How to manipulate the stimulus position precisely for online experiment

Online experiments
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Thanks for your reply, I assume that [0.343,0.5] is for size in height units? So far If I understand well, there has no solution yet to convert directly from pixels to height see this post. At the moment, my solution is to set the size as (0.69,1) as the aspect ratio of the stimulus is 3:4. Then adjusting the target position to match the previous fixation cross of [0,0] by doing trial and error until it looks acceptable on my computer. It is probably not the most elegant and systematic way to handle it.