bug: horizontal scrolling is inverted for Allegro5 backend with touchpads

[nobody-special666]

Branch:
docking + tables merged

Back-end/Renderer/Compiler/OS
Allegro5 / Linux / Synaptics touchpad

Back-ends: imgui_impl_XXX.cpp + imgui_impl_XXX.cpp (or specify if using a custom engine/back-end)
imgui_impl_allegro5.cpp

Operating System:
Ubuntu 20.04

Issue:
I was testing with the imgui_demo compiled using the Allegro5 backend
and my touchpad configured for natural scrolling and I noticed the horizontal scrollbars
are all moving in the opposite direction to what they should be. Vertical scrollbars are working fine.
Here is a patch to correct.
P.S: I don't know if this would break mouse wheels and haven't tested it with Allegro5 backend on other platforms.
Thanks for ImGui

diff --git a/examples/imgui_impl_allegro5.cpp b/examples/imgui_impl_allegro5.cpp
index 4a467c30..1f328732 100644
--- a/examples/imgui_impl_allegro5.cpp
+++ b/examples/imgui_impl_allegro5.cpp
@@ -332,7 +332,7 @@ bool ImGui_ImplAllegro5_ProcessEvent(ALLEGRO_EVENT* ev)
         if (ev->mouse.display == g_Display)
         {
             io.MouseWheel += ev->mouse.dz;
-            io.MouseWheelH += ev->mouse.dw;
+            io.MouseWheelH -= ev->mouse.dw;
             io.MousePos = ImVec2(ev->mouse.x, ev->mouse.y);
         }
         return true;

[ocornut]

and my touchpad configured for natural scrolling

Could you clarify what this means? Does toggling the settings means you get opposite values in the ev->mouse.dw field ?

How do SDL and GLFW behave for you in term of horizontal scrolling direction?

[nobody-special666]

Yes I mean the sign for the ev->mouse.dw field appears to be inverted,
but it's coming from Allegro5 that way so I just flipped it.
I tested demo with SDL2 and GLFW and scrolling direction is working fine without change.
I'll try test it on a different windows laptop and see if I can reproduce the error.

[ocornut]

Yes I mean the sign for the ev->mouse.dw field appears to be inverted,

My question is, can you confirm if altering your OS/WM/X11 setting changes the sign of any value reported to Allegro?

Allegro Windows:

      case WM_MOUSEHWHEEL: {
         if (accept_mouse_event()) {
            int d = GET_WHEEL_DELTA_WPARAM(wParam);
            _al_win_mouse_handle_hwheel(d, false, win_display);
            return TRUE;
         }
[....]

void _al_win_mouse_handle_hwheel(int raw_dw, bool abs, ALLEGRO_DISPLAY_WIN *win_disp)
{
   int d;
   int new_w;

   if (!installed)
      return;

   if (!abs) {
      raw_mouse_w += raw_dw;
   }
   else {
      raw_mouse_w = raw_dw;
   }

   new_w = al_get_mouse_wheel_precision() * raw_mouse_w / WHEEL_DELTA;
   d = new_w - mouse_state.w;
   mouse_state.w = new_w;

   generate_mouse_event(ALLEGRO_EVENT_MOUSE_AXES,
      mouse_state.x, mouse_state.y, mouse_state.z, mouse_state.w, mouse_state.pressure,
      0, 0, 0, d,
      0, (void*)win_disp);
}

Allegro X11:
https://github.com/liballeg/allegro5/blob/5.2.6/src/linux/lmouse.c

static void wheel_motion_handler(int x_button, ALLEGRO_DISPLAY *display)
{
   int dz = 0, dw = 0;
   if (x_button == Button4) dz = 1;
   if (x_button == Button5) dz = -1;
   if (x_button == 6) dw = -1;
   if (x_button == 7) dw = 1;
   if (dz == 0 && dw == 0) return;

GLFW Window:

        case WM_MOUSEHWHEEL:
        {
            // This message is only sent on Windows Vista and later
            // NOTE: The X-axis is inverted for consistency with macOS and X11
            _glfwInputScroll(window, -((SHORT) HIWORD(wParam) / (double) WHEEL_DELTA), 0.0);
            return 0;
        }

GLFW X11:

            // Modern X provides scroll events as mouse button presses
            else if (event->xbutton.button == Button4)
                _glfwInputScroll(window, 0.0, 1.0);
            else if (event->xbutton.button == Button5)
                _glfwInputScroll(window, 0.0, -1.0);
            else if (event->xbutton.button == Button6)
                _glfwInputScroll(window, 1.0, 0.0);
            else if (event->xbutton.button == Button7)
                _glfwInputScroll(window, -1.0, 0.0);

Looking at all this it seems Allegro does the opposite of GLFW but is consistent (at least accross X11/windows) in doing so and therefore your patch seems correct..

[ocornut]

Merged with 009276b, thank you,