Re : Drawing OpenGL into PGraphics with alpha 0.5f produces extremely faint output, more like 0.1f

The blending calculations that the renderer does behind the scenes are quite straightforward, if you look at the blendModeImpl() function in PGraphicsOpenGL all it does is basically to call glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA), so you should be able to define your own custom blending mode by using those other gl calls, maybe there is something else you need to do in the shader code? I can look at it in more detail later.