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　　gbiv
　　Newbie
　　Registered: 07/03/04
　　Posts: 39
　　Loc: ISU VRAC
　　Hello,
　　I'd like to create a RTT using an FBO which I then use as a texture on a billboard. My problem is that I'm not able to generate a texture that has "alpha" values. For instance if it were a tree that I rendered in the FBO, I'd try to set my clear color to (0,0,0,0) and then render my tree model in the FBO to create the texture. The output however has the tree surrounded by black rather than "clear". Is this the correct behavior or should I be able to setup the FBO properly to create a the texture properly?
　　thanks for any input.
　　biv
　　#237025- 04/14/0801:38 AM Re: FBO's and alpha[Re: gbiv]
　　k_szczech
　　OpenGL Pro
　　
　　
　　Registered: 02/20/06
　　Posts: 1026
　　Loc: Poland
　　Explain "tree surrounded by black". Do you mean that your entire texture is black, or do you see black edge aroud the tree?
　　If you're experiencing the 'black edge' problem, then it's because pixels of texture are filtered with black background pixels. The simplest solution would be to enable alpha testing when rendering tree from texture and set alpha test to (GL_GREATER, 0.95f).
　　The nicest solution would be to perform blur on the texture after you rendered tree into it. Texels with alpha >0 should copy their color to neighbour texels that have alpha = 0, but alpha should not be copied.
　　#237030- 04/14/0804:28 AM Re: FBO's and alpha[Re: k_szczech]
　　babis
　　Contributor
　　Registered: 12/09/07
　　Posts: 76
　　Loc: Hull, UK
　　For the 'black edge' problem :
　　It's even nicer solution,if you can afford it, to render first the blurred tree on the texture ( without writing to alpha as k_szczech said ) &then layer upon it the 'normal' rendered tree. This way you have no blurring on the inside (where alpha == 1)
　　#237032- 04/14/0806:17 AM Re: FBO's and alpha[Re: gbiv]
　　Dark Photon
　　Regular Contributor
　　Registered: 10/06/04
　　Posts: 191
　　Loc: Arlington, TX
　　 Originally Posted By: gbiv
　　I'm not able to generate a texture that has "alpha" values....The output however has the tree surrounded by black rather than "clear".
　　This will solve your problem:
　　Tom Forsyth's Blog(See Pre-multiplied Alpha article)
　　Bottom line: stop using SRC_ALPHA / ONE_MINUS_SRC_ALPHA blending. Use ONE / ONE_MINUS_SRC_ALPHA blending, and pre-multiply the alpha into the color components. Edited by Dark Photon (04/14/0806:19 AM)
　　#237051- 04/14/0801:19 PM Re: FBO's and alpha[Re: k_szczech]
　　gbiv
　　Newbie
　　Registered: 07/03/04
　　Posts: 39
　　Loc: ISU VRAC
　　Sorry for the late responses here. By that I mean, the tree renders as expected on the textured quad however the entire quad is visible. The portion of the quad that is not the tree (not just the border of the tree) is black (or whatever I set my clear color to) rather than completely transparent, so the "illusion" of the billboarded trees is lost because you can tell it's just a textured quad. Did that make sense or do I need to post some pics for clarity?
　　#237054- 04/14/0802:02 PM Re: FBO's and alpha[Re: gbiv]
　　babis
　　Contributor
　　Registered: 12/09/07
　　Posts: 76
　　Loc: Hull, UK
　　ok so you either got no alpha at all, or you have but you don't use it. How do you render to the texture? &how do you draw it afterwards?
　　#237057- 04/14/0803:03 PM Re: FBO's and alpha[Re: babis]
　　zed
　　OpenGL Guru
　　Registered: 07/06/00
　　Posts: 2444
　　Loc: S41.16.25 E173.16.21
　　 Quote:
　　were a tree that I rendered in the FBO, I'd try to set my clear color to (0,0,0,0) and then render my tree model in the FBO to create the texture. The output however has the tree surrounded by black rather than "clear".
　　yes thats the correct behaviour
　　glClearColor(...) clears the buffer to that color, it doesnt make that color invisible
　　to make the black invisible u have to set those pixels alpha values to say 0.0 (+ the trees pixels alpha values to 1.0) + then do alphatesting or blending etc
　　i suppose u could in a shader make black invisible by going
　　if ( dot( vec3(0.0), texture_value ) >0.99 )
　　discard;
　　but #A its pretty messy + #B its gonna be slower
　　#237087- 04/15/0805:46 AM Re: FBO's and alpha[Re: zed]
　　Dark Photon
　　Regular Contributor
　　Registered: 10/06/04
　　Posts: 191
　　Loc: Arlington, TX
　　 Originally Posted By: zed
　　 Quote:
　　I'd try to set my clear color to (0,0,0,0) and then render my tree model in the FBO to create the texture. The output however has the tree surrounded by black rather than "clear".
　　yes thats the correct behaviour
　　glClearColor(...) clears the buffer to that color, it doesnt make that color invisible
　　Well, he said he cleared to 0,0,0,0, which has alpha = 0, which by convention is 100% transparent. Something else much be wrong. Such as rendering with BLEND disabled or some such.
　　babis, check the alpha values in your edge texels in your texture and make sure their alpha is 0. Also check that when you're blending this texture onto the frame buffer you have BLEND mode enabled:
　　 Code:
　　
　　glEnable ( GL_BLEND ) ;
　　glBlendFunc ( GL_ONE, GL_ONE_MINUS_SRC_ALPHA );
　　glBlendEquation ( GL_FUNC_ADD ) ;
　　#237096- 04/15/0808:32 AM Re: FBO's and alpha[Re: Dark Photon]
　　gbiv
　　Newbie
　　Registered: 07/03/04
　　Posts: 39
　　Loc: ISU VRAC
　　Hello all, thanks for the replies. I've tried most of these but with no success. I think this has to do with my FBO setup. However, I'm not sure how to get around the problem if the clear color isn't setting alpha values in the FBO:
　　Here are some screen shots of my problem:
　　http://www.vrac.iastate.edu/~biv/fbo_clear_color_problem/clearcolor_black_alpha_0.tiff
　　http://www.vrac.iastate.edu/~biv/fbo_clear_color_problem/clearcolor_red_alpha_0.tiff
　　The flag is textured with the results of rendering the fire to my FBO. No matter what I set the clear color to, the texture doesn't seem to have any alpha information from the FBO outside of the object that I'm rendering (in this case the fire) so even if my texenv is set to REPLACE, the texture still retains the clear color as is shown on the flag. My texture is of format RGBA and is a color buffer attachment to my FBO so I think this is correct...
　　I'm not sure how to access those texels without adding some extra pass to post process the texture info as zed suggested, and I agree, this could get messy. Can anyone else reproduce this problem with a simple FBO rtt? This could help me narrow down if the problem is in my code or not.
　　Again thanks for the input here.
　　biv
　　#237103- 04/15/0801:22 PM Re: FBO's and alpha[Re: gbiv]
　　zed
　　OpenGL Guru
　　Registered: 07/06/00
　　Posts: 2444
　　Loc: S41.16.25 E173.16.21
　　 Quote:
　　Well, he said he cleared to 0,0,0,0, which has alpha = 0
　　i believe those alpha values are the destination alpha values eg GL_DST_ALPHA
　　i think what gbiv wants is SRC_ALPHA (ie from the texture)
　　ok looked at the screenshot i think he wants
　　A/ first to draw textureA(fire) into a FBO textureB
　　B/ draw this textureB onto a polygon onscreen
　　perhaps try
　　A/ set the textureB alphavalues from the textureA alphavalues (many ways of doing this)
　　B/ draw this texture onscreen with GL_DST_ALPHA, GL_ONE

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