Drawing two gratings with blendmode='add'

OS ( Win10):
PsychoPy version (v2023.2.3):
Standard Standalone? (y/n) y
What are you trying to achieve?: Create stimuli

Hi all

I am trying to replicate a study, namely Exp. 1 from Notman et al., 2005, Cognition.
In this research, two gaussian windowed gratings are added to produce the stimuli. What varies between stimuli is the spatial phase of the second grating.

When I use the OpenGl blend mode = add in the builder (or coder), the result is:

I have found previous comments referring to one (now solved) issue with text stimuli.
I have also seen some suggestions, but I cannot verify their validity.
I tried changing the library to pygame, it does not work.

So, is there any way to add two gratings (by not changing their opacity and specifically by using the ‘add’ mode)?

P.S. I have been able to reproduce the stimuli, more or less, by playing around with the second grating’s opacity, and using the ‘avg’ mode. But I need to be precise, it’s psychophysics after all.
P.S.2 I include the file I created for playing around with the gratings. You need to press “1” or “2” on the keyboard.
Categorization_Stimuli_Forum.psyexp (56.8 KB)

Thank you all for your time.
Fotis A. Fotiadis