W11/DX9 color alpha doesn't show correctly i.e. ColorEdit4

[ClaudiuHKS]

Version/Branch of Dear ImGui:

Version 1.91.8 WIP (19172), Branch: master

Back-ends:

imgui_impl_win32.cpp + imgui_impl_dx9.cpp

Compiler, OS:

W11 + VS2022

Full config/build information:

Dear ImGui 1.91.8 WIP (19172)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: _WIN32
define: _WIN64
define: _MSC_VER=1943
define: _MSVC_LANG=201402
--------------------------------
io.BackendPlatformName: imgui_impl_win32
io.BackendRendererName: imgui_impl_dx9
io.ConfigFlags: 0x00000003
 NavEnableKeyboard
 NavEnableGamepad
io.ConfigNavCaptureKeyboard
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x0000000F
 HasGamepad
 HasMouseCursors
 HasSetMousePos
 RendererHasVtxOffset
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 1536.00,793.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

Details:

My Issue/Question: No matter how I set alpha value (0-254), the alpha window is still containing white & grey squares. Only if alpha is 255 the white & grey squares are being opaque and displaying the color.

Screenshots/Video:

Minimal, Complete and Verifiable Example code:

No response

[PathogenDavid]

This was broken by cd6c83c, specifically by this change to ImLerp<T>:

-template<typename T> static inline T ImLerp(T a, T b, float t)                  { return (T)(a + (b - a) * t); }
+template<typename T> static inline T ImLerp(T a, T b, float t)                  { return (T)(a + (b - a) * (T)t); }

Not sure what warning it's trying to silence, but that change is definitely not right. It's causing ImAlphaBlendColors to break, which is the only usage of ImLerp with a type other than float:

imgui/imgui.cpp

Lines 2567 to 2574 in 7ae7c90

	IMGUI_API ImU32 ImAlphaBlendColors(ImU32 col_a, ImU32 col_b)
	{
	float t = ((col_b >> IM_COL32_A_SHIFT) & 0xFF) / 255.f;
	int r = ImLerp((int)(col_a >> IM_COL32_R_SHIFT) & 0xFF, (int)(col_b >> IM_COL32_R_SHIFT) & 0xFF, t);
	int g = ImLerp((int)(col_a >> IM_COL32_G_SHIFT) & 0xFF, (int)(col_b >> IM_COL32_G_SHIFT) & 0xFF, t);
	int b = ImLerp((int)(col_a >> IM_COL32_B_SHIFT) & 0xFF, (int)(col_b >> IM_COL32_B_SHIFT) & 0xFF, t);
	return IM_COL32(r, g, b, 0xFF);
	}

[ocornut]

Thank you David for finding this! The warning it aimed to fix is listed as #8241

[ocornut]

Fixed by 2af26b7, thank you David!