Drawing a shader generated PGraphics onto a non OpenGL PGraphics object

[r.jaoui]

I am writing this answer to close subject : apparently to apply a shader, something has to already have been drawn onto the PGraphics object, weither it is P2D or not. The simple fix was to apply a background color before applying the shader, weither it is a modifyer fragment shader or a “rendering” shader that draws “on top” of the previous pixels.