Converting pixels to cm?

[ilkekav]

I added credit card scale to my online task, but I previously determined the line lengths in pixels. Now how do I convert them to centimeters?

[wakecarter]

If you have something you want to be 2.5 cm wide then give it a width of 2.5*x_scale

[ilkekav]

Okay, so in this case, I should delete elif and el statements, and is this how it’s supposed to look?

[wakecarter]

Those are for some sensible starting values before the credit card gets resized. Don’t pick cm as your online units. Personally I think it’s best to use height, but you could use pix. You could delete the bits you don’t need or leave them there for screen scale to check what units you are using.

[ilkekav]

I mean, we want everyone to see exact line lengths, not proportional values. So, in this case, I think centimeters would be right. Why did you think I should use height or pixels?

[wakecarter]

Are you running the study online?

If so, then you don’t know the size or the participant’s monitor. The cm units only work if you have told PsychoPy the size of monitor being used. Screen scale is designed to allow you to work out the participant’s screen size in pix or height units by resizing a credit card (common object of known size)

[ilkekav]

Yes, it’s going to be an online study. You mean adding a code that assumes the size of the monitor? Can we assume that the standard is 1080 x 1920? Can we do it like that?

[wakecarter]

I wouldn’t recommend assuming the size of the monitor in pixels. Have you tried the screen scale demo?

https://run.pavlovia.org/Wake/screenscale/

Here are some threads which might help.

How to use screen scale routine

Hello, I’m trying to build an online experiment and I need to adjust the resolution for every participant individually. I’m using screen scale routine (this one: Pavlovia), but the resolution doesn’t seem to change after this routine. I’ve tried inserting code component in other routine which contains an image (the code is: expInfo[‘Window_size’] = [x_scale, y_scale]), but the image size is not changing. I suppose I’m doing something wrong, how could this be solved?

Consistent image size across screens

URL of experiment: https://run.pavlovia.org/aelreym/concurrent_schedule_task/html Description of the problem: In the routine “cigarette length” I ask participants to estimate the real length of a cigarette by clicking on the appropriate image. So I have images of cigarettes with the real length of cigarettes and some deviations. In my builder experiment I have set the units of my images to “cm” but this doesn’t work online. I have changed it to “from exp settings” and this does adapt the size…

Way to get monitor information like size when running exp online?

Description of the problem: I am trying to figure out how to get the monitor size of each participant that takes my study online and store it as a variable in my excel data files. How do I do this?

[ilkekav]

Yes, I added that to my experiment, but this is just for making sure that everyone will receive the same length of stimuli, right? So after adding this, I should also make some adjustments to make sure that everyone receives the same length in cms, if I’m understanding correctly. I mean I’m not sure how to convert the measurements to centimeters at this point after adding the credit card reference.

[ilkekav]

hey, if I’m missing something, can you please explain how to convert @wakecarter

[Tango]

Hi,
If you put units in pixels, credit card scale routine will make sure that all your participants have the same length/width in cm no matter their screen size/resolution. You don’t have to convert back to cm.

[ilkekav]

Thank you, we converted the the predetermined pixels to cms considering our own computer (1080px), we calculated how many centimeters the pixels we previously determined correspond to. Now we have our distributions in cms. How to proceed from here? I mean adjusting the line lengths based on scaling factor derived from screen scale??

[Tango]

As you have calculated in cm the size of the stim that you want the online particpants to see, all you have to enter in the size of your stim component is: (the_size_in_cm_you_want * y_scale). y_scale is the scaling factor that the routine give to each participant (which is of course different for each screen size and resolution).
For example: if the y scaling factor of one participant’s screen is 55.6 pixels per cm and you want him to see a 3cms line, then the size in pixels is 355.6 (3 y scaling factor)

[ilkekav]

Okay so considering line_rect_len is the name of the desired line lengths in cm in my spreadsheet, should I just write it like this:

line_length_px = thisTrial[‘line_rect_len’] * y_scale

Or:
line_length_px = line_rect_len * y_scale

And here:

[Tango]

Yes exactly, “line_length_px” in you code is the specific size in px that match the size of “line_rect_len” in cm for one particular screen !
I’m not totally sure about how you built your experiment so I can’t tell if this line of code is at the right place (as long there are no bugs it should be fine)