For a long time - like a very long time (right back to the early GZDoom days I think) - sometimes, on some maps, perhaps in just some places in some maps, when you look straight up you can see a black shape near the top of the skybox.
Example (on Imgur - so Brits won't be able to see it):
Spoiler:
Forgive the vignette effect, I was using a zoom mod to.. well, zoom in on it.
It can happen when using GLDEFs 6-sided skyboxes, constructed map skyboxes and even traditional texture skyboxes.
When I've had a map that does it, I've tried moving architechture around, throwing junk lines into the map (kind of like how Raven did when trying to get polyobjects to render properly with Vanilla Hexen), moving the entire map to a different part of the available space, and so on. I've also gone through as many menu options as I could think of too.
The problem is, I'm just stabbing in the dark because I don't know what causes the problem, so I don't know how to fix it.
So, does anyone know what causes this?
Any recommendations for being able to eliminate/minimise it when it happens?
The only thing I have noticed that might provide a clue - if you are using a map-built skybox with a skybox camera, and haven’t disabled ambient occlusion, you can see the ambient occlusion effect at the corners of lines in the skybox, but when you move to a position where the shape is visible, the ambient occlusion effect in the skybox disappears. If you move so that the shape goes, the AO effect returns. (I have checked several of the SSAO cvars and they had no impact on the shape.)
So far, the only thing that has "worked" is a sky that has a lot of black on it so the shape is less visible - but that's obviously not a suitable solution in many cases.
Oh! Good suggestion. I don't know why, but I hadn't tested that. I use Vulkan by default these days - and that shows the shape. If I switch to OpenGL the shape is not there.
And, yes, I remember that white shape thing being discussed. (As I mentioned, I also remember discussing black shapes in the sky before too - before even the white shape thing).
As an aside, I'm surprised at the frame-rate drop I get with OpenGL versus Vulkan - which is, obviously, unrelated to the shape.
I am surprised that it shows up when Vulkan is selected, I would have expected OpenGL to cause the artifact.
I made a few maps (just single sector) with various kothic skies and also just single colour skies to see what happens on my end, without an artifact showing up.
Would you mind providing a link to the culprit?
Have you tried vkDoom?
I'd love to link to the culprit but I'm having real difficulty in getting something that shows the problem in an easily portable format. The map, as it is, requires a lot of resources to run (too much to upload - it's actually a modified version of a Blade of Agony map I have been trying to get running on the latest GZDoom, just for my own amusement). If I start trimming stuff out, I also seem to lose the ability to show the effect easily. [Edit: I just noticed that if I load the map up in unaltered in GZDoom with the texture resources missing, I can get the sky-shape, but if I use UDB to put default textures on places with unknown texture names, the shape goes away.]
[edit]Ack! forget the stuff in the spoiler. For some reason, deleting the sector I mentioned used to reliably change whether the shape appeared or not, but now I can't replicate that. I haven't restarted my computer or anything - just the symptoms seem to have changed. It still happens, just less reliably.
Spoiler:
In fact, I've tracked things down to one little sector that, if I remove it, the black shape goes away. There is *NOTHING* particularly interesting about this sector. In fact, if I move it around (even to a totally different area of the map), change the direction of its lines, remove any properties that it has, remove the tag that it has, remove any items in it, change its heights, etc. etc. In fact, even if delete it and recreate it as a totally new sector in a slightly different place, with different properties, none of that stops the shape appearing. As soon as I delete it entirely, the shape in the sky goes. If I undo the delete (or just draw a new sector) the problem returns. If I add other sectors nearby, no difference. And, like I said, I can delete it and draw a totally new one - still the same.
And it's really weird. This is a big, complicated map that I have added to and changed a lot before, during and after noticing the shape. But this one little innocuous sector somehow seems to be responsible. I only noticed that it seems to be related because I had the player start spot in it, but when I moved the start spot to a location where the shape could be seen easily for demonstration purposes, I decided to delete the sector, and that's when I noticed that it made a difference. Note: the player start spot (or anything) does not have to be in the sector, and I have moved the start spot well away from the sector but, still, if I delete the sector, the shape goes, if I keep it, the shape is there.
I'm also struggling to get it to appear across different GZDoom versions. Some builds show it, some do not. I haven't been able to get it to show in the latest build yet, but I'm not sure if that means it's something that's been fixed, or if it's just that the delicate balance that makes this happen has been altered slightly.
If I can get something uploadable that shows the problem reliably, I will post a link to it, but I've been trying for over an hour this morning already.
I know, as the saying goes, "you can't debug a video" but I thought I'd throw this up just so you can see it in motion - and how specific it can be to position.
I did one with the default Doom SKY1 and then with a map-constructed skybox (that starts at about 17 seconds) which shows that there is a flicker of the ambient occlusion as the shape appears/disappears. Initially I reported that the AO would go entirely but I think that's because I was using a skybox with all-white textures on its sides and 255 brightness - which might just have been so bright that it masked the effect.
After looking at your video several times I tried a few scenarios with different skies and complex sectors. I even replayed some of the Boa maps. But nothing like that showed up in the sky.
I wonder, that is a fairly irregular shape with a 90° angle, could that be superimposed onto the sky somehow?
The way the AO changes with the white skybox, it seems like something's going wrong with the sky portal setup when you move around. The specific way the AO changes makes it look like (to me, anyway) the skybox is suddenly "popping out" of the sky portal and being rendered alongside the normal geometry.
