OpenGL to DirectX translation - alpha blending

Question

I'm trying to translate an OpenGL renderer into DirectX9. It mostly seems to work, but the two don't seem to agree on the settings for alpha blending. In OpenGL, I'm using:

glDepthFunc(GL_LEQUAL);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);

and never actually setting the GL_DEST_ALPHA, so it's whatever the default is. This works fine. Translating to DirectX, I get:

device->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_INVSRCALPHA);

which should do about the same thing, but totally doesn't. The closest I can get is:

device->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
device->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_DESTALPHA);

which is almost right, but if the geometry overlaps itself, the alpha in front overrides the alpha in back, and makes the more distant faces invisible. For the record, the other potentially related render states I've got going on are:

device->SetRenderState(D3DRS_LIGHTING, FALSE);
device->SetRenderState(D3DRS_ZENABLE, TRUE);
device->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
device->SetTextureStageState(0, D3DTSS_ALPHAOP, D3DTOP_MODULATE);

At this point, I feel like I'm just changing states at random to see which combination gives the best results, but nothing is working as well as it did in OpenGL. Not sure what I'm missing here...

Answer 1

The alpha blending itself is performed correctly. Otherwise, every particle would look strange. The reason why some parts of some particles are not drawn, is that they are behind the transparent parts of some other particles.

To solve this problem you have two options:

1. Turn off ZWriteEnable for the particles. With that, every object drawn after the particle will be in front of it. This could lead to problems, if you have objects that should actually behind the particles and are drawn afterwards.

2. Enable alpha testing for the particles. Alpha testing is a technique to remove transparent pixels (given a certain threshold) from the target. This includes the ZBuffer.

Btw. when rendering transparent objects, it is almost always necessary to sort the objects to solve ZBuffer issues. The above solutions work for some special cases.