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OpenGL Separate alpha blend enable states for MRT

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Hello everyone, My application uses D3D9. I have a main render target into which the scene is rendered, and an additional render target used for writing some data which I'm using later. My problem is that when the scene is rendered with alpha blending enabled, my second RT gets blended as well, which produces meaningless values. Is there a way in D3D9 to enable alpha blending only for a specific RT? The D3D docs don't mention such an option, but I know that in OpenGL it's possible with some extension (DrawBuffers2 I think), so it's not a hardware limitation. Thanks.

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D3D10 you mean I guess...
That's the answer I was afraid of... I can't move to D3D10. Isn't there some hack to do this in D3D9, like a secret format or something (even NV-only will be fine)? I know there are formats which don't undergo any post pixel shader operations, but for some reason I couldn't get them to work properly.

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Whoops, definitely meant to put "D3D10" there. :P

Anyway I don't know of any hacks or vendor-specific stuff to get around this. You could do the blending yourself in the shader by ping-ponging between render targets, but that's not exactly cheap...

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I think that though alpha blending is enabled for all render targets, the alpha of a specific target will be used for that target's rendering. In that case, just setting the alpha to 1 for the second RT will solve the problem. You're losing one output component, but that shouldn't be an insurmountable problem.

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How about when the second RT is a float target (say R32F)? What would be considered as alpha here?

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Quote:
Original post by eigers
How about when the second RT is a float target (say R32F)? What would be considered as alpha here?


It will still use the alpha value of the output, even if that alpha component isn't being stored anywhere.

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For the source it's understood, because the pixel shader always outputs a 4-component vector, but what about the destination, which doesn't store the alpha component at all? Is it always expanded to 1?

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Actually no, I don't really have a problem there. But I do have a problem with blending modes which use the destination color (e.g. D3DRS_DESTBLEND = D3DBLEND_ONE), as I don't want the second RT blended at all.
It seems that there really is no way around this in D3D9...

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What about premultiplied alpha ?

RT1 : I want src to be multiplied by alpha and dst to be multiplied by 1-alpha and both be added together : final1 = (1-alpha1) * dstcolor1 + alpha1 *srccolor1
RT2 : I want src to be overwritten over dst : final2 = 1 * srccolor2 + 0 * dstcolor2

As a blend op you take D3DOP_ADD obviously.
As blend source you take D3DBLEND_ONE (<- important in the premultiplied alpha case)
As blend dest you take D3DBLEND_INVSRCALPHA

when you write color 0 in your pixel shader you do :
mul r0.rgb, r0.rgbb, r0.aaaa
mov oC0, r0

Assuming ro.aaaa contained alpha1 and r0.rgb contained srcColor1.rgb
oC0.rgb then contains srccolor1.rgb * alpha1
oC0.a then contains alpha1
the result of the equation is oC0.rgba * ONE + destcolor1 * (1 - oC0.a)
or srcColor1.rgb * alpha1 + destcolor1.rgb * (1 - alpha1) which is what you wanted.

when you write color 1 you :
mov r1.a, constant(1.0,1.0,1.0,1.0)
mov oC1, r1

oC1.rgb contains srcColor2.rgb
oC1.a contains 1
the result of the equation is oC1.rgb * ONE + destcolor2 * (1 - oC1.a)
or srcColor2.rgb + destColor2.rgb * 0 which is also what you wanted.

The only problem with this approach obviously is that you've replaced the alpha value. But that's a general problem with blending is that it always assume the fourth channel of anything is going to be the blending factor (and that is also true for single RT case) which means in alpha blending situations you can only effectively write 3 components at a time per rendertarget.. Use more RTs (in MRT mode) if you want to write more.

LeGreg

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Thanks for this interseting idea.

However, as ET3D pointed out, each RT is blended with its own alpha, so I don't really have a problem with the SRCALPHA/INVSRCALPHA type of blending. I just set the source alpha to 1 when writing to oC1, and so the destination is overwritten.
My problem is with blending states where the factor of the destination color is not related to the source at all (such as D3DRS_DESTBLEND = D3DBLEND_ONE), which are generally used for glowing effects. I don't see how I can overcome this without knowing the current destination color, which of course I can't. Am I'm missing something here?

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Quote:
Original post by eigers
However, as ET3D pointed out, each RT is blended with its own alpha, so I don't really have a problem with the SRCALPHA/INVSRCALPHA type of blending. I just set the source alpha to 1 when writing to oC1, and so the destination is overwritten.
My problem is with blending states where the factor of the destination color is not related to the source at all (such as D3DRS_DESTBLEND = D3DBLEND_ONE), which are generally used for glowing effects. I don't see how I can overcome this without knowing the current destination color, which of course I can't. Am I'm missing something here?


Didn't I just tell you how ? Sorry if that wasn't clear..

The problem you had was that you were using SRCALPHA/INVSRCALPHA, where what you wanted was the premultiplied alpha.

If you want to replace, you set output alpha to one (without changing the source color). If you want to add (src+dst as in the glow effect) you output alpha to zero : that way dest is multiplied by (1-zero) before being added to the srccolor (that you don't modify either). You can also emulate the srcalpha/invsrcalpha case by premultiplying (hence the premultiplied alpha name) the srccolor by the alpha before outputing it.

A thing that doesn't work with this approach is if you want different alpha per channel to affect the dest color before combining (say dest.r * (1- alpha1), dest.g * (1 - alpha2), dest.b * (1 - alpha3) ). Well that level would require an extra color being output and used in the blend equation, which is only possible in dual blend mode using the Direct3D10 API.

LeGreg

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