DoubleSided transmission broken with WEBGL_multisampled_render_to_texture (Oculus Browser)

[mrxz]

Description

When transmissive materials are set to DoubleSided the transmissive objects are rendered in two passes (see: #25502). The back side of the transmissive objects are rendered into the transmissiveRenderTarget while using the intermediate result of the transmissiveRenderTarget. This works as the target is multi-sampled and uses two textures under the hood.

However, when the EXT_multisampled_render_to_texture is supported by the browser, it is used instead of the two textures approach. In this case rendering the back-sides fails as source and destination textures are the same.

[.WebGL-0x20170a200]GL ERROR :GL_INVALID_OPERATION : glDrawElements: Source and destination textures of the draw are the same.

Reproduction steps

1. Setup a scene with a double sided transmissive material
2. Open in a browser that supports WEBGL_multisampled_render_to_texture
3. Notice the back-side not being rendered and error(s) logged in the console

Code

const geometry = new THREE.SphereGeometry( 20, 64, 32 );

const material = new THREE.MeshPhysicalMaterial( {
	metalness: 0,
	roughness: 0,
	ior: 1.5,
	envMap: /* some equirectEnvMap */ hdrEquirect,
	transmission: 1.0,
	side: THREE.DoubleSide,
} );

mesh = new THREE.Mesh( geometry, material );
scene.add( mesh );

Live example

The bug is present in both webxr_vr_sandbox and webgl_materials_physical_transmission.

Screenshots

Without WEBGL_multisampled_render_to_texture	With WEBGL_multisampled_render_to_texture

Version

r163

Device

Headset

Browser

Chrome

OS

Android

[Mugen87]

Yeah, we were hoping to circumvent the feedback loop by always using a multisampled render target, see #26177. It seems there is no way around a separate solution for the EXT_multisampled_render_to_texture case.

In #26177, I have mentioned one possible approach based on copyTexSubImage2D(). Meaning copy the contents of the transmission framebuffer into a separate texture and use it as the value for transmissionSamplerMap. Do you think this approach goes into the right direction or are there other options?