BackBuffer ALPHA channel doesn't work on Android devices

Hi,

After recent update from Unity 4.1.x to 4.5, I've found quite critical issue with frame buffer on Android devices.

Repro:
1. Set camera clear flags to solid color, as rendering path use Forward.
2. Set the clear color ALPHA channel to something other than default 255, for example set it to 0
3. Everything works on PC, alpha channel is cleared to 0
4. Build an Android build and observe the alpha is cleared to default 255

This did work on Unity 4.1.2 for sure. It's critical because it brings serious inconsistencies between editor and device builds...

update: Looks like the alpha channel is not used at all. Even shaders can't blend with frame buffer alpha on Andorid, alpha is just always ONE.

Using 32-bit Display buffer from build settings.
Both devices have Android 4+

Test devices for example Galaxy S2, Nexus 7, ...

 

It seems to be even worse. If I create a full screen quad with shader, which writes zeros to FrameBuffer's alpha channel, result is the same. It even looks like Unity guys changed framebuffer format for Androids so it doesn't even use/have alpha channel anymore?

Shaders really can't blend with frame buffer's alpha anymore. That is quite pissing. You could do very nice effects with that. And if you use it in editor, it works, so ES2 emulation doesn't work properly if that was intentional change.

 

I am using MEAT5000 s amazing custom shader and have encountered similar issues.
I have been able to test on more devices and can confirm that for the Nexus 10 pad and a Nexus phone as well as the Samsung Galaxy Note, that the Alpha is always set to 1 when the build is run on those devices where as the correct alpha value will be used fine in unity editor.
Additionally I have also tested the same shader on the Sony Experia Z, Sony Experia Z1, Sony Experia Z Ultra and the Cubot One and these devices render the alpha channel values correctly using the exact same shader.
My initial conclusion is that the GPUs / or Drivers in certain android devices does not or no longer supports alpha < 1 in shaders;
I have tried allowing 24bit depth buffering as well as 32bit and that doesnt correct the issue.
Any pointers from community members with more service level/hardware knowledge of different Android devices is probably whats required to resolve this issue

 

Hi, I just want to point out, that it's most likely because of recent changes in Unity. My Galaxy S2 has the same Android v4.0.4 which I use for a long time. With Unity 4.1.x - 4.2.x build, alpha works correctly. With Unity 4.5.x it's always set to one. So same device, same game, different Unity version....

It would be great if someone from Unity devs could make a statement on this.. they must know about this obvious bug. I just wanna know when is it going to be fixed.

 

I did try Unity 4.5.1p4 but result is the same. How long could it possibly take to fix this....

 

Still not fixed in Unity 4.5.2p1.

 

You can vote for this issue now: http://issuetracker.unity3d.com/iss...channel-is-always-set-to-1-on-android-devices

 

Could somebody from Unity have a look into this? I even sent you a scene with 100% repro and very simple shader demonstrating the bug. It's all part of 620787. Or some time estimate would be nice, something.

 

"The only way to solve this problem right now (that I can see) is to render to an intermediate rt and then blit to the screen as a last step. This might be something we will do behind the scenes in the future if we can find a way of detecting buggy compositors. But as of now you will have to do this yourself."

Can you give example code of how to do this?

 

Thank you for the reply. I mentioned the possibility it might have been "by desing" in my earlier posts. The problem is that right now there is a difference between editor and device. If it is changed by design, editor should work the same way. So you shouldn't be able to change the alpha channel in editor as well (if working on Android platform with emulation for example).

Using separate RT is probably the first thing I would came up with as a workaround. Many users will not be able to use this because it is a PRO feature. Actually having alpha channel "free" for blending ops allowed nice effects even without PRO version. In my opinion, It's kind of step backwards, because engine should not limit users in such a basic thing as using alpha channel on back buffer. Especially if source codes are not available to public.