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    Hdri backdrop and Gi is broken?

    Can someone pls explain this to me - I drop an hdri backdrop in my scene and a ppv to control some aspects of the raytracing. I see no Global illumination whatever I do so I go and insert a directional light again no GI visible...when I hide the hdri backdrop my Gi is visible and controllable? Is this broken? or is there something else I'm missing..


    #2
    For anyone facing the same problem I have found the solution; If you go into the HDRi backdrop geometry and search for raytrace,,,,turn off the visible in raytracing...!

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      #3
      Hi ADemetriades ,

      Great to hear it and thanks for sharing the solution!

      However, I think if you put it invisible for RT, it won't be visible on reflections, in a mirror, for example. Correct me if I'm wrong.

      In fact, I noticed something strange with brackdrops and HDRi "invisible" background when using unlit materials, as I published here: https://forums.unrealengine.com/deve...-broken-things but no one was opining. I think there is a problem in 4.23 with this kind of background, like if they would be partially casting shadows.

      Regards!
      RTX RayTracing easy global settings tweaker

      Advanced HDRi background tool + 64 HDR images

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        #4
        Originally posted by ADemetriades View Post
        For anyone facing the same problem I have found the solution; If you go into the HDRi backdrop geometry and search for raytrace,,,,turn off the visible in raytracing...!
        Keep in mind that if you do that, you will lose visibility of the HDRI backdrop for reflections too, and you will get low res rasterized reflections instead:
        https://forums.unrealengine.com/deve...99#post1658799
        https://www.artstation.com/artist/rawalanche

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          #5
          yes it reverts to ssreflections..the best is to implement ssgi and leave rt reflections on if you need good reflections or use RT GI and ssreflections...so basically there is no gi in the hdri backdrop....

          However in the docs it states clearly "Also, keep in mind, that additional Ray Tracing features can be enabled and set in a Post Process Volume, when adding one to your scene. You can control Ambient Occlusion, Global Illumination, Translucency, and Reflections."
          So does this mean this is a bug...?

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            #6
            Originally posted by ADemetriades View Post
            yes it reverts to ssreflections..the best is to implement ssgi and leave rt reflections on if you need good reflections or use RT GI and ssreflections...so basically there is no gi in the hdri backdrop....

            However in the docs it states clearly "Also, keep in mind, that additional Ray Tracing features can be enabled and set in a Post Process Volume, when adding one to your scene. You can control Ambient Occlusion, Global Illumination, Translucency, and Reflections."
            So does this mean this is a bug...?
            It's mentioned in my thread too. Basically there's an issue of geometry behind the distance threshold of skylight still being considered for ray tracing. Not sure if that would technically be a bug, it's more like a design flaw which goes much deeper. The huge problem unreal has that it has no way to specify some "Environment/World" shader like traditional DCC apps have. Basically a full dome light source of infinite distance.

            UE4 can do it to some degree but it doesn't work. First of all, because the texture slot of the Skylight specified cubemaps accepts only cubemaps, not actual textures, such as EXR. Second of all because once Skylight uses Specified Cubemap, it does not display it also for eye rays/direct visibility shader, so it ends up useless in most cases. Now that UE4 is adding ray tracing, these design flaws are becoming far more problematic.
            https://www.artstation.com/artist/rawalanche

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              #7
              I agree that the hdri backdrop is only useful if you want to use for a quick way to display something but as a real workhorse it has its limitations...in fact when I toggled the ray tracing on/off I got some very weird and unexpected results...on one occasion even though I had GI running when off and I changed shader color the gi displayed h the color of the previous shader! On another occasion I got gi spilling unnaturally on the ground...like everything turned red from the shader....

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                #8
                I think that the implementation is the worst in imho. For example if you start using command lines and then go to ppv the whole system breaks down as there is no communication b/w the two and I have no control of the rt even after I go back to cmd line...its not the cost of using that bothers me because you can use some features and turn off others but its the way its implemented in engine that creates the biggest issues for using it...its not a smooth process!

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                  #9
                  I agree ADemetriades !

                  Maybe it happens with any other non-RT option too? (To enable it through cmd and stop being able to enable/disable it from the PPV anymore).

                  I have a rule: If I can modify a setting from a specific place, I will only use that place to modify that setting. Not very versatile but secure, at least
                  RTX RayTracing easy global settings tweaker

                  Advanced HDRi background tool + 64 HDR images

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                    #10
                    I do the same but it sucks!

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                      #11
                      Hi ADemetriades , with UE4.23.1 do you experience the same issue? For some reason, today I have tried a similar situation and it's like it works fine now.
                      RTX RayTracing easy global settings tweaker

                      Advanced HDRi background tool + 64 HDR images

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                        #12
                        hi
                        i had the same problem, here is a tip to solve it: locate the hdri backdrope material and change its blend mode to masked ans add the nodes as showed in the image.
                        Click image for larger version

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                          #13
                          Originally posted by kaddouri mohamed View Post
                          hi
                          i had the same problem, here is a tip to solve it: locate the hdri backdrope material and change its blend mode to masked ans add the nodes as showed in the image.
                          Hi kaddouri mohamed !

                          Thanks, but I have made a try and I think it doesn't work. In fact, if you are sending a value of 1 in both pins, it's the same as being it Opaque in the Blend Mode. In my try, however, I though your change was working, as I noticed some changes in the illumination but, after setting it again as it was by default, the illumination was the same. After this, I discovered that, if you maintain it by default and, after placing the Backdrop blueprint, you move it around the scene, it will "update" its illumination and will start to globally illuminate, however (one more time), the global illumination thrown it's very strange, even it changes as you move the backdrop around the scene, and it's like sending light rays from the bottom. Could you confirm it and upload some images of your results, if you consider that they are correct, please? (You can make some good tests with a similar scene as Ademetriades posted in the first post).

                          Best regards!
                          RTX RayTracing easy global settings tweaker

                          Advanced HDRi background tool + 64 HDR images

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                            #14
                            Miguel1900 it turn out you only need to set the HDRI material blending mode to masked and it worked for me, this is my video using HDRIbackdrop you can see the reflection and GI working fine.

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                              #15
                              Miguel1900 the video:https://www.youtube.com/watch?v=1ZvGJTWdC4k

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