Transparency issues with 3d particles and 3d models, libgdx

Question

I got some strange issues with transparency and 3d particles. A short vid to illustrate: https://youtu.be/ZHKI1X3MjhY As you can see I have a 3d particle effect, fire burning. Inside it is a 3 model with no alpha blending and it shows just fine. then in the far distance there is a small skeleton (with blending and alphatest turned on) and it also shows just fine through the fire. Then I turn camera and look at the warrior skeleton and it just disappear and instead you see what is behind him. I turn camera again and the mage skeleton also vanishes, but you can see the trees a bit further away just fine and they have the exact same settings for blending and alpha test. If I move the character say 20 yards away it also starts showing through the fire effect.

So it seems to have something to do with distance from the 3d particle effect...

The 3d particle batch is an extended BillboardParticleBatch like this:

protected Renderable allocRenderable(){

         BlendingAttribute ba=new BlendingAttribute(GL20.GL_SRC_ALPHA, GL20.GL_ONE,1f);

         Renderable r = super.allocRenderable();
         r.material = new Material(   ba,
//               new DepthTestAttribute(GL20.GL_LEQUAL, 0.0f, 0.5f, true),
//                r.material.set(new FloatAttribute(FloatAttribute.AlphaTest, 0.0f),
               TextureAttribute.createDiffuse(texture));

         return r;
   }

All the characters and the trees are created with following attributes:

if (alpha) {
         FloatAttribute floatAttribute = new FloatAttribute(FloatAttribute.AlphaTest, 0.5f);
         BlendingAttribute blendingAttribute = new BlendingAttribute(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA, 1f);
         for (int i = 0; i < bulletEntity.modelInstance.materials.size; i++){
               bulletEntity.modelInstance.materials.get(i).set(blendingAttribute);
               bulletEntity.modelInstance.materials.get(i).set(floatAttribute);
         }

      }

The models are drawn first then the particles, I tried changing order but no difference. I have tried a lot of different setups for alphatest, depthtest and blendingattribute but can not find anything that works.

EDIT I removed the Blending attribute from the 3d-models and now it looks as it should regarding the particle effect. However I need most materials on my character models to have blending set..

Anyone got any clue why this is happening when I enable blending?

I also tried to use the BillboardParticleBatch without extending it in case I had done something there but the effect then is even worse. All models with blending enabled appear in-front of the particle effect even though they stand behind it.

Answer 1

ModelBatch sorts your render calls (check this link, really, it is a must read), to avoid incorrect behavior (as you're experiencing). The actual sorting/rendering happens at the call to ModelBatch#end. By default it uses the DefaultRenderableSorter, which is a default implementation. Of course, because that implementation isn't aware of your scene, it might not fit exactly your needs.

The DefaultRenderableSorter tries to guess the location of each model based on their transformation matrix. Based on that location and the camera's location it will sort them so that:

• First all opaque objects are rendered from front to back (because whatever is behind an opaque object isn't visible anyway, so that reduces unneeded calls to the fragment shader).
• Secondly all transparent objects are rendered from back to front (because as soon as a transparent object is rendered then everything that is rendered after that and is behind it, will not be visible).

To decide whether an object is transparent, the BlendingAttribute#blended member is used. (So you could, if you really wanted to, set that member to false to force it to be treated (sorted) as if it was opaque)

So, the order in which you call ModelBatch#render is not necessarily the order in which they are actually executed. If you want to force to render whatever you've added to the batch in between, then call the ModelBatch#flush(). Of course, doing this a lot defeats some of the purpose of ModelBatch in the first place.

Instead you could implement your own RenderableSorter which has more knowledge about your scene and can therefor do a better job sorting than the default implementation. (however if flush() works for you and there's no other issue, then just flush might be the easiest solution for you).

That said, there a various other solutions you could try as well. E.g. the regions of the particles are fully transparent, so the fragment shader might as well discard those all together. Try adding FloatAttribute.AlphaTest with a value of 0.5f to the particles. If that messes with your blending then gradually reduce the value to e.g. 0.05f.

Also, you could add a DepthTestAttribute with depthMask set to false (new DepthTestAttribute(false)). This will prevent the particles from writing to the depth buffer. (but also might cause other things to show in front of the particles).