Compressed vs. uncompressed image files

[David McFarlane]

I am still thinking about this. As I understand from the manual (most of my knowledge of DirectRT comes from reading the manual rather than actual usage), all the stimuli for each trial are fully cached during the ITI before the trial. I.e., all the image files for the upcoming trial are read from disk during the ITI, and after that DirectRT simply flips screens during the trial. In that case, any delays from disk I/O or image decompression happen during the ITI and do not affect timing within trials. So any added delays from disk I/O or image decompression would only potentially add to the ITI, and if the ITI is already long enough then no one will notice the difference between .bmp, .jpg, .gif, or whatever, and researchers should use whatever format is most convenient for them.

Also as I recall, if the specified ITI is too short then DirectRT will lengthen it as needed, and will record this time in the output file. That gives us a way to run the test of, say, .bmp vs. .jpg. First make two copies each of several image files in both formats (should really make several versions of each .jpg at different compression rates, etc.). Make a simple DirectRT input file that just shows one image per trial, and set the ITI to 0. I think this works even better if you use stimulus lists, then you can swap .bmps for .jpgs just by changing the name of the stimulus list files. Anyway, run the input file and look at the actual ITIs that result under the different conditions.

I will not have the time to run this test myself, I just wanted to specify a testing methodology in case anyone wants to try.

-- David McFarlane, Professional Faultfinder

[jarvis24]

Yes on all counts (as far as I can tell), and I agree this would make a nice test. Would love to hear/see the results if someone could try it. Otherwise. I'll try it myself when I get the chance and will report back on the results.