImGui::Begin (name=0x55555e3e8b10 "View/C6DC8F45/8C64E6D8", p_open=0x0, flags=19663623) at ./third-party/imgui-docking/imgui/imgui.cpp:6876
#8  0x000055555d5a4bc4 in ImGui::BeginChildEx (name=0x0, id=2355422936, size_arg=..., border=false, flags=19663619) at ./third-party/imgui-docking/imgui/imgui.cpp:5410
#9  0x000055555d5a4d5d in ImGui::BeginChild (id=2355422936, size_arg=..., border=false, extra_flags=788992) at ./third-party/imgui-docking/imgui/imgui.cpp:5442
#10 0x000055555d640522 in igBeginChildID (id=2355422936, size=..., border=false, flags=788992) at ./third-party/imgui-docking/cimgui.cpp:141

The top of the backtrace refers to this code:
https:/ocornut/imgui/blob/v1.86/imgui_draw.cpp#L1066-L1068
(well specifically these ones in the docking branch code)

Problem is most probably caused by radius being a tiny number (2.38418579e-07) leading to some zero-division

It's hard to narrow down the exact cause, as the crash only happens sometimes when randomly panning around in a large node graph - but I'm hoping the backtrace is enough to narrow down what causes this exception?

[ocornut]

We don't well support SetWindowFontScale() admittedly, it is kind-of an old artifact but I understand it has use.
In this case, it leads to non-integer sizes which we also don't correctly support (specific bug reports are welcome).

While a little of a workaround, I think in ScrollbarEx() it would be reasonable to change the if (bb_frame_width <= 0.0f || bb_frame_height <= 0.0f) line to if (bb_frame_width < 1.0f || bb_frame_height < 1.0f). Can you confirm if that fixes it for you?

On another side, I think we could probably to fix/clarify cases of using near-zero rounding values. That radius value will probably fail our const int radius_idx = (int)(radius + 0.999999f) kludge in _CalcCircleAutoSegmentCount(). I wonder if fallback CircleSegmentCounts[0] should be ~0 instead of 0 ?

[dbr]

While a little of a workaround, I think in ScrollbarEx() it would be reasonable to change the if (bb_frame_width <= 0.0f || bb_frame_height <= 0.0f) line to if (bb_frame_width < 1.0f || bb_frame_height < 1.0f). Can you confirm if that fixes it for you?

Tried making that change it didn't seem to help; in ScrollbarEx the bb_frame_width is ~6 and height is 10.0 - I guess the problem window isn't tiny, isntead it is just small enough that with the padding it ends up too small

#4  0x000055555d5ad11c in ImGui::ScrollbarEx (bb_frame=..., id=3031077347, axis=ImGuiAxis_Y, p_scroll_v=0x7ffffffe5308, size_avail_v=10, size_contents_v=21, flags=288) at ./third-party/imgui-docking/imgui/imgui_widgets.cpp:1002
(gdb) print bb_frame_width
$6 = 6.00000048
(gdb) print bb_frame_height 
$7 = 10
(gdb) print grab_rect.Min
$8 = {x = 5.99999952, y = 164}
(gdb) print grab_rect.Max
$9 = {x = 8, y = 169}
(gdb) print style.ScrollbarRounding
$10 = 12

Poking around further, it seems like in ImDrawList::PathRect the scrollbar rounding is the style-configured 12.0, and it gets scaled to the very-small float by this code:

rounding = ImMin(rounding, ImFabs(b.x - a.x) * ( ((flags & ImDrawFlags_RoundCornersTop)  == ImDrawFlags_RoundCornersTop)  || ((flags & ImDrawFlags_RoundCornersBottom) == ImDrawFlags_RoundCornersBottom) ? 0.5f : 1.0f ) - 1.0f)
rounding = ImMin(rounding, ImFabs(b.y - a.y) * ( ((flags & ImDrawFlags_RoundCornersLeft) == ImDrawFlags_RoundCornersLeft) || ((flags & ImDrawFlags_RoundCornersRight)  == ImDrawFlags_RoundCornersRight)  ? 0.5f : 1.0f ) - 1.0f)

The tiny number coming from - 1.0 part:

(gdb) print b.x
$36 = 8
(gdb) print a.x
$37 = 5.99999952
(gdb) print (b.x - a.x) * 0.5 - 1
$38 = 2.384185791015625e-07

If I make the following ch ange it does stop crashing, although the change feels.. pretty arbitrary:

 void ImDrawList::_PathArcToFastEx(const ImVec2& center, float radius, int a_min_sample, int a_max_sample, int a_step)
 {
-    if (radius <= 0.0f)
+    if (radius <= 0.5f)

[ocornut]

We investigated this with @thedmd and concluded that < 0.5f was fine.
Pushed fixes 9e0517a
(We considering using < _FringeScale or < _FringeScale * 0.5f but will want later to eventually introduce a separate variable for that purpose, in order to facilitate scaling.)