ParticleEmitter does not work correctly with RenderState.BlendMode.Off.

[capdevon]

I opened this issue to keep track of the forum discussion:
https://hub.jmonkeyengine.org/t/problem-setting-renderstate-blendmode-off-on-particleemitter/45215

Changing the default "BlendMode" of the "particle material" from Off to another and then back to Off, something seems to break.
Also happens when setting it directly to Off without setting it to something else first.

Here is the test case:

package com.test.main;

import com.jme3.app.SimpleApplication;
import com.jme3.effect.ParticleEmitter;
import com.jme3.effect.ParticleMesh;
import com.jme3.light.DirectionalLight;
import com.jme3.material.Material;
import com.jme3.material.RenderState.BlendMode;
import com.jme3.math.ColorRGBA;
import com.jme3.math.FastMath;
import com.jme3.math.Vector3f;
import com.jme3.renderer.queue.RenderQueue;
import com.jme3.system.AppSettings;

/**
 * 
 * @author capdevon
 */
public class Test_BlendModeIssue extends SimpleApplication {

	private ParticleEmitter emit;
	private float angle = 0;
	private float timer;
	private float refreshRate = 1f;
	private int mIndex = 0;
	private BlendMode[] modes = { BlendMode.Off, BlendMode.Alpha };

	/**
	 * 
	 * @param args
	 */
	public static void main(String[] args) {
		AppSettings settings = new AppSettings(true);
		settings.setResolution(800, 600);

		Test_BlendModeIssue app = new Test_BlendModeIssue();
		app.setShowSettings(false);
		app.setPauseOnLostFocus(false);
		app.setSettings(settings);
		app.start();
	}

	@Override
	public void simpleInitApp() {
		flyCam.setDragToRotate(true);
		flyCam.setMoveSpeed(20);
		viewPort.setBackgroundColor(ColorRGBA.DarkGray);

		createParticleEmitter();
	}

	@Override
	public void simpleUpdate(float tpf) {
		timer += tpf;
		if (timer > refreshRate) {
			timer = 0;
			mIndex = (mIndex + 1) % modes.length;
			BlendMode bm = modes[mIndex];
			fpsText.setText("BlendMode: " + bm.toString());
			emit.getMaterial().getAdditionalRenderState().setBlendMode(bm);
		}

		angle += tpf;
		angle %= FastMath.TWO_PI;
		float x = FastMath.cos(angle) * 2;
		float y = FastMath.sin(angle) * 2;
		emit.setLocalTranslation(x, 0, y);
	}

	private void createParticleEmitter() {
		emit = new ParticleEmitter("Emitter", ParticleMesh.Type.Triangle, 300);
		Material mat = new Material(assetManager, "Common/MatDefs/Misc/Particle.j3md");
		mat.setTexture("Texture", assetManager.loadTexture("Effects/Smoke/Smoke.png"));
		emit.setGravity(0, 0, 0);
		emit.getParticleInfluencer().setVelocityVariation(1);
		emit.getParticleInfluencer().setInitialVelocity(new Vector3f(0, .5f, 0));
		emit.setLowLife(1);
		emit.setHighLife(1);
		emit.setImagesX(15);
		emit.setMaterial(mat);
		rootNode.attachChild(emit);

		System.out.println("Default BlendMode: " + mat.getAdditionalRenderState().getBlendMode());
	}
}

[stephengold]

It's true that the initial setting of the emitter material's additional blend mode is Off, but that mode isn't applied when particles are rendered---not initially, anyhow.

I added a print statement in GLRenderer.applyRenderState() like so:

        // Update the blend equations and factors only on a mode change for all the other (common) blend modes.
        } else if (state.getBlendMode() != context.blendMode) {
            changeBlendMode(state.getBlendMode());
            System.out.println("applied mode = " + state.getBlendMode());

Here's what I observed:

1. Initially, the applied mode alternated between AlphaAdditive and Alpha. The additional mode (Off) was never applied. (Presumably the AlphaAdditive came from "Particle.j3md".)
2. After the additional mode was set to Alpha, the applied mode became steadily Alpha.
3. After the additional mode was set back to Off, the applied mode alternated between Off and Alpha. That was when the app seemed to "break" visually.

RenderState.setBlendMode() is more complex than its name implies:

public void setBlendMode(BlendMode blendMode) {
        applyBlendMode = true;
        this.blendMode = blendMode;
        cachedHashCode = -1;
    }

Before setBlendMode() is invoked from simpleUpdate(), applyBlendMode is false, so the value of the blendMode field is irrelevant. Once you've set the mode this way, there's no mechanism to reset the RenderState to a pristine state. It will keep overriding the J3MD until the Material is garbage collected.

I don't see an actual bug here, but perhaps Wyatt's use case could benefit from adding a RenderState.reset() method to the Engine.

[stephengold]

Not seeing any immediate need for a reset() method, so closing this issue.