Seperate Alpha Blending

[SirShane]

Hi guys,

I'm currently working on porting a buffered GUI system over to Ogre, and I have a problem regarding separate alpha channel blending.

Direct3D (And I assume OpenGL as well) supports doing alpha blending on both the color channels and the alpha channels as separate operations. Normally you only need to do blending on the color channel, but when rendering to a render target with an alpha channel on it, you can have issues where the alpha channel is not blended properly and gets replaced.

This is an issue for my GUI because it renders alpha blended polygons into a texture which has an alpha channel. Because Ogre does not seem to support separate alpha channel blending, it produces holes in the texture where the polygons have transparency.

Are there any plans to support separate alpha channel blending? If not, I can probably do it myself and provide a patch. I just want to make sure I'm not going to be re-inventing the wheel.

Thanks.

[xavier]

alpha_op_ex?

[SirShane]

I'm not sure exactly what alpha_op_ex is. I don't think it's what i'm looking for. The specific functionality i'm seeking is provided in OpenGL with the function glBlendFuncSeparate and in Direct3D with the state mode D3DRS_SEPARATEALPHABLENDENABLE. It tells the device to blend the alpha and color channels seperately.

[Kencho]

I'm not sure exactly what alpha_op_ex is. I don't think it's what i'm looking for.

How can you tell it's not what you're looking for if you haven't read the docs?

It indeed fits your description, as there is both alpha_op_ex and colour_op_ex.

[SirShane]

I've done a search through the Ogre source code for D3DRS_SEPARATEALPHABLENDENABLE in the Direct3D renderer (It does not exist in my copy). If alpha_op_ex sounds like what i'm describing, then I'm probably not explaining it correctly. From what I understand, alpha_op_ex is a fixed-function state for managing the way alpha is blended between texture layers. That's not what i'm looking for. What i'm looking for is a way to manage how scene blending is performed for the alpha channel, seperately from the color channel. This has nothing to do with texture unit blending.

[xavier]

You can control the blending for the scene separately as well:

http://www.ogre3d.org/docs/manual/manual_16.html#SEC41

[SirShane]

scene_blend applies blending to the color channel and the alpha channel using the same algorithm, as far as I can tell. I need a way to set the algorithm used separately: one algorithm for the color channels (RBG) and another for alpha.

For example, we might have the same scene_blend command and then add an additional scene_blend_alpha, which acts like scene_blend but only for the alpha channel.

I apologize if Ogre already provides this functionality, but as far as I can tell it doesn't. The documentation does not talk about it at all, and the source code does not appear to have the code necessary to make it work.

[haffax]

Hi SirShane,

I guess you're right, this is indeed not in Ogre. scene_blend is combined, and alpha_op_ex only works between texture units / vertex colours but doesn't respect the current scene pixel.

Your proposal sounds good to me and it doesn't change the old behaviour, so it doesn't break anything. I can't speak for the team but a patch is probably welcome.

[SirShane]

Haffax,

Thanks. I'm working on a patch right now and I'll submit it as soon as I test it for both GL and D3D.

[sinbad]

Yep, we don't do this yet, I've never really thought about it. Guess it's handy for RTT renders, look forward to the patch.

Good to see you back after so long

[SirShane]

Hi Sinbad,

I've submitted a patch for this feature here: http://sourceforge.net/tracker/index.ph ... tid=302997