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Rasa Bhattarai

Thank you fro your reply. I want to create an experiment by myself just as the author of the above paper. Initially, I have created a stimulus in unity and using Tobii eye tracker to calculate the gaze points. I have read so many papers and it mentions about the importance of visual angle in such calculation. I really need help on this. My concerns are :-
1) In my experiment, an object(circle) is shown at center of screen. I can calculate the screen coordinates at this point using unity and it shows (600,280). Now after 2 seconds the position of the object changes to (800, 280) then to (600, 400) etc. My question is: Based on these screen coordinates position, how do I calculate visual angle ?
2) I have read the formula for visual angle. From the formula, i can calculate the distance between screen and observer but how do I know about the size of the object on the screen ? It is measured in pixels but the formula requires same unit of measurement for size of object and distance between visual display and observer. How do I convert the screen coordinates point to physical unit ?
Any help would be highly appreciated. Thank you.