Light2D subtractive mode not working [GLES3]

[ghost]

Godot version:

b63c506

OS/device including version:

Win10 64-bit

Issue description:

The subtractive mode on Light2D seems to be broken, producing a full fill of some color. It starts to partially work when it begins to overlap with another Light2D texture.

Steps to reproduce:
Make a Light2D and choose subtractive mode.

Minimal reproduction project:

3.1 Light2D Issue.zip

[akien-mga]

GLES2 or 3?

[ghost]

GLES3. I'll check 2.

[ghost]

GLES2 is fine.

[akien-mga]

Same issue in 3.0.6. It's worth noting that it's broken in editor, but works fine in game.

[MadEqua]

I took a look at this and found the culprit. It's a matter of blending and alpha values.

These are the alpha values after rendering the above sprite and the lights:

Black is 0, Gray is 1 and White is 2.

The alpha of 0 makes the image disappear on the editor.

This brings me to the following questions:

• Does the lighting pass really needs to affect the alpha values on the color buffer? If yes, then it should only affect the pixels on the light texture, not all of the light bounds. If no, then this would fix the problem.
• Does blending really needs to be enabled when drawing the viewport quad on the editor? Disabling blending would hide the problem, but I'm not sure if those unbounded alpha values are really supposed to be there.

Need some input on these questions in order to fix this. :)

[clayjohn]

@MadEqua

1. I think the light should affect alpha, it may be used for color masking to an external Viewport to do pixel operations, so there are circumstances where you would want to alter alpha of the color buffer. But it should have no effect outside of the light texture.

2. Blending isn't needed for the Viewport draw in the editor, nor is it needed for the final blit to the back Framebuffer.

Thanks for tackling this issue. :)

[MadEqua]

Thanks @clayjohn. That answers perfectly my questions. Will submit a PR soon.