Overlaying 3d scene over 2d bitmap using ofFbo

I’m clearly misunderstanding a bit of what’s going on with ogl framebuffers.

The concept I’m trying to build is quite simple. I’ve got a little 3d scene with a camera and I want to overlay it over a bitmap. The only *maybe* weird hangup here is the resolution I’m building for which is 1080x3840 (two portrait 1080p displays stacked high).

So up in setup I’m allocating my framebuffers (and I don’t think I even need two, but just for grins and troubleshooting…)

fpo.allocate(1080,3840);  //framebuffer for 3d scene  
bgfpo.allocate(1080,3840); //framebuffer to hold 2d image  
bgi.loadImage("bg0.jpg"); // background image  

then down in draw (which is a bit disorganized on account of general troubleshooting)

    ofBackground(255, 0, 0);  // give me a red background to better see what's going on  
    ofClear(1.0f, 1.0f, 1.0f);  
    bgi.draw(0,0);  // draw the background image into the bgfbo  
    ofClear(1.0f, 1.0f, 1.0f);  
    em.update(); // my particle emitter, all the objects get added to the scene here  
what I image should happen here is that the background gets drawn,   
        then I turn on alpha blending (the 3d scene should have an alpha channel   
        (stencil buffer?) no? and then I draw down the 3d scene on top)  
    bgfbo.draw(0,0, bgfbo.getWidth(), bgfbo.getHeight());  
    fbo.draw(0,0, fbo.getWidth(), fbo.getHeight());  

What I imagine here is I’m painting down the 2d bitmap background, then turning on alpha and then painting down the alpha masked 3d scene.

However what i get is just my 2d background image. If I go ahead and remove the bgfbo.draw call I get my 3d particle system all nice and alpha masked against the red background.

I know I’m missing something very obvious, but it’s been many years since I’ve had to deal with openGL on this level so any help would be appreciated.


Try doing ofDisableDepthTest(); before drawing your second fbo to the screen. Does that work?

Also, I don’t think you even need fbo’s to accomplish what you’re trying to do. Just draw the image to the screen first and then draw your particles. You might need to disable depth test before drawing the image & then enabling it again before drawing the particles. Using fbo’s for this is gonna give you a performance hit.

Yup I came to all the same conclusions after some hacking. Thanks a lot though!