Why do thin straight lines look dashed after a slight crop?
Asked 4/12/2016
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2 answers
0
After taking a photo of a street reflected in a large fisheye mirror, I noticed the telephone wires looked normal in the original image. But after applying a very small crop (under 10%, with aspect ratio preserved), the wires appeared as dotted or dashed lines. Why does this happen, and how can I fix or avoid it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
1
A. There is a good chance that the issue is only in your display software (meaning not in the picture). If the number of pixels in the display is different of the number of pixels in the picture, the software needs to make it fit somehow. Depending on the level of sophistication in the code, it simple skips every Nth pixel row, which results in potentially the only pixel row with the telephone line being not displayed.
B. (Less probable - the result would look different) If you save your picture as JPG, you need to understand that JPG is not generally lossless; most save dialogs give you the option to lose between 0% and 99%. The default is mostly 20% loss, but it depends of course on your software and settings. So if you edit anything and then save, you lose 20%, next open and save another 20%, etc.
Originally by user46784. Source · Licensed CC BY-SA 4.0
user46784
10y ago
0
Generated from our catalog & community — verify before relying on it.
This is usually a display/resampling issue, not damage to the actual photo.
When you crop, the image dimensions change. If your software then shows the cropped image at a reduced zoom level, it has to resample the image to fit the screen. Very thin features like telephone wires may fall between sampled pixel rows/columns, so the viewer effectively skips parts of the line and it appears dashed.
How to check/fix it:
- View the image at 100% (1:1) zoom and see if the wires are still broken.
- Try a different viewer or editor; some programs use better scaling algorithms than others.
- Export at high quality if using JPEG, since repeated JPEG saves can add artifacts, though that usually looks different from dashed lines.
- For critical thin-line detail, avoid repeated resize/save cycles.
So the crop itself probably didn’t create dotted lines; it changed the pixel dimensions, which made your software display the thin lines poorly at that zoom level.
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