Render To Texture & Trails Not Disappearing

I am trying to implement fireworks with point sprites and then drawing trails by rendering the fireworks to two different textures, and then drawing them back to each other every second time with alpha set to < 1, making each previous draw fade away more and more.

It looks nice and depending on what alpha I use when drawing the previously rendered texture the trails gets longer or shorter, as expected. However, on most devices (not all) the trails never disappear completely. They leave a very transparent but still noticable trail after they should have faded away.

This is what I do currently:

depending on a boolean called second I set the framebuffer to render to one of two textures called buff1 and buff2.

((GL11ExtensionPack)gl).glBindFramebufferOES(GL11ExtensionPack.GL_FRAMEBUFFER_OES, myFBO.get(0));
if (second){
    ((GL11ExtensionPack)gl).glFramebufferTexture2DOES(GL11ExtensionPack.GL_FRAMEBUFFER_OES, GL11ExtensionPack.GL_COLOR_ATTACHMENT0_OES, GL10.GL_TEXTURE_2D, buff2.texture.textureId, 0);
} else {
    ((GL11ExtensionPack)gl).glFramebufferTexture2DOES(GL11ExtensionPack.GL_FRAMEBUFFER_OES, GL11ExtensionPack.GL_COLOR_ATTACHMENT0_OES, GL10.GL_TEXTURE_2D, buff1.texture.textureId, 0);
}
gl.glViewport(0, 0, bufferSize, bufferSize);
gl.glClearColor(0, 0, 0, 0);
gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);
gl.glBlendFunc(GL10.GL_ONE, GL10.GL_ONE_MINUS_SRC_ALPHA);

Then I render the previously drawn texture on the current one, using a alpha value < 1. The buff object is just a helper drawing a quad with a texture. The draw function premultiplies the alpha to the other color channels.

// a = alpha, higher value = longer trails = more distinct leftovers
if (second){
    buff1.setColor(1, 1, 1, a);
    buff1.draw(gl);
    second = false;
else {
    buff2.setColor(1, 1, 1, a);
    buff2.draw(gl);
    second = true;
}

After that I do the drawing of the point sprite fireworks. Also using premultiplied alpha and gl.glBlendFunc(GL10.GL_ONE, GL10.GL_ONE_MINUS_SRC_ALPHA);

Finally I reset the frame buffer and have my new texture containing the old trails fading and the current fireworks on top drawn at 100% alpha. I then use this texture to draw to my scene together with all the other objects behind and in front of the fireworks texture.

As I said, almost everything is working, except for those almost transparent trails staying after the trails should have faded. The higher the alpha value drawing the previous buffer the more noticable the trail "leftovers" that stay forever.

I have been trying with different blending modes, pre-multiplied and not pre-multiplied alpha etc. But since this is an android opengl es 1.1 I think the blending should be GL_ONE and GL_ONE_MINUS_SRC_ALPHA since all bitmaps are premultiplied by default.

What am I doing wrong? Is there a better way to do this?

Thanks for any help, Anders

---

What am I doing wrong?

The way you sum downscaled (in their values) image is effectively a Power Series that exponetiate your alpha value. Such a power series will asympote to zero, but never reach it.

Is there a better way to do this?

Actually this is the right way to do it. You just forgot one last step: You want to have some threshold added, so that values below a certain alpha values get clamped to zero. Most easily this is obtained adding a GL_GREATER alpha test, with the threshold slightly above the alpha value of your trails you do want to remove.

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