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Dithered lod transition bug.

When using Dithered LOD transition with landscape grass using new Use Landscape Lightmap feature on iOS metal has nasty rendering error. I assume that lightmap coordinates are somehow corrupted during that transition. It seem that I can't disable dithered lod transitions completely for Landscape grass type. Without new Lightmap feature grass look totally wrong so I would want to see this fixed.

UE4.12.4 preview.

Product Version: UE 4.12 Preview
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asked May 23 '16 at 02:33 PM in Bug Reports

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avatar image Kalle-H May 24 '16 at 07:23 AM

This is how bug look. Screenshot taken from iPad pro. Where dither pattern is visible lighting is broken.

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ditherbug.png (1.8 MB)
avatar image Kalle-H May 25 '16 at 06:20 AM

Is there need for more info?

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2 answers: sort voted first

I really appreciate your patience while we determined the validity of this bug. I have gone ahead and entered a report for the issue, UE-32289. What we suspect is happening is that it's a UV or Z precision issue unique to iOS Metal.

Once the issue has been addressed by our engineers, the fix will be added to the release notes for fixed issues within an upcoming full engine or hotfix release.

Let me know if you have further questions or need additional assistance.

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answered Jun 21 '16 at 03:41 PM

avatar image Kalle-H Jun 22 '16 at 06:52 AM

I guess it's either wrong uv's for landscape ligthmaps or wrong lightmap binding.

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Hey Jenny Gore,

So the issue here is that Dithered LOD transitions only function when you have a platform that can support Temporal AA. The dithering effect is directly dependent on Temporal AA and mobile devices use a mobile specific Temporal AA that does not function the same, which means the dithering is expected

You can also have a read over the post below for additional information on Mobile Anti-Aliasing. It looks like MSAA is on our road map to have integrated.


Let me know if you have further questions or need additional assistance.

Thank you,


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answered May 25 '16 at 06:15 PM

avatar image Kalle-H May 26 '16 at 06:47 AM

Problem is not the dithering. Problem is that when LOD change happen landscape grass lightmaps are totally wrong. I am talking those really dark grass blades. This is caused by new "use landscape lightmap" feature which work fine until LOD change happen and then all grass that are swapping LOD's goes dark until LOD change is complete.

avatar image AndrewHurley May 26 '16 at 02:38 PM

Ah, I misunderstood the issue. In that case, could you provide me with some simple reproducible steps or a sample project so I can get the issue to occur on my end?

avatar image Kalle-H May 27 '16 at 07:41 AM

Simple project with landscape. Add Landscape grass type. Check Use landscape lightmap to ON. Make sure foliage mesh has LOD's. Scene should have some shadow casters so lighting has some variance. Materials can be super simple. When this is tested with iOS device using metal LOD changes have corrupted lightmaps.

avatar image AndrewHurley May 31 '16 at 02:13 PM

I just tested this with a simple set up on my end and was not able to reproduce the issue. Could you provide me with your project so I can test your exact issue on the iPad Pro we have here?

Thank you,

Andrew Hurley

avatar image Kalle-H Jun 01 '16 at 11:51 AM

I will need to make minimal test case then.

avatar image Kalle-H Jun 01 '16 at 02:25 PM

https://dl.dropboxusercontent.com/u/10960490/DitherBug.zip Minimal test case. Both terrain and grass use solid color and lighting bug when dithered is clearly visible at shadows.

avatar image Kalle-H Jun 08 '16 at 07:32 AM

What is status of this bug report? Problem is still clearly visible on 4.12.1 when using iPad pro.

avatar image Matthew J Jun 10 '16 at 02:44 PM

Hello Jenny,

Due to some technical issues it took a bit to get this tested. Unfortunately, whenever we attempt to deploy your project to our iPad Pro, the project never opens completely. It reaches the splash screen and then closes without ever showing the level itself. This same result does not occur on our other iOS devices (Such as an iPad Air and an iPhone 6. In case it matters, are you building from a Mac? Also, what iOS version is your iPad Pro on currently?

avatar image Kalle-H Jun 13 '16 at 10:09 AM

Just tested bug with other devices and it's clearly not related just to iPad pro. Exactly same outcome with iPhone 6s and IPad pro mini.

avatar image AndrewHurley Jun 14 '16 at 08:39 PM

Hey Jenny,

I had another peer run your test project on the iPad Pro version 9.3.2, and did not see the issue you are reporting. We worked directly with apple on the 9.3.2 release to get fixes in to the way metal handles rendering, and the issue you are seeing could be directly related to your iOS version. Try updating to see if that resolves your issue. Let me know your results.


Andrew Hurley

avatar image Kalle-H Jun 15 '16 at 08:46 AM

My iPhone is using 9.3.2 ios version and I can see same bug there. I will make another test project where the bug is more pronounced.

avatar image Kalle-H Jun 15 '16 at 11:06 AM

There is comparison shot how scene look when dither LOD change is not activated and when it is. I removed everything from scene that would distract to see this bug. It's quite clear that when LOD change is happening other LOD is using wrong lightmap data. Everything is looking great when camera is static and LOD's are not changing.

Link to project. https://dl.dropboxusercontent.com/u/10960490/DitherLodBug.zip alt text

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avatar image AndrewHurley Jun 15 '16 at 04:08 PM

Hey Jenny,

Apologies for not mentioning this earlier, but the person who tested internally, tested in the upcoming 4.12.3 hotfix, which is where he saw the issue resolved.

Would you mind running a quick test for me on your end in the 4.12.3 hotfix release once it comes out within the next couple days. We have all the information needed to write up a report if it comes to that, but I would like to see if the issue is resolved in the upcoming release.

Thank you,

Andrew Hurley

avatar image Kalle-H Jun 16 '16 at 07:00 AM

Thanks. I will test it when it comes out.

avatar image Kalle-H Jun 17 '16 at 07:09 AM

I just tested with 4.12.3 and problems is still there. This new test case should demonstrate problem very easily.

avatar image AndrewHurley Jun 17 '16 at 07:29 PM

So we are still having problems reproducing the issue on our end on the devices you mention. I will say there is a bug in place right now that looks very similar to your issue, and deals with Dithered LOD transitions showing a 'screen door like effect'. For tracking purposes the bug number is UE-31678.

If this is what you are seeing or referring to then it seems we have the bug in place. I have one question about the issue that might help us reproduce the bug. Are you able to see the issue within the Editor or is this strictly on the device itself?

Is there a way you could provide a test video of the issue occurring so we can see exactly what you are referring to?

Thank you,

Andrew Hurley

avatar image Kalle-H Jun 20 '16 at 08:39 AM

There is requested video. Captured from iPad pro using QuickTime Player. https://dl.dropboxusercontent.com/u/10960490/DitherBug.mov Bug is only visible on iOS. Using Metal preview on Mac Editor does not yield same bug.

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