Consistent image size and location for an online study

Hi :slight_smile:

I am working on a project where we will be presenting pairs of faces that appear to be looking at different objects (at the top or bottom of the screen) and appear in different locations (basically, closer to the object or further away). Part of this involves translating the stimuli used on a different platform into a psychopy experiment that works online.

I am struggling to figure out the best units to use so that the experiment looks appropriate on different screens. My understanding is that I could use either ‘height’ or ‘norm’ units but that a disadvantage of using ‘height’ is that this is based on the aspect ratio of the screen which may cause difficulty in defining the size and location of stimuli for different monitor types?

The two types of images (objects and faces) are of different sizes but the size of each image within the category (e.g. faces) is the same. I am also not sure how to convert the actual image size into either nom or height units outside of trial and error which would be time-intensive.

Basically, I think I need some advice on how which would be the best units to use for defining the size and location of the images and how to convert the images (for which I have the absolute size in pixels) into these units.

I have read through the documentation on the different unit types in psychopy and some previous forum posts on using these units with pavlovia but I am still quite confused, to be honest.

Thank you in advance for your time.

The advantage of height is that it will scale to different screens, as the size is proportional to the height of the screen, without stretching anything (a size of (1,1) is a square). The advantage of norm is that it will scale to both dimensions of a different screen (a position of (-1, -1) is the bottom left corner), however this also means that things will be stretched to the dimensions of the screen (a size of (1,1) is the half the size as the screen, if the screen is a rectangle then this will be a rectangle).

If you want a more hands-on way to visualise this, I’ve recently made a demo which allows you to change the position, size and units of a square to see the differences between units:

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