Why the line that set as 15cm is not 15cm?

If this template helps then use it. If not then just delete and start from scratch.

OS (e.g. Win10): MacOS version 12.0.1
PsychoPy version (e.g. 1.84.x): v2022.2.5
Standard Standalone? (y/n) If not then what?:y
What are you trying to achieve?:
I would like to draw a line with the polygon component. I set the Size as “[15,1]” and the Spatial Units as “cm”. But it is about 14cm, but not 15 cm, in the full screen of 19201080. And it is shorter in a full screen of 25601600. I am wondering if I need to set something so that it will always be 15 cm.

What did you try to make it work?:

What specifically went wrong when you tried that?:
Include pasted full error message if possible. “That didn’t work” is not enough information.

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Check the monitor definition for your display. PsychoPy needs to know its physical dimensions accurately to get absolute units like this to display properly.


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Hi thank you for the reply! I run the experiment on Mac. When I run, there was a warning saying that “User requested fullscreen with size [1280 800], but screen is actually [1440 900]. Using actual size”.

Honestly, I am a little confused. When I check “About this Mac”, it shows that the full screen size is [2560 1600]. But in “Display” it shows the screen size is [1440 900]. Also, in PsychoPy, when I cheked the “Full-screen Window”, the window size automatically becomes [1440 900] but it still gives me the above warning that I am requesting a window size of [1280 800]. I am not sure what the [1280 800] is and where can I change this setting considering PsychoPy already automatically shows that the screen size is [1440 900].

I also tried to set the Window Size as [2560 1600] in PsychoPy but the window was really big then. So now I am not sure which one is right size of the full screen.

Thank you so much for your time!

Pixels don’t have a standardized size. 2560 pixels could be 1 cm or 1 kilometer as far as the computer knows. It has no way of deducing the “real world” measures (cm) from pixels alone. So you have to tell it how tall and wide your display is in cm (for every screen the experiment is run on!). Only then it can calculate the correct dimensions in cm.
This info needs to be provided in psychopy’s monitor center.
You dont’t have to tell Psychopy the dimensions in pixels as this info is provided by the OS.

Hi thank you so much for the help and it works!!

I set the “Screen Width (cm)” as the width of the screen (exact the screen that displays, not include the frame around the screen!) and set “Size” as [1440 900] in Monitor Center.

Thanks again for the explanation!! It really helps a lot!