Multiply blend mode does not work with alpha

[RyannThi]

As the title says, multiply blending does not work with alpha on textures, nor does the SetImageState change its opacity.
Example:

Code:

SetImageState("image:ReimuFake_1", "mul+mul", Color(100,5,5,5))
Render("image:ReimuFake_1", 700, 155, 0, 1, 1)
SetImageState("image:ReimuFake_1", "", Color(255,255,255,255))
Render("image:ReimuFake_1", 675, 175, 0, 1, 1)

I apologize if there's any known method to avoid this. If there is, I'd love to know about it. And if this is intended behavior, I'd like to know why, too.

[Demonese]

In my considerations, the mul blending mode should be used with fully opaque images:

• with white representing a factor of 1.0
• black representing a factor of 0.0

If the alpha channel is taken into account, how does it participate in the calculation?

There are some possible calculations:

1. background-color * image-color * image-alpha ?
2. lerp(background-color, background-color * image-color, image-alpha) ?

I think these calculations are confusing... So I ignored the alpha channel.

[RyannThi]

In LuaSTG-Evo, the multiply blend mode works with alpha, so I assumed the same would apply for Sub 😭 It's confusing to me why it wouldn't, seeing as mul+rev can replicate a similar effect when manually inverting the used texture's color on an image editor.