Godot: Per Pixel Transparency in html5

[evandikt]

Godot version: 3.2 GLES2

OS/device including version: Linux 5.4.14-2-MANJARO

Issue description:
Hi

Per Pixel Transparency works in html5 export ?

get_tree().get_root().set_transparent_background(true) in my _ready
Display/Window/Per Pixel Transparency/Allowed => true
Display/Window/Per Pixel Transparency/Enabled => true

On export linux, everythings is good and transparent but in html5 i got a black background !
I change the export template to have background-color: transparent; in body html and #canvas

thx

[bruvzg]

Per pixel transparency is implemented on Linux, macOS and Windows only.

Display/Window properties do nothing on other platforms. set_transparent_background disables background rendering.

[muiroc]

Looks like something like this could work! 5d1cbc887bf94fcba714635e932fb61e44aec215

test export, html body blue background is visible
TransparentCanvasExport.zip

Not sure about the EmscriptenWebGLContextAttributes premultipliedAlpha option,
also I guess the alpha option should be setted only if required in the project preferences

[evandikt]

@muiroc i have to rebuild godot to get this bug fix ?

[Calinou]

@evandikt Yes, you need to download the Godot repository with @muiroc's commit then recompile the HTML5 export template by following the instructions in Compiling for the Web.

Once you've finished compiling, you can specify the generated ZIP as a custom (release or debug) export template in your project's HTML5 export preset.

[muiroc]

@evandikt meanwhile here's the debug-opt export template built from #36075 if you need it
godot.javascript.opt.debug.zip