Problem with alpha and polygons

**cronodragon** · 18-10-2005, 06:08 AM

I have this plane that is like a window with 4 glasses, and it's over a big plane. The big plane shows thru the window, but the cone is not rendered correctly. What could be happening, did I forget to set something?

These are the states I'm using:

Code:

  Device.SetRenderState&#40;D3DRS_AMBIENT, AmbientLight&#41;;

  Device.SetRenderState&#40;D3DRS_LIGHTING, iTrue&#41;;
  Device.SetRenderState&#40;D3DRS_DITHERENABLE, iTrue&#41;;
  Device.SetRenderState&#40;D3DRS_SPECULARENABLE, iTrue&#41;;

  Device.SetRenderState&#40;D3DRS_CULLMODE, D3DCULL_NONE&#41;;

  Device.SetRenderState&#40;D3DRS_ZENABLE, iTrue&#41;;

  Device.SetRenderState&#40;D3DRS_ALPHABLENDENABLE, iTrue&#41;;
  Device.SetRenderState&#40;D3DRS_SRCBLEND, D3DBLEND_SRCALPHA&#41;;
  Device.SetRenderState&#40;D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA&#41;;

  Device.SetTextureStageState&#40;0, D3DTSS_COLOROP, D3DTOP_MODULATE&#41;;
  Device.SetTextureStageState&#40;0, D3DTSS_COLORARG1, D3DTA_TEXTURE&#41;;
  Device.SetTextureStageState&#40;0, D3DTSS_COLORARG2, D3DTA_DIFFUSE&#41;;
  Device.SetTextureStageState&#40;0, D3DTSS_ALPHAOP, D3DTOP_SELECTARG1&#41;;
  Device.SetTextureStageState&#40;0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE&#41;;
  Device.SetSamplerState&#40;0, D3DSAMP_MINFILTER, D3DTEXF_LINEAR&#41;;
  Device.SetSamplerState&#40;0, D3DSAMP_MAGFILTER, D3DTEXF_LINEAR&#41;;
  Device.SetSamplerState&#40;0, D3DSAMP_MIPFILTER, D3DTEXF_LINEAR&#41;;

Regards!
-Marco

**Paulius** · 18-10-2005, 07:14 AM

You draw you?¢_Tre translucent thingy first, but it still writes to the Z buffer and you?¢_Tre cone doesn?¢_Tt pass the Z test. You should draw you?¢_Tre translucent after all solids, have Z writes disabled and preferably sort them back to front for a more correctly look.

**cronodragon** · 18-10-2005, 02:15 PM

Thanks a lot! It works sorting the objects.

What do you mean with "Z writes disabled"? I'm rather new to 3D programming.

LP · 18-10-2005, 02:35 PM

I'm not sure, but it seems that your cone is covered by transparent part of texture. If that is the case, then perhaps the best solution would simply to enable alpha-testing.

Check DirectX SDK:
http://msdn.microsoft.com/library/de...stingstate.asp
(This is what I don't like in PhpBB forums - since you can't put custom title to the link, it appears very long and ugly)

**cronodragon** · 18-10-2005, 02:40 PM

Thanks, I'll check.

And you can use the other URL syntax:

The nice link

Code:

&#91;url=http&#58;//msdn.microsoft.com/library/default.asp?url=/library/en-us/directx9_c/directx/graphics/programmingguide/gettingstarted/direct3ddevices/states/renderstates/alphatestingstate.asp&#93;The nice link&#91;/url&#93;

**Paulius** · 18-10-2005, 03:06 PM

Z testing tests if incoming fragments depth is lower than the one currently in the Z buffer (ensuring that stuff in the background doesn?¢_Tt get drawn on to of stuff in the foreground). Disabling Z writing mean?¢_Ts that nothing gets written to the depth buffer, this should be done for translucent polygons so that stuff that gets rendered later wouldn?¢_Tt be tested against the depth of transparent polygons.
I don?¢_~t use direct3D, but a quick googling gave me this
Device.SetRenderState(D3DRS_ZWRITEENABLE, false);
If you use alpha testing like lifepower suggested then you don?¢_Tt need to worry about sorting and a lot of commercial games do it like that, but this way you?¢_Tll be limited to complete transparency (no translucency) and there?¢_Tll be texture filtering artifacts on passing and non passing fragment edges.

**Clootie** · 18-10-2005, 08:34 PM

Originally Posted by Paulius

If you use alpha testing like lifepower suggested then ... there?¢_Tll be texture filtering artifacts on passing and non passing fragment edges.

Not really artifacts, but aliasing (if you set alpha ref to 1).

**marmin** · 26-10-2005, 04:05 AM

and Never set the Z near value to 0. this will give awkward distortions.

**cronodragon** · 29-10-2005, 09:21 PM

Ok, now I'm writing the code to solve the problem. It's easy to draw the solid triangles lists first and then the translucent ones, but now I have to order the translucent ones. I can think in several ways of doing that, but none that doesn't waste a lot of proccessing time. Any suggestion for a fast sorting method? Thanks in advance.

**Paulius** · 30-10-2005, 07:47 AM

There?¢_~s no perfect way to sort as this article explains. You shouldn?¢_Tt sort polygons but rather groups of them (translucent sphere could be drawn correctly from any angle if it?¢_Ts split into 8 groups). As long as these groups are small enough and do not intersect it should be passable to radix or quick sort them by groups centers distance to camera.