Addititive blending fbo

advanced
#1

Hi,

I am at the very end of my OpenGL powers, so I decided to ask for help. After playing around with planet rendering and fake atmospheric scattering and finally getting it to work, I tried to add a sun and observed some odd behaviour.
Please see here: https://www.youtube.com/watch?v=YxdZoh2iT6g&feature=youtu.be
My code is as follows, I’ve added comments to explain:

//----------------------------------------------------------
// code for atmospheric scattering
// I used a sphere bigger than the planet that has some lighting effects with shaders
// I record this in an fbo to blur it later on 
// I do not set blending mode during the recording of the fbo, just when drawing it 

atmScatter.begin();  
ofPushMatrix();
ofTranslate(ofGetWidth()/2, ofGetHeight()/2);
ofDisableBlendMode();
ofClear(0);
shader3.begin(); //  shader for lighting effect
shader3.setUniformBuffer("Light", atmosphereLight);
shader3.setUniformBuffer("Material", blue);
shader3.setUniformBuffer("NormalMatrix",
                         calcNormalMatrix(glowSphere.getGlobalTransformMatrix()*camera.getModelViewMatrix()));
shader3.setUniformTexture("tex", glow.getTextureReference(), 0);
glowSphere.draw();
shader3.end();
ofPopMatrix();
atmScatter.end();

//----------------------------------------------------------
// code for blurring the atmospheric scatter fbo

ofPushStyle();
ofEnableBlendMode(OF_BLENDMODE_ADD);
shaderBlur.begin();
fboBlurOnePass.draw(0, 0);
shaderBlur.end();
ofPopStyle();
ofDisableBlendMode();

//----------------------------------------------------------
// code for drawing the sun, loaded as a 2d image

ofPushMatrix();
ofPushStyle();
ofEnableBlendMode(OF_BLENDMODE_ADD);
ofTranslate(sun.getPosition()); // translate to sun position
ofRotate(ang, vec.x, vec.y, vec.z); // rotate to face camera
glow2.draw(-2400/2, -2400/2, 2400, 2400); // set size
ofDisableBlendMode();
ofPopStyle();
ofPopMatrix();

Visually, this draws correctly, only that the sun is somehow overlapped by the atmosphere scatter fbo.
If I draw the sun behind the atmosphere scatter fbo, this does not happen anymore, instead, if I zoom the camera such that it is positioned behind the sun’s orbit, the sun image will then overlap the atmosphere scatter fbo.
I have no clue as to why this happens, especially since the effects look as intended. I have individually looked at the amt scatter fbo - and it is just the sphere with a black background, as expected, and also at the sun image, which is the sun you see in the video with a black background. I have also tried to use the SCREEN blend mode, but to the same effect.

Thanks a bunch for any tips on this!

p.s.: the shaders have nothing to do with this, I commented them out and, sadly, the effect is still present.

#2

I’ve found out why this is happening, apparently 2 translucent objects and depth testing is delicate in OpenGL. See this article for reference: [alpha test][1]
[1]: https://www.opengl.org/wiki/Transparency_Sorting

I’ve written the shader for alpha testing and the issue is solved, with the added overhead that virtually all of the blurring effect is lost, leaving some jagged edges behind, that are visible especially when the sun passes behind.

#3

I was just going to suggest that you look at sorting before drawing – you get weird effects with transparency and depth and in this kind of situation I try to look at depth sorting so I draw stuff that’s further back first.

#4

Yeah, I ended up drawing the sun behind the atmosphere fbo, and determining when to enable or disable depth testing for the sun’s drawing - to avoid the sun coming in front of the camera and blocking the fbo.