CODEWITHSUNDEEP
pythonimagingpsychopy
Nolan Conaway
Nolan Conaway

Reputation: 2757

image rendering issue in psychopy

I am a long-time psychopy user, and i just upgraded to 1.81.03 (from 1.78.x). In one experiment, i present images (.jpgs) to the user and ask for a rating scale response. The code worked fine before the update, but now i am getting weird artifacts on some images. For example, here is one image i want to show:

actual image

But here is what shows up [screencapped]: shown image

You can see that one border is missing. This occurs for many of my images, though it is not always the same border, and sometimes two or three borders are missing.

Does anyone have an idea about what might be going on?

Upvotes: 1

Views: 295

Answers (1)

Nolan Conaway
Nolan Conaway

Reputation: 2757

I received this information from the psychopy-users group (Micahel MacAskill):

  1. As a general point, you should avoid using .jpgs for line art: they aren't designed for this (if you zoom in, in the internal corners of your square, you'll see the typical compression artefacts that their natural image-optimised compression algorithm introduces when applied to line art). .png format is optimal for line art. It is lossless and for this sort of image will still be very small file-size wise.

  2. Graphics cards sometimes do scaling-up and then down-scaling of bitmaps, which can lead to issues like this with single-pixel width lines. Perhaps this is particularly the issue here because (I think) this image was supposed to be 255 × 255 pixels, and cards will sometimes scale up to the nearest power-of-two size (256 × 256) and then down again, so easy to see how the border might be trimmed.

  3. I grabbed your image off SO, it seemed to have a surrounding border around the black line to make it 321 × 321 in total. I made that surround transparent and saved it as .png (another benefit of png vs jpg). It displays without problems (although a version cropped to just the precise dimensions of the black line did show the error you mentioned). (Also, the compression artefacts are still there, as I just made this png directly from the jpg). See attached file.

  4. If this is the sort of simple stimulus you are showing, you might want to use ShapeStim/Polygon stimuli instead of bitmaps. They will always be drawn precisely, without any scaling issues, and there wouldn't be the need for any jiggery pokery.

  5. Why this changed from 1.78 I'm not sure. The issue is also there in 1.82.00

Upvotes: 1

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