Fading out an impostor

Ilici · 2005-06-06T09:59:38

Ok I've got a problem with fading out an impostor billboard using some kind of alpha value. The problem is that I need (must) use the blending factors GL_ONE and GL_ONE_MINUS_SRC_ALPHA for the texture to blend correctly. This is because of the texture which is created by splatting a special texture for creating clouds (see Harris' cloud rendering articles). I can't use a different texture. Setting the blend factors to GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA will result in the billboard showing some nasty transparency effects. So is there a way to get my texture to blend with the framebuffer behind to fade from fully transparent to fully opaque? Simply setting alpha will whiten the billboard when alpha is anything less than one because of the blend factors. Is there some trick I can pull? I thought that the blending does Cr = Cs + Cd * (1 - As), but I need Cr now to fade out using As too. This is how it looks with SRC_ALPHA, ONE_MINUS_SRC_ALPHA: That's not very nice ! And how it looks good with ONE, ONE_MINUS_SRC_ALPHA: Much better but no fading possible: [Edited by - Ilici on June 5, 2005 4:40:32 AM]

Graphics and GPU Programming Programming

Started by Ilici June 04, 2005 07:01 PM

18 comments, last by GameDev.net 18 years, 11 months ago

Ilici

862

Author

June 05, 2005 01:33 PM

Quote:Original post by baldurk
What filtering method are you using? is it just linear? See if setting the background to white before rendering the cloud makes any difference at all anyway. Sometimes weird things work :).

I tried setting the background clear color to white when rendering the impostor and tried both ONE and SRC_ALPHA for the source factor, they don't work. My hat is all out of weird things to pull out [sad]

Here's the impostor texture with alpha, maybe someone can figure out how to fade it nicely:

baldurk

1,029

June 05, 2005 02:08 PM

Looks like the problem lies in your texture generation. Here's a blown up view of it in the gimp on my system:

I did a real quick erase of the black semi-transparent haze and whipped up an app to show the difference:

So it seems that black is creeping in there. I assume there's no black in your rendering of the cloud. I'm guessing here, but maybe when blending the rendered cloud with the 0,0,0,0 frame buffer created it? I don't know why it would still do that if you set the framebuffer to 1,1,1,0 though.

One solution I guess would be what I did and make it so that you get the texture data and loop through it going a=(r+g+b)/3 on it, but that's a lot slower.

Ilici

862

Author

June 05, 2005 03:20 PM

Well the edge is, though a little less visible, still present. The cloud must be blended with 1, 1 - SRC_ALPHA and I have no idea how to make it fade then. My simulation will have to do without switching from the impostor billboard to the 3d particles :(.

Maybe something could be done with the combiner but I'm not sure the combiner does it's thing after blending, otherwise it wouldn't work. I'm guessing what I want is this:

Result = (Incoming + Destination * (1 - IncomingAlpha)) * IncomingAlpha;

baldurk

1,029

June 05, 2005 03:50 PM

Quote:Original post by Ilici
Well the edge is, though a little less visible, still present. The cloud must be blended with 1, 1 - SRC_ALPHA and I have no idea how to make it fade then. My simulation will have to do without switching from the impostor billboard to the 3d particles :(.

Maybe something could be done with the combiner but I'm not sure the combiner does it's thing after blending, otherwise it wouldn't work. I'm guessing what I want is this:

Result = (Incoming + Destination * (1 - IncomingAlpha)) * IncomingAlpha;

Well the edge is still visible because I did a quick erase to remove the worst of it. If you removed all the black, then I'd think that it would disappear entirely.

hplus0603

11,917

June 05, 2005 04:08 PM

This is the difference between images generated for alpha (which would be mostly white), and images generated with pre-multiplied alpha (which is what you have). GL_ONE, GL_ONE_MINUS_SRC_ALPHA is the pre-multiplied alpha blending function. Just learn to love it and live with it -- it's the least incorrect options.

It's somewhat un-clear what you mean by wanting to "fade out" with pre-multiplied alpha. If you want to make some parts fade towards black, then just put black into the rendered image, but make the alpha strong.

enum Bool { True, False, FileNotFound };

OrangyTang

1,298

June 05, 2005 04:11 PM

I may be being stupidly obtuse here, but couldn't you just use ONE,ONE_MINUS as you are doing, then fade the vertex colours to black to fade out?

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Ilici

862

Author

June 05, 2005 04:13 PM

I mean I want it to fade from opaque to completely transparent.

Thanks to hplus0603 for sorting me out.

zedzeek

529

June 05, 2005 04:20 PM

try this
fade the color out as well

glColor4f(fade,fade,fade,fade)
draw cloud

Captain Courgette, eat more veg

Ilici

862

Author

June 05, 2005 04:33 PM

Quote:Original post by OrangyTang
I may be being stupidly obtuse here, but couldn't you just use ONE,ONE_MINUS as you are doing, then fade the vertex colours to black to fade out?

Quote:
try this
fade the color out as well

glColor4f(fade,fade,fade,fade)
draw cloud

Omg, that works! Seriously, I thought I tried that but it seems I did something wrong the previous time. Thanks to the both of you, 10x!

Shots:
Full Impostor: