Additive Blending Problem

Arthor · 2017-06-06T21:54:03

I'm having trouble with additive blending, Im trying to draw particles onto a black background additively so they build up to white at the center. However a single particle (point) is saturating to white immediately. Any RGB values seem to saturate to 1, this is not what I expected. Shader output color is: RGBA .5f, .1f, .1f, 1.0f Clear Color is: 0,0,0,0 No Blending: [attachment=35984:NoBlend.png] Blending (GL.BlendFunc(BlendingFactorSrc.One, BlendingFactorDest.One);) [attachment=35985:PinkBlended.png] I was not expecting white, I was expecting the same red as the first image. Am i misunderstanding additive blending? Or am I doing something else wrong? I've hacked my app to pieces trying to solve this. I've checked the shader output, I can change the colors and it matches up outside of blending. Blending is enabled, if i use a dark blue background with dark red particles I get pink/purple, its just 100% pin not dark pink. Im only drawing once, I'm not drawing the same particles on top of themselves several times. Its not just the particles, the whole scene whites out if I draw it with blending enabled. I've tried rendering to the backbuffer, an fbo and a multisampled FBO, same thing. Any other suggestions are welcome. Thanks, Arthor.

Graphics and GPU Programming Programming

Started by ArThor May 23, 2017 11:08 PM

12 comments, last by _WeirdCat_ 7 years, 8 months ago

21st Century Moose

13,459

May 25, 2017 05:47 PM

Another reason may be that clears are affected by the current write masks; if you have glColorMask (0, 0, 0, 0) when you clear, the color buffer will not be cleared! This will result in multiple frames work accumulating in the manner described by the OP.

Direct3D has need of instancing, but we do not. We have plenty of glVertexAttrib calls.

ArThor

Author

579

May 30, 2017 06:19 PM

I never worked out why GL.BlendFunc(BlendingFactorSrc.One, BlendingFactorDest.One); was giving me the results I got but I have worked around it.

I used GL.BlendFunc(BlendingFactorSrc.SrcAlpha, BlendingFactorDest.SrcAlpha); instead and manually set my alphas to 1.0f in the particles. This results in a 1 in the destination after a particle is written and then any source particles thereafter are also 1.0f, which is what I wanted in the first place, I do not understand why the result is different though. Perhaps I'm not understanding what GL_ONE is supposed to do or perhaps OpenTK is passing the wrong enums, but I'm just guessing. Hopefully this helps someone else if they get stuck though.

Thanks for all of the suggestions, helped me work though an alternative method.