TRANSLUCENT display incorrect.(use method in sample-transparent-view)

[dsafa22]

#4084
use android sample:sample-gltf-viewer
In ModelViewer.kt, use the method in sample-transparent-view,the code changed as blow(no other code change)
inSurfaceCallback :

swapChain = engine.createSwapChain(surface, SwapChain.CONFIG_TRANSPARENT)

In MainActivity.kt:
do not use sky box

//        readCompressedAsset("envs/$ibl/${ibl}_skybox.ktx").let {
//            scene.skybox = KtxLoader.createSkybox(engine, it)
//        }

and I set the view background #0f0 in layout

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/simple_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#f00">
....

It show as blow

not right.

[romainguy]

What's not right? The background is set to red and visible through the 3D view. What issue are you reporting?

[dsafa22]

What's not right? The background is set to red and visible through the 3D view. What issue are you reporting?

The face is missing some part.

[dsafa22]

[pixelflinger]

Is the face okay when you're NOT setting the view to transparent?

[romainguy]

Could you share the glTF file you are loading please?

[dsafa22]

debug in metal.

after colorGrading, the texture begin not right.

[dsafa22]

Could you share the glTF file you are loading please?

it is here.
#4084

https://github.com/google/filament/files/6596877/role_boy.glb.zip

[dsafa22]

the missing part same as the face node exceed the hair node.

[dsafa22]

Is the face okay when you're NOT setting the view to transparent?

Yes, it is OK when not set transparent.

BTW.
If the scene has skybox, whatever the view is transparent or not transparent, it is OK too.

[romainguy]

How did you modify gltf_viewer? When I remove the skybox and set the swap chain to transparent, everything renders properly:

[dsafa22]

How did you modify gltf_viewer? When I remove the skybox and set the swap chain to transparent, everything renders properly:

dest->setBlendMode(View::BlendMode::TRANSLUCENT);

[romainguy]

Ah I was able to repro. There's no need to set the swap chain to use CONFIG_FLAG_TRANSPARENT. What triggers the bug is setting the View's blend mode to TRANSLUCENT:

view->setBlendMode(View::BlendMode::TRANSLUCENT);

[romainguy]

@pixelflinger I forgot, is the app supposed to set the ClearOptions in this case? It looks like we're not properly clearing the depth buffer (or depth test is still incorrectly enabled).

[dsafa22]

Ah I was able to repro. There's no need to set the swap chain to use CONFIG_FLAG_TRANSPARENT. What triggers the bug is setting the View's blend mode to TRANSLUCENT:

view->setBlendMode(View::BlendMode::TRANSLUCENT);

yes, very smart!

[romainguy]

@pixelflinger I found the bug. It's caused by this line in the post processing material:

color.rgb /= color.a + FLT_EPS;

I'm not sure why it causes the problem yet. But it's fine when returning just before and it's broken when returning right after. I imagine we have bad values in color.a.

[romainguy]

Alright I found the issue. The problem is the asset. The face uses alpha masking with an alpha value that comes from the face texture. And the alpha channel of said texture looks like this:

So what's happening is that in "view transparent" mode the alpha of the alpha masking is used to blend and we correctly interpret it and the model disappears. The bug doesn't appear in opaque mode because we disable blending so all pixels are treated as alpha=1.

Please disable alpha masking on the face, it'll fix your issue and improve rendering performance.

[dsafa22]

Alright I found the issue. The problem is the asset. The face uses alpha masking with an alpha value that comes from the face texture. And the alpha channel of said texture looks like this:

So what's happening is that in "view transparent" mode the alpha of the alpha masking is used to blend and we correctly interpret it and the model disappears. The bug doesn't appear in opaque mode because we disable blending so all pixels are treated as alpha=1.

Please disable alpha masking on the face, it'll fix your issue and improve rendering performance.

Yes, I also suspect that this model has a problem, because other models don't have this problem. I'll try to modify it. Thank you.

[ShoutTree]

Hi, @romainguy, I also met this issue in version 1.15.0. Does that mean the issue is still not fixed? From your comments above I thought it was a model issue at first, but since the model is correctly rendered in other renderer(https://gltf-viewer.donmccurdy.com/), I concluded that it's still a filament blending issue. Seems filament does not correctly use the alpha masking. Am I right ?
Thanks.