I've been making a simple game in OpenGL, and implemented a screen fade-out using the old 'draw a black fullscreen quad and ramp up the alpha' trick. I'm doing all my shaders in linear space and using
GL_FRAMEBUFFER_SRGB to automatically apply gamma. When fading out the screen I noticed that the alpha blending also becomes nonlinear - to fade to ~25% brightness I need to use an alpha of ~0.95.
I fired up Unity for comparison, and its alpha blending also seems to work this way - when rendering in linear mode most of the opacity is bunched towards the top of the alpha range.
Is this expected behaviour? Is it correct from a PBR point of view? It seems very unintuitive. As a workaround, to get more intuitive blending, would it be reasonable to apply gamma correction of 0.45 to the alpha channel before blending?
EDIT: this is the result I get after blending a .95 alpha black quad over a pure white button. It could be bad gamma on my display as Simon F suggested in the comments, but it looks a lot brighter than 5% to me.