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Feature Request Support supplying lightmaps as "Texture" instances, rather than "Texture2D"

Discussion in 'Global Illumination' started by TheZombieKiller, Oct 23, 2020.

  1. TheZombieKiller

    TheZombieKiller

    Joined:
    Feb 8, 2013
    Posts:
    119
    An in-development project I'm a part of necessitates dynamic manipulation of lightmap textures. Currently this is being handled by using CustomRenderTextures and setting the unity_Lightmap shader property to them on individual renderers. This however is unwieldy to use, and complicates operations such as raycasting into the lightmap.

    I managed to throw together a (very dangerous) prototype that can forcefully store any texture type in the lightmaps array, and with the exception of erroring or crashing when the engine tries to clean up the texture instance it actually works perfectly fine (the textures even preview correctly in the lightmap inspector!).

    Code (CSharp):
    1. using System;
    2. using System.Collections.Specialized;
    3. using System.Runtime.InteropServices;
    4. using System.Runtime.CompilerServices;
    5. using UnityEngine;
    6. using Object = UnityEngine.Object;
    7.  
    8. class ConvertLightmapsToRenderTextures
    9. {
    10.     [StructLayout(LayoutKind.Sequential)]
    11.     class UnityObject
    12.     {
    13.         public unsafe NativeUnityObject* CachedPtr;
    14.  
    15.         // Partial version of the internal C++ Unity object layout.
    16.         // Due to being partial, this is only safe to use by reference!
    17.         public unsafe struct NativeUnityObject
    18.         {
    19.             static readonly BitVector32.Section s_MemLabelIdentifier = BitVector32.CreateSection(1 << 11);
    20.  
    21.             static readonly BitVector32.Section s_TemporaryFlags = BitVector32.CreateSection(1 << 0, s_MemLabelIdentifier);
    22.  
    23.             static readonly BitVector32.Section s_HideFlags = BitVector32.CreateSection(1 << 6, s_TemporaryFlags);
    24.  
    25.             static readonly BitVector32.Section s_IsPersistent = BitVector32.CreateSection(1 << 0, s_HideFlags);
    26.  
    27.             static readonly BitVector32.Section s_CachedTypeIndex = BitVector32.CreateSection(1 << 10, s_IsPersistent);
    28.  
    29.             public IntPtr* MethodTable;
    30.  
    31.             public int InstanceID;
    32.  
    33.             public BitVector32 BitFields;
    34.  
    35.             public RuntimeTypeIndex CachedTypeIndex
    36.             {
    37.                 get => (RuntimeTypeIndex)BitFields[s_CachedTypeIndex];
    38.                 set => BitFields[s_CachedTypeIndex] = (int)value;
    39.             }
    40.         }
    41.     }
    42.  
    43.     enum RuntimeTypeIndex : uint
    44.     {
    45.         WebCamTexture       = 239,
    46.         RenderTexture       = 244,
    47.         CustomRenderTexture = 245,
    48.         SparseTexture       = 246,
    49.         Texture2D           = 247,
    50.         Cubemap             = 248,
    51.         Texture2DArray      = 249,
    52.         Texture3D           = 250,
    53.     }
    54.  
    55.     static unsafe void ChangeTypeIndex(Object obj, RuntimeTypeIndex index)
    56.     {
    57.         Unsafe.As<UnityObject>(obj).CachedPtr->CachedTypeIndex = index;
    58.     }
    59.  
    60.     [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.AfterSceneLoad)]
    61.     static void Execute()
    62.     {
    63.         var lightmaps = LightmapSettings.lightmaps;
    64.        
    65.         for (int i = 0; i < lightmaps.Length; i++)
    66.         {
    67.             // Create a RenderTexture clone of the lightmap, as an example.
    68.             var lightmap      = lightmaps[i].lightmapColor;
    69.             var renderTexture = new RenderTexture(lightmap.width, lightmap.height, 0);
    70.             Graphics.Blit(lightmap, renderTexture);
    71.  
    72.             // Trick engine into thinking that the RenderTexture is a Texture2D.
    73.             ChangeTypeIndex(renderTexture, RuntimeTypeIndex.Texture2D);
    74.             lightmaps[i].lightmapColor = Unsafe.As<Texture2D>(renderTexture);
    75.         }
    76.  
    77.         // Ensure original type indices are restored before the game quits, this is presumably
    78.         // because the engine calls the Texture2D cleanup function on each lightmap despite
    79.         // them not actually being Texture2D objects. It's also possible that this is the result
    80.         // of C++ virtual call optimization if we assume the engine stores them as Texture2D fields.
    81.         Application.quitting += () =>
    82.         {
    83.             for (int i = 0; i < lightmaps.Length; i++)
    84.             {
    85.                 var lightmap = lightmaps[i].lightmapColor;
    86.  
    87.                 // If this lightmap is not actually a Texture2D, then
    88.                 // its type index must be restored to prevent a crash.
    89.                 switch (lightmap.GetType())
    90.                 {
    91.                 case Type type when type == typeof(WebCamTexture):
    92.                     ChangeTypeIndex(lightmap, RuntimeTypeIndex.WebCamTexture);
    93.                     break;
    94.                 case Type type when type == typeof(RenderTexture):
    95.                     ChangeTypeIndex(lightmap, RuntimeTypeIndex.RenderTexture);
    96.                     break;
    97.                 case Type type when type == typeof(CustomRenderTexture):
    98.                     ChangeTypeIndex(lightmap, RuntimeTypeIndex.CustomRenderTexture);
    99.                     break;
    100.                 case Type type when type == typeof(SparseTexture):
    101.                     ChangeTypeIndex(lightmap, RuntimeTypeIndex.SparseTexture);
    102.                     break;
    103.                 case Type type when type == typeof(Cubemap):
    104.                     ChangeTypeIndex(lightmap, RuntimeTypeIndex.Cubemap);
    105.                     break;
    106.                 case Type type when type == typeof(Texture2DArray):
    107.                     ChangeTypeIndex(lightmap, RuntimeTypeIndex.Texture2DArray);
    108.                     break;
    109.                 case Type type when type == typeof(Texture3D):
    110.                     ChangeTypeIndex(lightmap, RuntimeTypeIndex.Texture3D);
    111.                     break;
    112.                 }
    113.             }
    114.         };
    115.  
    116.         LightmapSettings.lightmaps = lightmaps;
    117.     }
    118. }
    (I'm almost hoping that the sheer hackiness of this workaround will convince somebody at Unity to take a look at implementing this properly in the engine :D)

    The problem is that I absolutely do not want to rely on a hack like this, as it's reliant on implementation details within the engine (and would break if, for example, the internal layout of the Object class changed).

    Would it be possible for this functionality to be added to the engine within the Unity 2021 (or even 2020) release cycle? Perhaps via the addition of APIs along the lines of:

    Code (CSharp):
    1. public sealed partial class LightmapData
    2. {
    3.     public Texture polymorphicColor { get; set; }
    4.     public Texture polymorphicDir { get; set; }
    5.     public Texture polymorphicShadowMask { get; set; }
    6. }
    The Texture2D property variants could either throw or return null if another texture kind is used.
     
    Lars-Steenhoff and Kolyasisan like this.
  2. LeonhardP

    LeonhardP

    Unity Technologies

    Joined:
    Jul 4, 2016
    Posts:
    2,357
    Moving this to the GI forum.
     
    TheZombieKiller likes this.
  3. TheZombieKiller

    TheZombieKiller

    Joined:
    Feb 8, 2013
    Posts:
    119
    Correct, more specifically a CustomRenderTexture so I can make changes to the lightmap at runtime without paying texture copy costs.
     
  4. rasmusn

    rasmusn

    Unity Technologies

    Joined:
    Nov 23, 2017
    Posts:
    86
    Hi TheZombieKiller! Let me start by saying that that is one creative hack. That must have been interesting to do. That being said, I agree that it is (too) dangerous.

    Can you elaborate on what you mean by this?
    As a general rule we never change APIs in Unity versions that have entered beta. This is to reduce likelihood of introducing instabilities. So this rules out adding this to 2020.2 and earlier.

    For future versions I agree that your proposal could theoretically work. However, the major downside that I see is that it allows all textures of type
    Texture
    in
    LightmapData
    which means that people would be able to assign for example Cubemap and Texture3D without getting compilation errors. Obviously, this is not good since the lightmap shader code expects flat 2D textures. This issue could be solved by restructuring the inheritance hierarchy of
    Texture
    types but that would be a major change that would require a significant refactoring effort and possibly break/change a lot of APIs.

    Of course, you could argue that people could just keep using the old "safe" LightmapData.lightmapColor and we could encourage only advanced users to use the polymorphic properties that you suggest (e.g.
    public Texture polymorphicColor
    ). However, my gut feeling is that would cause more confusion and misuse than benefits in this case. We try out best to write the best docs possible but our experience is that many users still don't read them. Compare this to how rare this use-case is (to my knowledge anyway) and that arguably means that this is not a worthwhile change.

    We will keep your proposal in mind though and reevaluate as we learn more.
     
    Last edited: Nov 5, 2020
    TheZombieKiller likes this.
  5. TheZombieKiller

    TheZombieKiller

    Joined:
    Feb 8, 2013
    Posts:
    119
    As I need to bypass the lightmaps array by using MaterialPropertyBlocks to override unity_Lightmap currently, the lightmaps array needs to be filled with dummy data which the Renderer components reference. Any third party code that deals with lightmaps also needs to be tweaked to account for this system (granted, if polymorphic texture properties were added they'd need to be tweaked regardless, just on a much smaller scale).

    That's completely understandable and I have no complaints there. Asking for this to arrive in 2020.2 is a huge stretch and I was expecting 2021.x to be a much more realistic request from the get-go.

    Yep, that's a very good point. I'd be perfectly fine with the properties just being typed as RenderTexture instead of Texture, since that'd be fine for my use case as I'm only using CustomRenderTexture instances.
     
    Last edited: Nov 5, 2020
    rasmusn likes this.
  6. rasmusn

    rasmusn

    Unity Technologies

    Joined:
    Nov 23, 2017
    Posts:
    86
    I didn't consider that solution, thanks for pointing that out.

    I assume that this is what you propose...?
    Code (CSharp):
    1.   public class LightmapData
    2.   {
    3.       public Texture2D color { get; set; }
    4.       public Texture2D dir { get; set; }
    5.       public Texture2D shadowMask { get; set; }
    6.  
    7.       public RenderTexture rtColor { get; set; }
    8.       public RenderTexture rtDir { get; set; }
    9.       public RenderTexture rtDhadowMask { get; set; }
    10.  
    11.       [...]
    12.   }
    13.  
    Let me know if this is not what you mean.

    I agree that that could work and also that it solves the problem of allowing too many types unsuitable textures (Cubemaps etc.).

    To iterate further on this design proposal, allow me to once again play the devil's advocate.
    1. Isn't it a problem that you have two properties that refer to the same underlying object? For example LightmapData#color and LightmapData#rtColor would refer to the same object. When you change one of them, the other also changes. To my knowledge this is a convention that haven't been widely used in Unity before, so I fear that this will lead to a lot of confusion for many users. Of course, you could solve this by letting #color and #rtColor point to two different objects/properties and then have a policy for which one to pick for shading. Hhowever, that would introduce ambiguity into the design: Why is one chosen over the other? Both can be set at the same time even though both cannot meaningfully be used at the same time. Ideally, we'd like to avoid this ambiguity/complexity by disallowing it in the design of the structure/type itself.
    2. I don't think that it is good practice to have two properties with names #color and #rtColor in this case. This naming suggest that one is a generalization of the other, but this is not the case here since Texture2D and RenderTexture have a "type sibling" relationship rather than a super/sub type relationship. Of course, we could fix this by renmaing the properties so that we have #tex2DColor and #rtColor but that would involve a breaking API change which is not exactly ideal either.
    I think both of these issues could be solved by changing the Texture type hierarchy to have an abstract type that denotes all textures that are 2-dimensional (such as the current Texture2D). However, that would require a lot of refactoring in Unity generally, and probably cause breaking API changes.

    I want to make it clear that I am not trying to obstruct your proposal. I just want us to be cognizant about any possible repercussions involved with the proposed design change. In my experience, seemingly subtle and innocent-looking changes tend to explode into complexity in unexpected ways and therefore it is usually worthwhile to play the devil's advocate game up front :).
     
    TheZombieKiller likes this.
  7. TheZombieKiller

    TheZombieKiller

    Joined:
    Feb 8, 2013
    Posts:
    119
    Correct, that's exactly it.

    I agree, this is a tough problem to think of a solution for. In the OP I suggested that the Texture2D properties could throw or return null if the other properties are set, but that wouldn't alleviate confusion in the API (rather, it could even make it worse if a third party library changed the properties without your knowledge).

    This would definitely be ideal in the long term, but a change of that scale does seem excessive to enable a use case as niche as this. That said, it'd have some pretty nice benefits for other projects I'm sure.

    Given that Unity 2020 has already begun introducing significant changes to the lightmapping APIs, I wonder if a better alternative in the short term could be to deprecate the LightmapData class in Unity 2021 and replace it with something else. The problem is figuring out what that "something else" should even look like.

    It's no problem at all! I never had the impression you were intending to obstruct anything. Rather I very much appreciate the responses, the design in the OP leaves much to be desired so it's only natural to want to explore alternative routes.
     
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  8. rasmusn

    rasmusn

    Unity Technologies

    Joined:
    Nov 23, 2017
    Posts:
    86
    I could be wrong, but I think the vast majority of our users would prefer the LightmapData class to not change (simply because it would cause their projects to break, or cause them to receive deprecation warnings). Even if I am wrong in this, then I am sure the majority would not be able to agree on what the replacement API should look like (as you also suggest).

    You have provided some interesting arguments and use-cases in this thread and I/we are grateful for you taking the time. But I think we need to hear similar requests from more users and/or we need to identify some large potential benefits before we can justify a change like this.

    Thanks for your feedback, I'll definitely keep it in mind going forward.
     
    TheZombieKiller likes this.
  9. TheZombieKiller

    TheZombieKiller

    Joined:
    Feb 8, 2013
    Posts:
    119
    That's completely understandable. The project in question that would benefit from the feature likely wouldn't be able to jump to Unity 2021 before release regardless, so right now this is merely an attempt to explore potential optimizations for lower end computers in a patch (as the texture copies aren't much of an issue for higher end devices.)

    I really appreciate you taking the time to have this discussion and look forward to any potential news on this in the future!
     
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